Recrystallization of single crystals of quartz

Tectonophysics - Elsevier Publishing Company, Amsterdam Printed in The Netherlands

RECRYSTALLIZATION

OF SINGLE CRYSTALS

OF QUARTZ1

B.E. HOBBS Department of Geophysics and Geochemistry. Canberra, A.C.T. (Australia)

Australian National University,

(Received February 14, 1968) SUMMARY Three types of experiment have been carried out to investigate the influence of stress, strain and initial crystallographic orientation on the recrystallization of single crystals of quartz. These experiments are: (1) stress annealing experiments, in which the specimen is loaded to a high differential stress at a relatively low temperature and the temperature then rapidly increased to a higher value while the differential stress is present; (2) annealing experiments in which the specimen is deformed at a low temperature and then heated under hydrostatic stress at a higher temperature; (3) syntectonic recrystallization experiments in which the specimen is deformed to high strains at a constant strain rate and high temperature. All experiments were conducted at 10 or 15 kbar confining pressure and at temperatures in the range 300 o - 1 ,400°C. Recrystallization does not take place in any of these experiments unless trace amounts of (OH) are present in the quartz structure. In both stress annealing and annealing experiments, nucleation of new grains takes place along narrow kink bands and the new grains have approximately the same orientation of c as that within the kink zones. The host orientation appears to exert a considerable control over the orientations of grains that ultimately grow to form an aggregate of polygonal grains. Grains in which c lies at 20° - 40° to the adjacent host c-axis grow fastest; grains in which c lies at 0” - 10” to the adjacent host c-axis rarely appear in the final preferred orientation. In syntectonic recrystallization experiments, nucleation and growth of new grains from submicroscopic regions does not appear to taKe place. Rather, subgrains that form in deformation bands at low strains increase their relative misorientations as the strain increases until an array oi diversely oriented grains with sharp grain boundaries develops. The resulting preferred orientations may be interpreted in terms of a host control similar to that observed in annealing experiments or as a tendency for new grains to form with c-axes at 50° to the axis of shortening. At 15 kbar confining pressure and temperatures above 800°C, cocsite nucleates and grows in highly strained single crystals of quartz. ‘Publication no.677. Institute of Geophysics and Planetary Physics. California, Los Angeles, Calif. (1J.S.A.) Tectonophysics,

6 (5) (1908) 353-401

University of

I(.53

INTRODUCTION

This paper presents the results obtained from studies of the recrystallization of single crystals of both natural and synthetic quartz under a variety of experimental conditions. The aim has been to delineate the mechanisms by which deformed single crystals of quartz recrystallize to form a relatively strain free aggregate of new grains and to determine the controls (if any) that the applied stress, the strain, and the host grain orientation exert on the lattice orientations of the newly recrystallized grains.

APPARATUS

AND EXPERIMENTAL

CONDITIONS

Three different types of experiment have been performed: (1) Experiments in which the single crystal is strained at a relatively low temperature, the temperature then being rapidly increased while a differential stress is present. In these experiments recovery and recrystallization of the quartz leads to a relaxation of the imposed differential stress. These are referred to as stress amlealiug experiments. (2) Experiments in which the initial straining is followed by an increase in temperature under a hydrostatic stress. These are referred to as aunealing experiments. (3) Experiments in which the crystal is strongly deformed at relatively high temperatures. Recrystallization takes place while the single crystal is being deformed at a constant strain-rate. These experiments are analogous to what the metallurgist would call hot work&g and to what the geologist would call syntecto?lic recrystallization. They are referred to here as syrztectonic vecrystallizatiolz experiments, Most experiments were performed in a solid pressure medium, constant strain rate apparatus designed by Griggs (1967) although a few were performed in the cubic apparatus (Carter et al., 1964). Experiments were performed at 10 - 15 kbar confining pressure and at temperatures in the range 300° - 1,400°C with strain rates of approximately 8 lo-“/set 8 * 10-7/sec. Summaries of the experimental conditions and results are presented in Tables I, II and III. Orientation conventions are as described by Carter et al. (1964, fig.%). As in other solid pressure medium deformation equipment developed to date (Carter et al,, 1964) considerable temperature gradients exist in the specimen during deformation. In the apparatus used here, these gradients have the same general character and magnitude as those described by Carter et al. (1964) and by Christie and Green (personal communication, 1968) for the cubic apparatus. Recently published work by Griggs (Griggs and Blacic, 1965; Griggs, 1967) has established the profound effect that trace amounts of (OH) in the quartz structure have on the mechanical properties of quartz. Similar profound effects have been found on the recrystallization behaviour of single crystals of quartz. For this reason, an attempt has been made to study the effect of (OH) concentration by deforming single crystals, whose initial (OH) content was known, in anhydrous jackets of AlSiMag 222, a commercial material composed of forsterite and periclase. In some experiments, water was introduced by using talc, pyrophyllite or kaolinite jackets, which break down in the range 600 - 830°C to give water and other products. l

3

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STARTING MATERIAL

Specimens were in the form of oriented, right circular cylinders cored from large, optically clear, untwinned crystals of both natural and synthetic quartz. Initial dimensions of the cylinders were 0.23 cm in diameter and between 0.5 and 0.8 cm in length. After each experiment, the specimens were sawn in half longitudinally using a wire saw. Half of the specimen was used to make an oriented thin section whereas the other half was used for study by transmission electron microscopy, As far as could be determined by etching, the natural crystals used were free from growth defects such as twinning, zoning or mosaic structure, By contrast, the synthetic crystals, although free from macroscopic twinning, possess a regular zoning parallel to the surfaces of the plate like seed crystals used to nucleate growth. The layers making up the zoned structure are predominantly parallel to (0001) and locally, to {1120). They arevisible when collimated light is passed through a thick slice but are generally not delineated by exposure to X- or Y - radiation. They apparently reflect differing concentrations of (OH) and perhaps Al (Cohen, 1960) and, on a small scale, are not planar, but are wavy in nature, reflecting the shape of the growth hummocks which develop on the surfaces of the growing crystal. A detailed study of the intrinsic defects present in the crystals used here has been made by Dr. A.C. McLaren using X-ray topographical techniques and his results will be published elsewhere. In many respects, the defects present in the synthetic crystals used are identical to those described by Lang and Miuscov (1967). Eifect

ofheat

tveahnent

011

sy?zthetic crystals

In advance it can be mentioned that reproducible recrystallization could only by produced in the synthetic crystals used, if they were given a heat treatment above 600°C prior to deformation. The treatment arbitrarily adopted here was 1.5 h. at 600°C and 15 kbar hydrostatic pressure unless otherwise indicated in Tables I, II and III. The effect of this heat treatment is u~nown but a number of possibilities exist: (1) The heat treatment at pressure and at temperatures above the critical temperature for hydrolytic weakening leads to the formation of dislocations around hard impurities by the mechanism of Jones and Mitchell (1958). Thus, a high density of dislocations may be formed prior to axial loading and this may alter the following deformation and recrystallization behaviour. Transmission electron micrographs (McLaren and Retchford, 1966) show no significant increase in dislocation density after heat treatment so that this possibility is excluded. (2) The heat treatment allows a different dislocation type or structure to develop during deformation thus enabling recrystallization nuclei to form more easily during subsequent treatment. This possibility seems unlikely since transmission electron micrographs of dislocations in both heat treated and untreated specimens after deformation are apparently identical (McLaren and Retchford, 1966). (3) Brunner et al. (1961) have shown that a distinct change in the infrared absorption of quartz results after heat treatment at 600°C and atmos-

PLATE 1

D 336

Tectonophysics.

6 (5) (196~~ 3T,.ir_iUl

F

Effect of heat treatment on synthetic crystal, X-13. A. X-13 stress-annealed 3 h at 800°C without a pre-heat treatment prior to loading. DT.258; crossed nicols; x 50. B. X-13 loaded at 300°C and 15 kbar confining pressure to a differential stress of 30 kbar after an initial pre-heat treatment at 600°C. The narro_w zones marked by fractures are kinks formed by slip on (OOOl}and on {lOlO}. DT.2’7’7; crossed nicols; x 200. C. X-13 stress-annealed for 1 h at 600°C. The kink zones shown in B have become clear and free from fracturing. Tiny nuclei are developed alongthe kink zones. DT.270; crossed nicols; x 200. D.X-13 stress-annealed for 30 set at 800°C. The major development of nuclei is along kink zones parallel to (1120) with minor development along zones approximately parallel to (0001) o The continuous bands parallel to { 0001) are growth bands accentuated by deformation and annealing. DT.291; crossed nicols; x 50. E. X-13 stress-annealed for 7 min at ‘7OOOC.Euhedral grain shapes are well developed. DT.358; crossed nicols; x 50. F. Polygonal aggregate formed by complete recrystallization of X-13single crystal. DT.284 stress-annealed for $ h. at 8OOOC.Crossed nicols; x 50. The direction of shortening in each case is parallel to the long edge of the page. c axis of host grain in plane of page and trending northeasterly. a* normal to page. Tectonophysics,

6 (3) (1968) 333-401

357

pheric pressure. This change is attributed to a permanent change in the positions of hydrogen atoms in the quartz structure and it may be possible that this change results in entirely different recrystallization kinetics. (4) The heat treatment homogenises the distribution of (OH) in the specimen so that the zoned distribution mentioned above is made more uniform. This possibility is supported by the nature of specimens observed in the optical microscope. Plate IA shows a specimen which has been deformed and then stress annealed for 3 h at 800°C without prior heat treatment. Plate ID shows a specimen given the same deformation but after heat treatment; it was stress annealed for only 30 set at 800°C. In the specimen without pre-heat (Plate IA) the growth bands are strongly delineated even though they were not visible between crossed polarizers prior to stress annealing. The boundaries of the growth bands are now asymmetrical kink planes and c changes by up to 5” across those boundaries. The different (OH) concentrations in adjacent layers have apparently resulted in different amounts or types of deformation so that the growth bands have acted as deformation bands during straining (Hobbs et al., 1966). Stress annealing has then developed the type of polygonizationl structure illustrated in Plate IA, The boundaries are sharp and their orientations are easily measured with a U-stage. Bubbles, presumably filled with water and approximately 11.1across, collect along those boundaries after annealing. Very few recrystallized grains form and those that are present are restricted to discrete growth bands. In the specimen with pre-heat (Plate ID) the growth bands are only faintly delineated; their boundaries are diffuse so that their orientation cannot be measured with a U-stage. Recrystallized grains are common and occur independently of growth bands. These observations seem to support this possibility and homogenization of (OH) distribution by heat treatment is tentatively adopted here. A, single crystal, X-13, supplied by Dr. D.B. Fraser of the Bell Telephone Laboratories was used for almost all of the experiments on synthetic quartz reported here. This crystal has an average water content equivalent to a hydrogen concentration of 5000 H/lo6 Si as determined from the 3n infra-red absorption. ‘Polygonization is used here to mean the migration of dislocations by glide or climb to form stable arrays. In the original use of the word by Cahn (1949) these arrays were normal to the active slip plane. Recent usage refers to the formation of any stable array not necessarily related to the predominant slip plane. The word is used here with this wider meaning.

Plate II Transmission electron micrographs of synthetic crystal X-13. A. Tangled dislocations in specimen loaded to 30 kbar at 300°C and at a IO-e/see, The dislocation density is greater than strain rate of 7.7 101u/cm2 (X 63,000.) B. Dislocation networks in specimen given same treatment as in a and then stress annealed for 1 h at 700°C. (Photographs by A.C. McLaren and J.A. Retchford) (x 52,000.) l

358

Tectonophysics,

6 (5) (1968) 353-401

PLATE

II

Tectonophysics,

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339

STRESS-STRAIN

CURVES

The natural crystals show the same decrease in yield point with increase in temperature as has been reported by Griggs (1967, fig3). Typical stress-strain curves for the various experiments performed are presented for the synthetic crystal, X-13, in Fig.1. This crystal shows the following features: (1) At low temperatures (300“ and 350°C; Fig.lC, D) crystals loaded in the Of, L Y and 11c orientations show linear stress-strain curves up to 30-40 kbar differential stress, H’owever, the slope of these curves is less than the expected slopes corresponding to a stiffness coefficient of about 1 * 1012 dynes/cm 2. On unloading crystals deformed at 300°C, the specimens are found to be permanently shortened by 3-4%. This means that only approximately 1% of the observed strain at this temperature is elastic; no lower yield point is detected. Plate IL4 shows a high density of dislocations, in

360

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Fig.1. Stress-strain curves for synthetic quartz crystal X-13. A. Single crystals loaded in the 0’ orientation at a strain rate 7.7 ’ 10eG set and 15 kbar confining pressure, B. Single crystals loaded in the 0’ orientation at a strain rate and 10 kbar confining pressure. 7.7 ’ IO-;rsec C. Single crystals loaded in various orientations at 300cC and confining pressure. The strain rate was 7.7 - f0-6/sec. D. Single crystals loaded in various orientations at 350°C and confining pressure. The strain rate was 7.7 + 10W6/sec. E. Single crystals loaded in various orientations at 900% and confining pressure. The strain rate was 7.7 - 10m7/sec. TecEonophysics, 6 (3) (1968) 353-401

of of 15 kbar 15 kbar 10 kbar

361

iz

0” 0+ Of 0+ 0+ Of 0+ 0+

Synthetic quartz X-13 preheat at 6OOOCfor 1; h

lm _Lm

:+

-+ 0 0+ +

Of

0+

Of 0+ 0+

0+ 0+ 0+

g:

0’

0’

0+ 0+ 0+

o+

Crientation

Specimen

15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

15 15 15 15 15 15 15 15 15 15 15

300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300

300 300 300 300 300 300 300 300 300 300 300

Loading Pressure temp. (OC) u S&bar)

Stress annealing experiments1

TABLE I

30 30 30 30 30 30 30 30 30 30

30

30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30

Max. differential stress (al - 03)kbar

700 800 800 800 800 800 800 800 800 800 800

300 500 600 600 600 600 600 600 650 650 650 650 700 700 700 700 700 700 700

Annealing temp. (“C)

!:: 16 h 4 days 10 h l$ min lh

2; h 30 set 1-1min 2+ min 4 min

2h ih lh 1; h 3h 4h 124 h 30 set 60 set 3 min 45 min 30 set 90 set 3 min 7 min &h _:h lh

7 days

Annealing time

423 268 291 295 354 302 284 287 318 341 324 320

11.0 12.9 14.2 14.0 12.0 13.8 13.9 12.7 16.0 14.8 14.8

278 290 270 274 299 292 303 399 400 366 349 289 361 294 358 283

398

Exp. no. DT

12.9 13.6 12.5 12.9 12.9 12.9 12.5 13.1 14.5 11.8 11.8 12.8 14.2 10.0 13.9 14-6 12.8 12.0 17.1

Strain2 (%)

81. mod. mod. motor left on during run.Compl. ext. sl. mod. mod. mod. compl. compl. compl. compl. ext. compl.

Sl.

Z** tr. tr.

tr. tr. tr. 61. tr. tr. tr. tr. tr.

tr.

RecrystaIlization3

IIc

30 30 30 30 30

15 15 15 15 15

650 650 650 650 650

0+ 0+ 1200 1300 1300 1400

1100

900

900 900

800

700

850 850 850 850 850 900 900 900 900

800

lh

5h 21 h l& h 4h 2$ h

44 h 3h lh 24 h 5h

60 see 90 set 2 min 5 min 5 min 20 set 30 set, 60 set lh lh

N

10

298 296 297 309 313

282

13.0 17.8 22.0 25.2 21.2 25.0

262 258 261 255 254

335 334 402 409 445 444 364 307 353 288 427

12.0 12.9 11.8 17.2 19.8

15.4 14.3 16.4 14.8

14.8 14.4 14.0 17.4

Sl.

none ; pre-heated at 1000cC for 10 h tr. sl. ext. mod.

tr. Sl. tr. mod. mod.

ext. ext. ext. compl.

compl.

ext. mod. ext.

lAll specimens loaded at a strain of 8*lO-‘/set in AlSiMg 222 jackets. 2The strain, E, is calculated from the increase in the cross-sectional diameter of the specimens and represents the maximum strain in the specimen. In experiments performed in the cubic apparatus (prefixed by the letter C-) the strain quoted is the mean strain assuming no bulging of the specimen. If no strain is quoted the specimen was not sectioned. 3tr. = trace; sl. = slight; mod. = moderate; ext. = extensive; compl. = complete.

0'

$

30

15

650

Of

Naturai quartz

30 30 30 30 30

13 15 15 15 15

300 300 300 300 300

Of 0+ 0+ 0+ Of

30 30 30 30 30 30 30 30 30 30 20

15 15 15 15 15 15 15 15 15 15 15

300 300 300 300 300 300 300 300 300 300 300

Synthetic quartz X-13, no pre-heat

0+

$ ;I

lr

$

0+ 0+

excess of 101°/cm2 , in a specimen loaded to 30 kbar at 300°C in the O+ orientation. The dislocations are tangled with no dislocation free areas present. They resemble the dislocation configurations seen in heavily cold worked metals. These stress-strain curves then, instead of representing an essentially elastic deformation, indicate a high linear rate of work hardening at these temperatures. (2) At 300°C, crystals loaded in the 1 m-orientation (Fig.lc), show the same features as described above. At 350°C (Fig.lD), the stress-strain curve is linear up to about I4 kbar where a “yield point” is detected. The crystal then work hardens until the curve assumes the same slope as that below the yield’point. (3) At 400°C (Fig.lA) the stress-strain curve exhibits a well defined yield point at about 7 kbar for a strain rate of 8 * 10m6/sec. 400°C is close to the critical temperature for hydrolytic weakening (48OOC) for this crystal as defined by the stress-relaxation method of Griggs (1967, fig.8). (4) From 600°C onwards, this well defined yield point and the overall character of the stress-strain curve is not significantly altered by increase in temperature to 900°C (Fig.lA, B). (5) After the well defined yield point, the work hardening rate is constant (Fig.lA, B, C). (6) Decreasing the strain rate to 8 * 10-7/sec (Fig.lB) lowers the well defined yield point by 18% at 400°C and by 25% at 9OOOC.The rate of work hardening is only slightly decreased by this ten-fold decrease in strain-rate. These effects are expected to continue with decreasing strain rate in a manner similar to that described by Heard (1963) for calcite.

STRESS

ANNEALING

EXPERIMENTS

In stress annealing experiments the specimen is axially loaded at relatively low temperatures to some high differential stress when the piston advance is stopped. The temperature is then rapidly increased to some higher value and held constant for varying periods of time. The time constant of the carbon furnaces used is so short (1-2 set) that the temperature distribution in the region of the specimen is probably constant within 5 set of increasing the power to the furnace. On raising the temperature, the differential stress decays in a way which depends on both the temperature and the (OH) content of the crystal. At the same time, the specimen undergoes a further shortening of about 4% due to relaxation of the deformation apparatus. This extra strain tends to be concentrated in the hot central part of the specimen so that the specimens bulge; the maximum total strain in this central part is generally about 14% shortening as shown in Table I. It is found that if (OH) is present in the quartz structure then nucleation of new grains takes place very rapidly while the stress is relaxing. These experiments, then, provide a means of nucleating grains in a stress field, while the crystal is deforming. In most experiments where nucleation occurred, the subsequent growth of these grains took place under small (@-3 kbar) differential stress and with little or no deformation taking place. The stress relaxation behaviour is illustrated in Fig.2. As in metals, (Oding, 1965) the relaxation consists of two distinct parts, an initial very rapid decrease in stress followed by a slow decrease to a low differential stress. 364

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10'

2

E3

4

6

8

12

10

t , 16’scc

Fig.2. Stress relaxation curves for single crystals loaded initially to 30 kbar at a strain rate of 7.7 * 10-s/ set and a confining pressure of 15 kbar, A. Synthetic crystal X-13 loaded at 300°C and relaxed at various temperatures. B. Natural crystal loaded at 65O“C and relaxed at various temperatures.

Experiments

on dry natural

crystals

Dry natural crystals of quartz were loaded at 65WC and 15 kbar in the ~~-orientation and in AlSiMag 222 jackets to a differential stress of 30 kbar, These specimens are characterized by the development of widely spaced basal deformation lamellae that are continuous across the specimen. On increasing the temperature to above 900°C, the specimen deforms predominantly by slip on {lOlO} with the widespread development of narrow kinked regions having boundaries approximately parallel to {OOOl] . Even though polygonization is common, leading to the development of low angle boundaries coincident with deformation lamellae (Plate IIIA), all deformation lamellae still show strong phase contrast effects even after heating at 1,400°C for 2i h. Thus, recovery is slow in dry quartz even at very high temperatures. The polygonization effects are discussed in detail on,p.378. During stress annealing, the basal lamellae are rotated out of their initial rational crystallographic orientation so that they tend to lie at angles up to 15O from {OOOl) in the central part of the specimen. They maintain Tectonophysics,

6 (5) (1968) 353-401

365

1‘

their rational orientation in the cooler e_ndparts of the specimen. The sense of rotation is consistent with slip on {lOlO) under the imposed stress system. A few small nuclei of recrystallized quartz formed after heating for 24 h at 1,200°C. 12 h at I,300°C produced a large number of grains but the result was not reproducible so that 4 h at I,300°C or 2t h at 1,400°C produced only sporadic nucleation. Thus, in dry natural quartz under these experimental conditions, recrystallization is sporadic and is apparently very slow at temperatures below l,400°C. Relatively strain free grains of quartz nucleate along the approximately basal lamellae or along planes nearly parallel to (1010) . (Plate IIIB,C). More rarely, nucleation occurs along the kink planes as shown inparts of Plate IIIB. Even from the earliest stages of growth the new grainsappear to be elongate parallel to their planar nucleation site and to have a rounded interface with the strained host grain. As these grains grow, they preserve this elongate, rounded shape until finally they meet neighbouring grains when an aggregate of polygonal grains develops (Plate IIID).

Experiments

on synthetic

crystals

Specimens of the synthetic crystal X-13 were loaded at 300°C and 15 kbar confining pressure to a differential stress of 30 kbar in AlSiMag 222 jackets. The 0’ orientation was used for most experiments but lr, Lm and 11c orientations were also used. For convenience, only O+-oriented specimens are discussed below. The details of deformation and recryst$llization for other orientations appear to be similar to the behaviour of 0 specimens. Specimens loaded in the O+ orientation, develop two sets of kinks (Plate IB), one approximately parallel to {OOOl} and formed by slip on { lOi0) and the other at 20° to (1120) and presumably due to slip on {OOOl}. These kinks are 2 -5~ wide and are spaced at 5 - 10~ with those parallel to {1120) more common. The kinks are asymmetric and, at this stage, are marked by fine fractures normal to the axis of compression, u 1. Apparently, the stress concentrations induced by asymmetrical kinking at relatively low temperatures have been relieved by fracturing (see Christie et al., 1964). Plate III Results of stress-annealing experiments, A. Result of annealing deformation lamellae. DT.297 stress annealed for 15 h at 1300°C. The long, continuous bands are parallel to {OOOl} whereas the short bands are parallel to {lOiO). Crossed nicols; x 50. B. DT.313 stress annealed for 2d h at 14OOOC.Nucleation of new grains occurs mainly along sub-basal lamellae with some nucleation along kink bands. Crossed nicols; X 50. C. DT.313 showing nucleation along sub-basal lamellae and kink boundaries. Crossed nicols; x 50. D. DT.297 showing polygonal aggregate formed locally upon completion of recrystallization. Crossed nicols; x 50. The axis of shortening is parallel to the long edge of the page in each case. c axis of host grain in plane of page and trending northeasterly. a* normal to page. 366

Tectonophysics, 6 (5) (1968) 35+461

PLATE

III

D

C Tectonophysics,

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367

The predominance of kink planes parallel to ~11~0) is in accordance with observations of Griggs and Blacic (1965) that at low temperatures, slip on (0001) is the dominant mechanism of deformation in quartz. The axis of kinking for kinks formed during slip on (0001) would presumably be that Q* axis normal to u1 whereas that for kinks formed during slip on llOiO}would be one of the a axes with low resolved shear stress (Christie and Green, personal communication, 1968). Specimens loaded in orientations other than 0’ develop intersecting sets of kinks due to slip on systems with high resolved shear stress. Details of these systems are presented by Christie et al. (1966) and by Christieand Green (personal communication, 1968).

Nucleation

of new grains

In O+ crystals the initial rapid relaxation of differential stress associated with an increase in temperature is accompanied by the relaxation of the internal stresses associated with kinking. The kink zones now appear as clear, lenticular regions in which the orientation of c differs by up to 45” from that in the host crystal (Plate IC, D). During this rapid stress drop, also, tiny (l-31.1) nuclei of recrystallized quartz appear along the boundaries of the kinked regions (Plate ID). These nuclei have approximately the same orientation of c as the material in the kinked regions although a wider spread of orientations is present. The nuclei occur as either discrete euhedral grains distributed in chains along the kink bands parallel to { 1120) or as euhedral grains or bulbous swellings along the kink bands parallel to {OOOl} . A definite incubation time for nucleation is noticeable below 65O*C and is of the order of days at 300°C. At temperatures above 650°C, nucleation of all new grains is completed within 30 sec. This period of nucleation is associated with the reorganization of dislocations so that the dense tangle illustrated in Plate IIA is converted to the polygonal networks seen in Plate IIB. By this stage the dislocation density has been reduced to 108/cm2 locally, the dislocations straightening and forming sub-boundaries. These sub-boundaries are often parallel to Y and z planes, the networks being composed of dislocations parallel to [a+c] and to a. It is possible that they may form by reactions of the type: (a + C)

[2ii3]+ a [i2io]-(a

+ c) [1123]

(McLaren and Phakey, 1965)

if some are pure twist networks. Details of the dislocation which accompany annealing will be given elsewhere.

Growth

rearrangements

ofnewgrains

With continued heating,_the grain sjze increases, the euhedral grains remaining euhedral with {loll} and (1010) developed as grain boundaries (Plate ID, E; Fig.3A), implying that these boundaries have the slowest rates of migration. This means that the new grains must have rather unique orientations relative to the host such that {loll} and {lOiO} have the lowest rates of migration. When the grains are large, prisms tend to be best developed.

368

Tectonophysics, 6 (5) (1968) 35~401

Fig.3. A. Histogram showing frequency of grain boundaries whose poles occur at angle, 8, to the c axis of new grains. 168 grain boundaries were measured in specimen DT.283. B. Calculated poles to Y and .z in newly recrystallized grains in DT.283. 168 poles. Contours 1, 2, 3% per 1% area. Full squares represent the orientations of the poles of host Y and z planes. C. Calculated a axes of newly recrystallized grains in DT.283, 84 poles. Contours 1, 3, 5,7% per 1% area, Full squares represent the orientations of the host a axes, D. Calculated a* axes of newly recrystallized grains in DT.283. 84 poles. Contours 1, 3, 5, 7% per 1% area. Full squares represent the orientations of a* axes of the host single crystal. E. Orientations of c axes of new grains in DT.268. Dots are c axes in large grains whereas the open triangles are c axes of grains which have remained small. The full square is the c axis of the host single crystal, Arrows in each case represent the direction of shortening. Tectonophysics,

6 (5) (1968) 353-401

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In Fig.3B, C, D are plotted1 the calculated poles to Y and z, a and u* for eyhedral grains growing in a single crystal stress annealed at 700°C in the 0 orientation. These show strong preferred orientations. In many grains, a z plane is common from host to new grain (Fig.3) but the patterns are best described as being derived from the equivalent host orientations by a rotation of 30-40° about an axis coinciding approximately with that a* axis2 of the host normal to al, If individual grains are considered, exact coincidence of a* from host to new grain is rare. As the average grain size increases, some grains remain small and are ultimately consumed or surrounded by larger grains. In Fig.3E are plotted the c-axes both of large grains and of grains which have remained small. The smaller grains tend to have their c-axes close to that of the host whereas the larger grains are more diversely oriented. Thus, there is a growth anisotropy of new grains such that grains with the orientation of the host crystal grow slowest whereas those with c oriented at 20-40° to the host c grow fastest. Investigation of the recrystallization behaviour with temperature and time reveals another aspect of the growth of new grains; at 900°C, recrystallization is completed (i.e., all of the strained host material is replacedby relatively strain free material) in 12 min. At 700°C the process begins rapidly but slows down at long times so that after 2; h the specimen is still only 75% recrystallized. At 650°C the process is slower still and at 600°C, although considerable numbers of nuclei form, the volume fraction recrystallized never increases above 5% even after heating for 121, h. At 300°C no more than 1% is recrystallized after 7 days. Assuming that the recrystallization process consists of two distinct parts, namely, nucleation of strain free grains and subsequent growth of these grains to impingement, Avrami (1939, 1940, 1941), Johnson and Mehl (1939) and Bailey (1963) have derived the expression: x = 1-exp(-Btm)

(1)

to describe the progress of recrystallization with time, t. In this expression, x is the volume fraction recrystallized and B and m are constants which depend on the model adopted for nucleation and growth during recrystallization. In the Avrami theory, a limited number of nucleation sites are assumed to exist at the beginning of recrystallization and these are steadily exhausted as the process continues. This assumption gives 3SrnS 4. In the Johnson and Mehl theory, nucleation takes place at a constant rate throughout recrystallization. This assumption gives m as a function of the nucleation and growth rates. Bailey assumes a model of bulge nucleation (Cahn, 1966) and derives values of m between 1 and 4.5. In all theories B varies with temperature according to: B = q exp(-Q/RT)

(2)

where Bg is a constant, Q is the activation energy for recrystallization, the gas constant and T is the absolute temperature. Fig.4 shows that eq.1 is apparently followed in these experiments

R is only

1

All projections used here are equal area and on the lower hemisphere. 2The patterns may also be described by rotations about various 5 ’ or [C+U]

370

directions.

Tectonoptiysics, 6 (5) (1968) 353-401

10

1

In(&) I6

10:

Id

1,

‘O t

(see)

Id

id

Fig.4. Rate of recrystallization of crystals deformed in the 0’ orientation at various temperatures. x is the volume fraction recrystallized at timet. at 800°C and above. As indicated above, at 700°C the expression may be followed for short times but the process slows down at long times. Below 650°C, the process virtually stops, especially at long times. This behaviour means that either the mechanism of recrystallization is changing with time or that the driving force for grain boundary migration is decreasing with time. The observation that grains stop growing before they impinge on neighbouring grains suggests that it is the driving force which is changing with time. Again, after 15 h at 600°C, the dislocation density has been reduced to about 108/cm 2 locally, the dislocations occurring in polygonal networks which separate large (0.5 - 1~) regions of dislocation free quartz (McLaren and Retchford, 1x6). Thus, at 600°C, the dislocation density may be reduced from greater than 1010/cm2 (Plate IIA) to less than IOS/cm2 without the volume fraction recrystallized increasing above 5%, again suggesting that the driving force for grain boundary migration is decreasing with time due to recovery. The observation that this retardation effect becomes more important at low temperatures implies that the activation energy for recovery is less than that for recrystallization, a situation which is common in metals. An identical retardation effect has been described for the annealing of aluminium by Vandermeer and Gordon (1963). They could show that recovery was responsible for retardation of recrystallization in this example. Tectonophysics, 6 (5) (1968) 353-401

371

Grain growth after After the grain size touching, the grain shape (Plate IF). The grain size now stops presumably because lated at grain boundaries. have failed.

impingement has increased to the stage where all grains are ceases to be euhedral and becomes polygonal increases much more slowly and finally, growth of the foreign material which has by now accumuAttempts to induce secondary recrystallization

Preferred orientation of the recrystallized grains For crystals deformed in the 0’ orientation, a typical pattern of preferred orientation of c of new grains is presented in Fig.5A. c of new grains tends to lie in the plane containingaland the host orientation but the host orientation tends to be neglected. A maximum is always developed 20°-40° from the host c orientation towards 61 with a sub-maximum equidistant from the host c away from 01. These same general features are again developed in crystals loaded lm and IIc (Fig.5B, C). Thus, for all orientations of the host c relative to 01:

(1) maxima tend to form 20°-40° from the host c-axis in the plane containing 01 and the host c; in crystals loaded hc a small circle develops with al as axis (2) the host c-axis orientation tends to bi! neglected by the new grains. Fig.SD shows the preferred orientation which develops in an 0’ specimen after one hour at 700°C with 5 kbar of differential stress present throughout recrystallization. This was accomplished by leaving the piston advance motor running as the temperature was increased from 300°C to 700°C. As the temperature was increased the differential stress dropped from the initial 30 kbar to 5 kbar and remained steady throughout the run. The specimen strained an extra 5% above normal in a stress annealing run during the recrystallization period. The pattern of preferred orientation which developed during this experiment (Fig.SD) is not significantly different to the pattern that develops during normal stress annealing experiments (Fig.5A). This suggests that the presence of a high differential stress and an environment which is deforming while recrystallization is proceeding, has little effect on the pattern of preferred orientation which develops during these experiments. These patterns of preferred orientation appear to be stable on continued heating after recrystallization is complete. Thus, DT.341, heated for 4f days at 800°C has a grain size and preferred orientation similar to that of DT.287 heated for 1 h at 8OOOC. ANNEALING

EXPERIMENTS

In annealing recrystallization experiments, the specimen is first deformed at some relatively low temperature and the deformed crystal is then heated at some higher temperature in a hydrostatic stress field. In metals (Beck, 1954) new grains nucleate and grow provided both the initial stored strain energy and the annealing temperature exceed some critical

372

Tectonophysics,

6 (5) (1968)

353-401

Fig.5. Preferred orientation of c axes of new grains formed by stressannealing of single crystals of X-13. The full square represents the c axis of the host crystal. Arrows represent the direction of shortening. A. JJT.287 deformed in the 0’ orientation and stress annealed 1 h at 8oO”c. 200 c axes. Contours 1, 5, 10, 15, 208per 1% area. B. DT.320 deformed in the 1 m orientation and stress annealed 1 h at 8oo°C. 250 c axes. Contours 1, 5, 10, 15, 20, 25%per 1% area. C. fIT.335 deformed in the 11c orientation and stress annealed 1 h at 800°C. 200 c axes. Contours 1, 3, ,5, 7, 11% per 1% area. D. DT.423 deformed in the 0 orientation and stress annealed 1 h at ?OO°C while deforming at a strain rate of 7.7 * 10V6/sec. 210 c axes. Contours 1, 5, 7, 10% per 1% area. value. The driving force for recrystallization is envisaged as the difference in free energy between the deformed host grain and the relatively strain free new grains. The annealing experiments described below have been performed using both dry natural quartz and the synthetic crystal X-13 in anhydrous AlSiMag 222 jackets and also dry natural quartz in hydrous jackets. The results of these experiments illustrate, above all, the profound effect that trace amounts of (OH) in the quartz structure have on the recrystallization behaviour of quartz. Experiments As

on dry natural

crystals

in a dry environment

indicated in Table JI, dry natural crystals

Tectonophysics, 6 (5) (1968)353-401

were deformed at 600°C, 373

:

TABLE II

lr Of lr

Natural quartz

lr

ir

lr

lr lr lr Lr

Lr

Lr

lr lr Lr

lr

;::

lr

Lr lr 1r Lr lr lr II c II c Lr lr

Of

o%r

Orientation

Specimen

Annealing experiments

15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 20 20 20 20 20 20 20 20 20 20 20 20 15 20-24 20-25 20

15

Pressure a3 kbar

18.8 N 30 16.9 17.1 18.7 18.3 21.3 20 18.3 22.4 11.0 18.3 13.9 18.0 26.7 8.5 4.8 22.2

520 520 520 520 520 520 520 520 540 600 600 600 610 650 650 690

26.7 21.0 28.0 13.8 25.0 26.7 16.2 25.0 21.0

13.4

800 800

800

650 900 900 900 650 650

650 650

600

Loading Strain’ temp. OC % AlSiMg AlSiMg AlSiMg AISiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg Talc Talc Talc Talc Talc Talc Pyr. Pyr. 4rr. Pyr. 4rr. Pyr. Pyr. Pyr. Pyr. Pyr. 4rr. Pyr. Pyr. Pyr. Pyr. Pyr.

Jacket

700 850 900 1,110 1,120 1,150 1,260 1,280 1,010 970 1,190 1,220 1,180 700 700 1,550

1,000 1,000

1,300 1,300 1,400 1,000 1,000

1,100 1,200

1,000 1,000

1,300 _

Annealing temp. OC

lh 21 min lh 20 min 59 min 64 min 20 min 61 min 32 min lh lh lh 20 min 33 min 100 min 62 min

lh 1; h

15 min lh 46 min 5 min 22 min

ext.

mod. ext. ext.

Sl.

ext. ext. ext. ext.

Sl.

mod. mod.

id.

none tr. ext. none sl.

none

Sl.

tr.

Sl.

tr. none none tr. tr. tr. none

5 min 2h 121, h 10 h 15 h

RecrystaIlization2

Annealing time DT.375 DT.430 DT. 229 DT. 230 DT. 339 DT. 342 DT. 379 DT.391 DT.376 DT.381 DT.387 DT.422 DT.440 DT.433 DT.426 DT.392 c.431 c.433 c.434 c. 435 C.436 C.422 d.437 C.438 C.264 C.446 c-445 c.447 C.235 C.116 c.110 c.419

Exp. no.

0+

lm* II c*

0+* 0+* 0+* 1 r*

.O’

0+ 0+

0+ 0+ 0+

$

15

20-25 20-23 20 20-25 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

500

750 800 800 900 400 400 400 400 400 400 400 400 400 400 400 400 350 350 350 350 350 12.6

5.3 6.0 20.2 14.8 20.8 21.9 21.3 22.9 27.2 20.4 20.5 21.9 25.9 21.1 25.3 19.5 15.2 18.3 22.2 11.8 7.1 AlSiMg

Pyr. Pyr. Kaolin AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg

PP.

900

900 900 900 900 900 900 850 850 850

900

900

900

820 880 960 1,000 700 800 800 900 900

2h

66 min 13 min lh 13 min 10 h 4h 4h ih !h ih lb h 3-h 3h 3h 2h lh SO set lh 2f min 23 min 24 min ext.

mod. mod. sl. ext. ext. sl. sl. sl.

Sl.

mod. mod. sl. tr. tr. sl. sl. sl. sl. sl.

v. sl.

DT.273

c-111 C.124 C.432 Cl45 DT.250 DT. 242 DT. 244 DT.240 DT.246 DT.237 DT.24-i DT. 243 DT. 245 DT.249 DT.234 DT.397 DT.353 DT.322 DT. 446 DT.450 DT. 451

lThe strain, E, is calculated from the increase in the cross-sectional diameter of the specimens and represents the maximum strain in the specimen. In experiments performed in the cubic apparatus (prefixed by the letter C-) the strain quoted is the mean strain assuming no bulging of the specimen. If no strain is quoted the specimen was not sectioned. ?The sequence in recrystallization is denoted by: tr. = trace; sl. = slight; mod. = moderate; ext. = extensive; compl. = complete. *Heated for 11 h at 600°C and 15 kbar, hydrostatic stress prior to deformation.

Synthetic quartz x-125

Synthetic quartz X-13’

Lr

lr _Lr lr 0+ 0+

650°C and 900°C inAlSiMag222 jackets prior were deformed in the 0’ and 1 Y orientations being heated at 15 kbar hydrostatic pressure range l,OOO” - 1,400°C. Significant amounts detected in any of the experiments.

to annealing. The specimens and shortened up to 28% before and at temperatures in the of recrystallization were not

Specimens loaded at 600-650°C Specimens loaded at 600-650°C at a strain rate of 8 - 10-6/sec in the 0’ and IY orientations are characterized by the development of long, continuous deformation lameilae parallel to {OOOl) . These are fairly regularly spaced at 0.25 - 0.5 mm and, when viewed in phase contrast, adjacent lamellae often show opposed contrast as illustrated in Plate IVA. In the microscope used, the low intensity side of a deformation lamella viewed in phase contrast illumination corresponds to a local increase in refractive index. This, in turn, as pointed out by Christie et al. (1964) may be correlated with the extra half plane side of an array of basal edge dislocations locked in their glide plane, The alternating contrasts observed in Plate IVA indicate a considerable amount of order in the slip process such that dislocations of opposite sign were moving in adjacent deformation lamellae. In many examples, the long continuous basal lamellae are the boundaries of deformation bands, in that short lamellae approximately parallel to { lOiO] are present between adjacent (0001) lamellae. These are “bands of secondary slip” in the sense of Honeycombe (1951). Extensive fracturing is present, the individual fractures terminating on the tension (low phase contrast intensity) side of basal lamellae. Little difference in the thin section appearance of the quartz is apparent after annealing for 2 h at l,OOO°C. Fracturing is still common and the deformation lamellae are still associated with large phase contrast effects Plate IV Results of annealing natural crystals. A. Deformation lamellae in DT.229, a natural crystal loaded at 650°C and annealed for 2 h at 1000°C, dry (phase contrast illumination). The termination of cracks (NE-SW trending bright features) in the high intensity (tensile) side of lamellae is clearly visible; X 200. B. Deformation bands bounded by basal deformation lamellae in a natural crystal loaded at 650°C and annealed for 124 h at 1000°C dry. Strings of small recrystallized grains run across the bands. Rosettes similar to those observed by Christie et al. (1964), are distributed along the deformation lamellae. DT.230; crossed nicols; x 50. C. Newly recrystallized grains along deformation bands in a natural crystal loaded at 69O’C and annealed for 62 min at 1,550’C wet. C. 419; crossed nicols; X 50. D. Newly recrystallized grains with ragged ends, elongate parallel to {OOOl) in a natural crystal loaded at 520°C and annealed for 61 min at 1,280°C wet. C.438; crossed nicols; x 50. The direction of shortening in each case is parallel to the long edge of the page. c axis of host grain in plane of page and trending northeasterly. a normal to page. 376

Tectonophysics, 6 (5) (1968) 353-401

PLATE IV

Tectonophysics,

6 (5) (1968) 353-401

377

where fracturing has not relaxed the local stresses. No nucleation of new grains is observed. After annealing for 12$ h at l,OOO°C, the local stresses have largely been relaxed so that fracturing is not common. Phase contrast effects are still strongly developed along lamellae but apparently local stresses are less than the tensile fracture strength of unconfined quartz. Associated with this relaxation of stress is a change in the c-axis orientation across individual lamellae (Plate IVB). This is apparently due to the migration of prismatic dislocations to the {OOOl) lamellae to form stable dislocation walls with small (2“ - 4O) misorientations of c across them. These misorientations are such that c tends to remain normal to the band boundary in those bands which show low intensity phase contrast effects and lies at 88” - 86O to the band boundary in bands which show high intensity effects. This is a geometrical consequence of the nature of the array of basal dislocations; c must lie at a higher angle to the array on the extra half plane side than on the other side. The observed misorientations indicate (Read, 1953, pp.174 - 175) a density of 106/cm for prismatic dislocations and of 5 * 104/cm for basal dislocations. Plate IVB also shows a number of rosette shaped structures which are identical to those discussed by Christie et al. (1964). Specimens loaded at 900°C Specimens loaded at 900°C at a strain rate of 8 * IO-%ec in the Ot and 1 Y orientations, show a greater variety of planar structures than those deformed at 600° - 65OOC. Those structures which have the characteristic features of deformation lamellae are approximately parallel to {OOOl,] { lOi0) and {lOil]. Th e b asal lamellae are long and relatively continuous and can be traced from the cool ends of the specimen whei-e they are strictly basal, to the hot central region of the specimen where they lie at loo - 18O from {OOOl) and such that their pole lies at a lower angle to 01 than c. In-the hot regions of the specimen, short discontinuous lamellae parallel to {lOlO} are plentiful and slip on fhese planes has apparently been responsible for the observed irrational nature of the approximately basal lamellae. The_ approximately basal lamellae also form kink boundaries for slip on jlOlO}. Also present are a limited number of kinks resulting frdm [c+a] (1011) slip. No fracturing occurs on unloading indicating that at 900°C recovery takes place at such a rate during deformation that no high stresses are generated at kink boundaries or other deformation bands. The effect of annealing these specimens is to produce similar recovery effects as have been described above; no recrYstalliza_tion is observed. Lamellae approximately parallel to (0001) and to {lOlO) become low angle boun_daries associated with slight misorientation of c. Lamellae parallel to { 1011) disappear very rapidly and leave no trace of their former existence other than the presence of kinks. Kink boundaries become very sharp accompanied by slight increases in misorientation across them. All of these effects can be attributed to polygonization.

378

Tectonophysics, 6 (5)(1968)353-401

Experiments

on dry natural crystals

in a wet environment

In order to examine the effect of water on the recrystallization of natural quartz, a number of experiments were performed using hydrous jackets such as pyrophyllite, kaolinite or talc which break down in the range 600° - 830°C to yield water together with other products. Many of these experiments were performed in the cubic apparatus by H.W. Green. The specimens were loaded at 500° - 900°C and then annealed at temperatures above the break down temperature of the jacket material. Inmany instances considerable recrystallization occurred within one hour even at temperatures as low as 9OOOC.This contrasts with the results obtainedin a dry environment where only sporadic recrystallization was observed even after +’ h at l,400°C. The presence of water, then, during recrystallization has a very considerable influence on the process. Plate IVC, D show recrystallization of a single crystal loaded in the Lr-orientation. The recovery effects described above for dry quartz develop rapidly so that marked changes (loo - 15O) inthe orientation of c develop across the boundaries of deformation bands approximately parallel to {OOOl). New grains of quartz nucleate at the boundaries of deformation bands and grow faster parallel to the boundary than normal to it so that elongate grain shapes tendto form(Plate IVD). Often, a grain which nucleates at one boundary of a deformation band, will grow until it is the same width as the band but will not cross either boundary of the band. It continues to grow, with rather ragged ends, parallel to the band. At high temperatures an aggregate of polygonal grains develops. Orientation of new grains The patterns of preferred orientation of c which develop (Fig.6A, B) are similar to those that form in the stress annealing recrystallization of X-13; the host orientation tends to be neglected and maxima are developed at 20° - 40° to,the host c-axis orientation in the plane containing 01 and the host c-axis. In these natural crystals,however, the maximum concentrations of c lie at high angles to ~1 whereas in X-13 maxima were equally or better developed close to cl. Experiments

on synthetic

crystals

in a dry environment

Single crystals ofX-13 were loaded at 400°C in the 0’ orientation and at 350°C in the O’, lr, 11 c and lrn orientations. Details of the amount of strain induced prior to annealing in AlSiMag 222 jackets at 15 kbar hydrostatic pressure are indicated in Table II. Specimens loaded at 4OO’C in the Ot orientation develop two intersecting kinks (Plate VA), one due to slip on (0001) in an a direction and the other due to slip on (lOi0) in the c-direction. In general, no recrystallization occurs in the central region of these specimens on annealing but is concentrated in regions where the temperature during deformation was less than 400°C (Plate VA). The area recrystallized is sporadic and nonreproducible from one experiment to the next. It appears that at 400°C, not enough strain Tectonophysics, 6 (5) (1968) 353-401

379

Fig.6. Preferred orientation. of c axis of new grains formed by annealing single crystals in the 0’ and Lr orientations under various conditions. The full square represents the c axis of the host grain. Arrows represent the direction of shortening, A. C.438, a natural crystal deformed in the lr orisntation in the cubic apparatus and annealed 61 min at 128O“C wet. 210 c axes. Contours 1, 5, 9, 13%per 1% area. B. C.419, a natural crystal deformed in the lrorientation in the cubic apparatus and annealed 62 min at 1550°C wet. 300 c-axes, Contours 1, 3, 4% per 1% area. C. DT.234, X-13 deformed in the Ot orientation and annealed 2 h at 900°C dry. 250 c axes. Contours 1, 3, 5,+‘7, 9Wper 1% area. D. DT.322, X-13 deformed in the 0 orientation and annealed 1 h at 900°C. 210 c axes. Contours 1, 5, 10, 15, 2O%per 1% area. E. DT.273, X-125 deformed in the 0’ orientation and annealed 2 h at 900°C. 228 c axes. Contours 1, 3, 5, 7, 9% per 1% area. Plate V Specimens loaded at 350-5OOOC. A. X-13 loaded at 400°C in the 0’ orientation and annealed 2 h at 900°C dry. DT.234; crossed nicols; x 15. c axis of host grain in plane of page and trending northwesterly. a* normal to page. B. X-13 loaded at 350°C in the 1 m orientation and annealed 2; min at 850°C dry. DT.450; crossed nicols; x 53: c axis of host grain in plane of page and horizontal. a normal to page. C. X-125 loaded at 500°C in the 0’ orientation and annealed 2 h at 900°C dry. DT.273; crossed nicols; x 53. The direction of shortening in each case is parallel to the short edge of the page. c axis of host grain in plane of page and trending northwesterly. a* normal to page. 380

Tectonophysics, 6 (5) (1968) 353-401

Tectonophysics,

6 (5) (1968) 353-401

381

energy is stored at 2% shortening, to promote recrystallization during annealing. Specimens loaded at 350°C develop numerous intersecting kinks similar to those formed during loading at 300°C (Plate VB). On annealing at high temperatures widespread nucleation and growth occurs although the recrystallization process is much slower than in the stress annealing experiments at the same annealing temperature. Otherwise, the details of nucleation and growth appear to be identical to those described for the stress-annealing experiments. Fig.GC, D present the c-axis orientations which develop during the recrystallization of X-13 loaded at 400°C and 350°C respectively in the 0’ orientation. The patterns are almost identical to those produced during the stress-annealing experiments (Fig.5A, D). Thus, the patterns of preferred orientation which develop are independent of the presence or absence of a differential stress during recrystallization, indicating again that a differential stress has little influence on the pattern which forms during recrystallization in these experiments. For comparison, Fig-GE shows the t-axis pattern formed during the recrystallization of a synthetic crystal X-125* loaded at 500°C in the O’orientation and annealed 2 h at 9OOOC.The specimen develops widespread asymmetrical kinks approximately parallel to illz0) during loading and nucleation occurs along these kinks during annealing (Plate VC). The c-axis pattern is similar to that developed in the annealing of natural crystals in in a hydrous environment; the host c-axis orientation is completely neglected and a maximum of c-axes of new grains is developed at a high angle to 01. SYNTECTONIC

RECRYSTALLIZATION

EXPERIMENTS

Single crystals of both natural and synthetic quartz have been shortened up to 50% at lo-15 kbar and at temperatures in the range 400°-950°C (see Table III). As can be seen from fig. 3 of Griggs, 1967, dry natural quartz can only beshortened a few percent at 650°C and at 15 kbar confining pressure before a stress difference of 35 kbar is reached. This corresponds to a fracture strength of 45 kbar for the carbide pistons used. By heating the specimen at 950°C and 15 kbar for 3 h in a talc jacket prior to deformation enough water, liberated from the breakdown of the talc, is incorporated in the quartz structure to induce hydrolytic weakening at 950°C (Griggs and Blacic, 1967). High strains may then be achieved without high differential stress (Griggs, 1967). Specimens of the synthetic crystal X-13, were deformed in anhydrous AlSiMag 222 jackets. In most of the experiments, the strain was markedly inhomogeneous (Plate VIA), the specimen being so weak that the carbide piston punched through the quartz once the cross-sectional area of the specimen exceeded that of the piston. Even so, strong preferred orientations of new grains are present but the results are ambiguous so that it is not possible to distinguish between control of the preferred orientations by the host, by the imposed stress system of by a combination of the two. *X-125 has an (OH)-content of 500 p.p.m.H/Si and a hydrolytic

weakening

temperature

of 93ooc.

382

Tectonophysics,

6 (5) (1968) 353-401

10 ;;

11 15

:; 10 10 lo

Of ;::

$

$ Im iy

$

47.9 43.6 45.8 44.2 46.0 51.4

900 900

46.2

45.0 37.6 19.0

48.0 43.9

42.8 32.8

900 900 900

900

900 900

900 900

15 15

Of Of

lY

0+ $

25.0 54.7 41.7 43.5 45:4

e(%Y

10-5 10-5 10-5 10-5 10-5 10-6 10-6 10-6 10-5 10-6 10-6 10-6

10-5 10-5 10-6 10-6 10-5 10-5 10-5 10-5 10-5

10-b

Strain rate per set

AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg

AlSiMg Talc AlSiMg AlSiMg AISiMg AlSiMg AlSiMg AlSiMg AlSiMg AlSiMg

Jacket

ext. mod. mod. ext. mod. ext.

81. coesite in high strain areas extensive coesite annealed 1 h at 900QC, 10 kbar ext. recrystallization mod. Sl. mod. mod.

none pre-heated 3 h at 950°C, 15 kbar sl. coesite in high strain areas pre-heated 1; h at 600°C, 10 kbar tr. tr.

Remarks amt. of recrystallization2

407 406 403 404 408 419 410 411 425 420 438 434 441

383 437 424 415 388 405 396

379 395

DT no.

1The strain, E, is calculated from the increase in the cross-sectional diameter of the specimens and represents the maximum strain in the specimen. In experiments performed in the cubic apparatus (prefixed by the letter C-) the strain quoted is the mean strain assuming no bulging of the specimen. If no strain is quoted the specimen was not sectioned. 2The sequence in recrystallization is denoted by: tr. = trace; sl.=slight; mod. = moderate; ext. = extensive; compl. = complete.

Synthetic quartz x-13

900 950 .900 400 600 600 800

lr lr

Natural quartz

Pressure Temp. o 3(kbarf (“Cl

experiments

15 15 15 10 10 10 15 16

Orientation

Specimen

Syntectonic recrystallization

TABLE III

Specimens

deformed

at 10 kbar confinirlg $wes.swe

Specimens of the synthetic crystalx-13 were deformed at strain rates of lo-5/set and 10-s/ set at 10 kbar confining pressure and temperatures in the range 400” - 900°C.. The specimens were shortened up to 50%, the experiments lasting up to 14 h at a strain rate of lo-5/set or up to six days at 10-6/sec. Significant amounts of recrystallization occurred only at 800° and 900°C, the grain size in the slower strain rate experiments being about ten times that in the faster experiments. No recrystallization occurred before about 30% shortening at 900°C presumably because not enough stored strain energy was present to drive the process. At 10 kbar the (Y- /3 transition lies at 800°C so that the recrystallization process to be described below occurred in the P-field. In regions of low strain, a number of wide deformation bands form within the crystal. These bands, in specimens loaded in the O’-orientation, resemble kinks formed by slip on {OOOl} and on {lOiO>, in the sense of rotation of c within the bands (Plate VIB). No deformation lamellae are formed but instead an array of subgrains develops (Plate VIB, C). Some subgrains tend to be-rectangular in shape with sides approximately parallel to (0001) and to {1120). (See regions A and B in Plate VIA). This shape is Plate VI Syntectonic recrystallization experiments. A. Synthetic crystal X-13 shortened 50% at 900°C, 10 kbar, and a strain rate of 1O-s/ sec. Original orientation was 11c. The punching action of the upper piston is clearly shown. Unrecrystallized host material next to the pistons is relatively undeformed and has the initial orientation of the specimen consists of an array of subgrains; the average orientation of c in this region remains parallel to the axis of shortening, The regions A and B contain rectangular subgrains with sides trending parallel to {OOOl} (horizontal) and to {llsO} (vertical). Subgrains within deformation bands are irregular and have the same shape and size as the newly recrystallized grains. DT. 441; crossed nicols; x 15. B. Deformation band in X-13 shortened about 30% at 800°C and a strain rate of 10-s/ set in the 0’ orientation. The approximate charge in the orientation of the c axis is indicated. Ragged subgrains tend to occur within the deformation band whereas more equant, rectangular subgrains occur outside of the deformation band. DT.388; crossed nicols; x 50. C. A deformation band in X-13 shortened 46% at 900°C and a strain rate of 10-s/ set in the 0’ orientation. The lower left hand corner of the figure shows host material which passes rather sharply into an array of recrystallized grains. These grade into an array of subgrains and finally into relatively strainfree host quartz in the upper right hand corner. DT.434; crossed nicols; x 50. D. DT.396 loaded at 900°C, 15 kbar, and a strain rate of 10-s/ sec. The original orientation of the single crystal was 0’. The central area of the specimen is composed of blades of coesite. Regions of relatively low strain are composed of recrystallized quartz or of subgrains. Crossed nicols; x 15. The direction of shortening in each case is parallel to the long edge of the page.

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PLATE

VI

D

C Tectonophysics,

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3 85

independent of the initial relative orientation of the host c -axis relative to al suggesting that these subgrain walls do not represent stable arrays of dislocations normal to the active slip plane. Other subgrains are raggedin shape with sides often trending at high angles to {OOOlj (Plate VIB, C). These two distinct shapes of subgrains tend to develop in different regions within the host crystal and the two regions often join along a sharp boundary which is the deformation band boundary, the ragged subgrains occupying the regions of higher strain. Over large regions of the specimen, the misorientation between adjacent subgrains is approximately 5O but on approaching a deformation band boundary, misorientations increase rapidly to loo - 30° (Plate VIB, C; Plate VIIA, B). In examples where large misorientations occur between adjacent subgrains, a fine substructure may be visible at the boundary suggesting that this high misorientation takes place gradually over a distance of several microns. At low strains, the misorientations between adjacent subgrains within the deformation bands remains at about 5O. The subgrain boundaries are not simple boundaries so that the lattice either side of the boundary are not related to each other by a single axis of rotation normal or parallel to the boundary. As the strain increases, the misorientations between subgrains in the deformation bands increases from lo - 5O to 20° - 30° (Plate VIIA, B) at 50% shortening. Locally, misorientations as high as 90° may develop. The subgrains outside of the deformation bands maintain lo - 5O misorientations independently of the amount of strain (Plate WA; Plate VIID) but the cumulative effect of these small changes can lead to large changes in the orientation of c at 50% shortening. In O-and 1 ~-specimens, c is rotated towards 01 over large regions of the specimen (Fig.‘7A,D). In lm-specimens c remains at approximately 90’ to al (Fig.7G); also 11c-specimens tend to be stable with respect to the orientation of c so that the 11c-orientation is maintained outside of the deformation bands as deformation continues (Fig.75). The recrystallized grains observed within the deformation bands at high strain (Plate VI, VII) are sharply delineated from their neigbours or the host material by large orientation differences and sharp grain boundaries (see Plate VII Deformation band. A. Edge of a deformatioh bandinDT.441. Relatively strain free host quartz occurs at the base of the figure and this grades into an array of ragged subgrains and then finally into “newly recrystallized grains” which have the same shapes and size distribution as the ragged subgrains. Crossed nicols; x 50. B. Same view as in A,,but in a slightly different orientation relative to the nicols. The shapes of the ragged subgrains are shown as well as the subgrain shapes in the relatively undeformed host; x 50. C. Deformation band in DT.434. One large grain, A, is growing into a matrix of subgrains resulting in a serrated grain boundary. Crossed nicols; x 50. D. Deformation bands and recrystallized grains in X-13 loaded at 900°C and a strain rate of IO-%ec. Original orientation was 1 ~2. The margins of the deformation bands are strongly serrated and in some places (for example at A), the material in one deformation band is growing into the adjacent band. DT.420; crossed nicols; x 50.

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PLATE

VII

particularly Plate VIIA, B). These new grains are largely free from undulatory extinction and hence are “strain free” in usual petrographic terminology. This-is so even though the grains have formed under a non-hydrostatic stress and in an environment which was undergoing considerable strain. However, in large areas of the specimens the “new grains” still preserve the same shapes and general size distributions as are present within adjacent areas of subgrains (Plate VIIA, B). There is a complete gradation from definite subgrains with a slight orientation difference from neighbouring sub-

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grains and a diffuse grain boundary to “new grains” with sharp grain boundaries and a large orientation difference from the neighbouring material. There is no evidence in these examples to suggest that nucleation and subsequent growth of sub-microscopic regions is the mechanism of recrystallization. Rather, the process appears to be one in which adjacent subgrains increase their relative misorientations during deformation until an array of highly misoriented grains is developed. There is not enough evidence from these experiments to see if this misorientation process continues indefinitely Fig.7. Relationships between the c axes of newly recrystallized grains and the c axes of the host crystal in syntectonic recrystallization experiments. The measurements were collected throughout the specimen and not only in the central homogeneously strained area as in Fig.8. All specimens loaded at 900°C and 10 kbar. The arrows represent the direction of shortening of the specimen. A. DT.411 loaded in the 0’ orientation. 200 c axis orientations measured adjacent to new grains in host crystal. The full square represents the initial host orientation of c. Contours 1, 5, 10, 15, 20% per 1% area. B. DT.411. 200 c axes in newly recrystallized grains. Measurements taken adjacent to those of A. Contours 1, 3, 5% per 1% area. C. Histogram showing frequency, V, of angles, 9, between c axes of new grains and c axes of immediately adjacent host crystal. 200 angles. DT.411. D. DT.438 loaded in the lrorientation. 96 c axis orientations measured adjacent to new grains in host crystal. The full square represents the initial host orientation of c. Contours 1, 5, 10, 15, 20% per 1% area. Maximum 34% per 1% area. E. DT.438. 96 c axes in newly recrystallized grains. Measurements taken adjacent to those of D. Contours 1, 2, 4, 6% per 1% area. F. Histogram showing frequency, V, of angles, 8, between c axes of new grains and c axes of immediately adjacent host crystal. 96 angles, DT.438. G. DT.420 loaded in the 1 morientation. 112 c axis orientations measured adjacent to new grains in host crystal. Original orientation of c lay at base of the maximum indicated. Contours 1, 5, 10, 16, 20% per 1% area. H. DT.420 112 c axes in newly recrystallized grains. Measurements taken adjacent to those of G. Contours 1, 3, 6, 9. 12% per 1% area. I. Histogram showing frequency, V, of angles, 8, between c axes of new grains and c axes of immediately adjacent host crystal. 112 angles. DT.420. J. DT.441 loaded in the 11c orientation.155 c axis orientations measured adjacent to new grains in host crystal. Original orientation of c lay in the maximum indicated. Contours 1, 5, 10, 15, 20% per 1% area. Maximum 29%per 1% area. KDT.441. 156 C axes in newly recrystallized grains. Measurements taken adjacent to those of J. Contours 1, 2, 4, 6%per 1% area. L. Histogram showing frequency, V, of angles, 0, between c axes of new grains and C axes of immediately adjacent host crystal. 155 angles. DT.441.

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389

with continued deformation or if a steady state situation is finally reached. As the strain increases, the deformation bands appear to increase in width so that more and more subgrain boundaries are made into high angle boundaries. In some areas of thin sections, the grains do not have the same shapes and size distributions as in adjacent areas occupied by subgrains. Here, the grains are generally larger than the average subgrains and are irregular in shape with highly serrated boundaries (Plate VIIC). In areas where such grains are in contact with subgrains the boundaries are cuspate into the boundaries of the subgrains (Plate VIIC). This structure is similar to that produced during strain induced boundary migration in metals (Beck and Sperry, 1950), where stored strain energy drives grain boundary migration, the rate of migration being highest in local regions (such as subgrain boundaries) where the stored strain energy is highest. The process has been described in single crystals by Bailey (1963) and has been referred to as the bulge rzucleation model for recrystallization by Cahn (1966). An example where this process appears to be operating between adjacent orientations of host quartz is illustrated in Plate VIID. The shapes of grains which have developed by this process are not polygonal (Plate VI, VII) and grain boundaries possess many re-entrant angles. These shapes probably have arisen by some grains growing in the subgrain matrix and impinging, without appreciable subsequent adjustment of boundaries, under the influence of interfacial tensions. Preferred orientations of stew grains Fig.8 presents patterns of preferretd orientation of c of recrystallized grains in single crystals loaded in the 0 , 1 Y, lrn and ))c orientations. In each example, the measurements were taken from the central region of the deformed sample where the strain appears to be homogeneous and simply related to the axis of shortening of the specimen. Measurements of host caxis orientations for each specimen are presented in Fig.7 together with orientations of c in recrystallized grains immediately adjacent to the host measurements, and histograms showing the frequency of angles between c of the host grain and that of immediately adjacent recrystallized grains. Each pattern of preferred orientation in Fig.8 is characterized by maxima which lie on a small circle of approximately 50° angular distance from the axis of shortening of the specimen. It appears that each pattern is a combination of two tendencies: (1) The tendency for the host c-axis orientation to rotate towards the axis of shortening (or in -L m specimens, to remain at 90° to the axis of shortening). (2) The tendency for new grains to form which have c at 30“ - 50” to the new host orientations. There appears then to be a host control over the orientations of new grains and this control is similar to that observed in stress annealing and annealing experiments although certainly the control is not as precise as in these latter experiments. An alternative explanation is that there is a tendency for new grains to form with c at approximately 50° to 01. The present experiments do not distinguish between these two possibilities.

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Fig.8. Preferred orientation of c axes of new grains formed during syntectonic recrystallization of X-13 loaded at QOOOCand 10 kbar confining pressure. The arraws represent the direction of shortening. A. DT.411 loaded in the 0’ orientation, 213 c axes. Contours 1, 3, 5% per 1% area. B. DT.438 loaded in the1 Y orientation. 155 c axes. Contours 1, 3, 5, 7% per 1% area C. DT.420 loaded in the 1 m orientation‘ 110 c axes. Contours 1, 5, 7% per 1% area. D. DT.441 loaded in the I\c orientation. 250 c axes. Contours 1, 3, 5, 5, 7% per 1% area. Specimens deformed at 15 kbar confining pressure Specimens deformed at 15 kbar confining pressure and temperatures in the range 800° - 950°C, are notable for the widespread occurrence of coesite, the most spectacular of these being that pictured in Plate VID, where individual crystals of coesite are up to 2 mm in length, In most of these experiments the differential stress on the specimen is not well known because of the punching action of the piston into the specimen. However, an upper value for the differential stress can be fixed since this punching action can only increase the measured force on the specimen, In the specimen shown in Plate VIID where coesite is well developed, the maximum differential stress on the specimen was 9 kbar. Thus, the mean stress given by 1 vii is 18 kbar which is 12 kbar from the quartz-coesite boundary at 900°C as determined hydrostatically by Kitahara and Kennedy (1984). Tectonophysics,

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391

The coesite nucleates in the high strain regions of crystals whichhave been shortened 30% and spreads throughout the crystal as the strain increases to 50% shortening. In weakly deformed parts of the crystal, recrystallized quartz is present. It is possible that the stored energy of deformation has been responsible for the nucleation and growth of the coesite outside of the hydrostatic stability field (Kitahara and Kennedy, 1964). DISCUSSION The der:elopment

of preferred

orientation

The recrystallization process during the annealing and stress annealing of single crystals of quartz is identical with that in metals during annealing in that it involves nucleation of new relatively strain free grains and the subsequent growth of some or all of these nuclei to form a polycrystalline aggregate with a distinct lattice preferred orientation. Considerable controversy has existed in the metallurgical literature as to whether this preferred orientation owes its origin to: (a) development of nuclei with this preferred orientation followed by equal growth of all nuclei (Burgers and Louwerse, 1931); or (b) development of randomly oriented nuclei followed by growth of only some nuclei which bear a special orientation relationship to the host material (Barrett, 1940). The present state of these two hypotheses in metals is briefly discussed below. Preferred nucleation hypothesis Orowan (1954) was the first to point out that the classical theory of homogeneous nucleation cannot apply to the formation of recrystallization nuclei in metals. In this theory a thermal fluctuation results in the spontaneous formation of a new region of unstrained lattice which is large enough that during its continued growth, the decrease in volume free energy offsets the increase in surface free energy. The region is then able to grow and is called a nucleus. Orowan showed that in copper, the energy required to create such a thermal fluctuation was much larger than that possible for the nucleation process. Modifications were introduced by Burke and Turnbull (1952) to include heterogeneous nucleation but it appears that, at least for relatively pure metals, the theory is incapable of predicting the results of experiment. Orowan (1954) suggested that the nuclei developed in situ from pre-existing regions of the deformed lattice. Similar suggestions were made by Beck (1949) and by Cahn (1950) who postulated that polygonization of regions of high dislocation density was the mechanism of formation of the regions. This particular mechanism has been adequately documented by Bollman (1959), Walter and Koch (1963) and in particular, by Hu (1963a) who demonstrated subgrain coalescence as a mechanism of nucleation, On this basis, preexisting cells which have formed during deformation, are modified to form subgrains during recovery and, at the same time, adjacent subgrains rotate until they adopt a common orientation. This rotation increases the number of high angle boundaries in the subgrain array and the process continues until the growing subgrain encounters a region of quite different orientation.

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It is observed that these high angle boundaries can migrate readily and subsequent growth is often quite rapid leading to a newly recrystallized grain. Theoretical aspects of subgrain rotation have been discussed by Li (1962). Another nucleation mechanism which has been described in detail is the bulge nucleation model (Cahn, 1966). This model was proposed by Bailey (1960) and by Bailey and Hirsch (1962) and detailed descriptions of the process are given by Bailey (1963). In this model, high angle boundaries on pre-formed regions suddenly bulge and grow under the influence of local differences in stored energy. Hu (1963b) has suggested that this model may be identical to his grain coalescence model. Thus, in some materials, pre-formed regions do form the nuclei although in the subgrain coalescence model substantial reorientations of these regions occur during nucleation (Hu, 1963a, fig.30). However, this simple hypothesis does not adequately explain the development of lattice preferred orientations during annealing in metals. The situation has been recently reviewed by Beck and Hu (1966). Anisotvopic growth hypothesis It has been adequately demonstrated that in f.c.c.lmetal single crystals undergoing recrystallization, grains with with respect to the host, have the greatest rate of growth. Thus, in the recrystallization of aluminium single crystals, if randomly oriented nuclei are first introduced, a strong preferred orientation results after growth related to the host orientation by 40° rotations (Kohara et al., 1958). This observation holds only for impure metals with trace amounts of soluble impurities; in pure metals growth anisotropy may be absent (Aust and Rutter, 1963). The effect of impurities has been reviewed by Gordon and Vandermeer (1966) and by Beck and Hu (1966). In b.c.cr metals the orientation for fastest growth is a rotation of 25O or 30° about a common axis and in hexagonal metals the rotation is 30° [OOOl]. Kronberg and Wilson (1949) were the first to suggest that 40° boundaries in f.c.c. metals owed their high mobility to the fact that this orientation has a relatively high number of the atomic sites in the new grain coincident with a sublattice of the host grain. They postulated that this coincidence of atomic sites facilitated exchange of atoms from one lattice to the other and resulted in the high growth rates observed for these boundaries. Such boundaries of good fit are often called Kronberg-Wilson or “specialn boundaries. Much of the recent metallurgical literature has attempted to rationalise observed orientation relationships on the basis of the Kronberg-Wilson suggestion even though strict Kronberg-Wilson boundaries are rarely observed and departures of up to l2O from special relationships are more the rule. Gordon and Vdndermeer (1966) have pointed out that even a few degrees departure from a special relationship results in considerable misfit so that the Kronberg-Wilson suggestion does not strictly agree with observed boundary relationships. Further, Li (1961,1963) has indicated that strict Kronberg-Wilson boundaries should be relatively im mobile since, intuitively, one would expect the lack of porosity associated with a region of nearly perfect fit, to inhibit diffusion rather than enhance ‘f.c.c. = face centered cubic; b.c.c. = body centered cubic. Tectonophysics, 6 (5) (1968) 353-401

393

it. The situation has been rationalized by Gordon and Vandermeer (1966)who suggest that boundaries with bad fit (and hence high porosity) may be associated with impurity clouds that inhibit boundary migration whereas boundaries close to special orientations are not so porous and hence have higher mobilities. Even though strongly anisotropic growth has been demonstrated this observation alone is not capable of explaining all observed preferred orientations, particularly the development of different preferred orientations from specimens given identical cold deformation but different annealing treatments. Nucleation in annealed quartz single crystals The first stage in annealing, as revealed by transmission electron microscopy, is the development of regions 0.5 - b across bounded by dislocation arrays in which dislocation lines are parallel to [a-tc] and to a. As indicated, these arrays are parallel to Y and z planes. It is possible that these subgrains represent nuclei which grow by a process analogous to the Hu-Li process but electron microscope studies on much larger areas are required before details of the nucleation process are obtained. It is possible though to check if the observed preferred orientations could develop from the growth of small pre-formed regions. The intimate association of grains of a particular orientation of c with kinked regions of a similar orientation of c suggests that these preformed regions, if present, could have developed by a process of kinking of the active slip planes. Fig.9 shows the predicted orientations of c in kinked regions in crystals of various orientations with respect to al , The agreement with observed patterns of preferred orientations of+c is almost perfect. The model is also supported loaded at low1 temperatures where by the observation that 0 , or Ircrystals basal slip predominates, contain a predominance of new grains with c close to al whereas those loaded at high temperatures, where prismatic slip predominates, contain a predominance of new grains with c approximately normal to 9. Although the agreement for predicted c -axis patterns is good, for other crystallographic axes the agreement is poor in that the crystallographic direction parallel to the postulated axis of kinking in the host is rarely coincident with the same directioy in the recrystallized grain (see Fig.3). Thus, in crystals loaded in the 0 -orientation, new grains with their c-axis near 01 should have an a*-axis in common with the host if they have developed simply from pre-formed kinked regions. Fig.3D indicates that a* of the new grains lies up to 30° from a* of the host and only rarely are the two coincident. If these grains have developed from pre-formed regions by a Hu-Li process there must have been some rotation of the subgrainsabout an axis close to c during recovery. Growth of new grains Lack of knowledge of the orientations of nuclei makes interpretation in terms .of anisotropic growth uncertain. However, the presence of more diversely oriented grains in the early rather than in the late stages of recrystallization suggest that anisotropic growth is playing a significant part ‘The terms high and low temperature here are relative to the critical temperature for hydrolitic weakening (Griggs, 1967).

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Fig.9. Predicted redistribution of c axes of host crystal by kinking on slip systems with high resolved+shear stress. A. Crystal loaded in the 0 orientation. Path-1 is for a {OOOl} slip, path 2 is for c 00103 slip and path 3 for [~+a] (1010) slip. B. Cr stal loaded in the lm orientation. The paths shown are for [c+uJ {loll{ and [ctu] {Olilj slip. C. Crystal loaded in the IIc orientation. Path 1 is for [c+u] {lo&} and [c+u] (0111) slip whereas path 2 is for[c+a] (1122) and [~+a] {2112} slip.

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in the process. This conclusion is confirmed by observations on grains that remain small; grains with orientations of c within loo of the host c-axis tend to remain small and may be completely absent from the final pattern of preferred orientation, whereas grains with c oriented at 20° - 40° to the host c-axis make the greatest contribution to the final pattern. There is not enough data from optical measurements to decide if particular crystallographic axes are common between the host and the recrystallized grain but it appears that either a* (Fig.SD), a (Fig.5B) ory or z (Fig.3B) may be approximately coincident depending on the intitial relative orientations of host c and of al. Lack of knowledge of the early stages of nucleation and growth of grains precludes definite statements regarding the origin of observed preferred orientations. The answer might be obtained from a study of large areas by transmission electron microscopy. This necessitates the preparation of thin slices approximately 0.1~ thick and this has not yet proved successful. The indication is that the orientations of preformed regions are slightly changed during recovery and that these then act as nuclei in a manner analogous to the Hu-Li process. Anisotropic growth is probably also a dominant process such that new grains with c oriented at 20 o - 40° to the host c have the fastest rate of growth.

Effect

of water

on the recrystallization

process

The experiments described here indicate that dry quartz will not recrystallize except at temperatures close to the melting point. This inability to recrystallize may be due to structural or to kinetic reasons or to combinations of the two. Stnictural reasons for the lack of recrystallization The recrystallization process in single crystals consists of two distinct parts, namely, the nucleation of new strain free grains and the subsequent growth of these nuclei to ultimately form a polycrystalline aggregate. Hu (1963a) has shown that the nucleation process may be strongly structure dependent so that silicon-iron which has been heavily rolled in the (001) < llO> orientation recrystallizes with great difficulty whereas rolling in the {OOl) orientation enables recrystallization to proceed with considerable ease. Hu (1963) has shown that a different microstructure results from these two treatments; the (001) rolling produces a uniform tangle of dislocations with no cell structure developed. On annealing, a uniform network structure forms with no local areas of large misorientation. Thus, this material undergoes recovery but no recrystallization ensues because of the lack of nucleus formation. The (001) rolling produces thin deformation bands which have a distinct cell structure within them. During annealing, subgrains coalesce and ultimately expand into a growing grain. Thus, the deformation bands act as nucleation sites and this material recrystallizes readily. The observation that natural quartz crystals fail to recrystallize in a dry environment yet recrystallize readily in a wet environment, even though the initial deformation treatments are identical, rules out structural reasons for the lack of recrystallization in the dry state. 396

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Kinetic reasons for the lack of recrystallization The influence of kinetic factors on either or both of the nucleation and growth stages of recrystallization may contribute to the lack of recrystallization in dry quartz. A common observation in metals (Vandermeer and Gordon, 1963) is that recrystallization is slowed or inhibited by recovery competing with recrystallization for the stored energy of deformation. However, both optical and electron microscopic observations suggest that recovery is very slow in the absence of (OH) from the quartz structure. Thus, 2 h at 1,Ooo~C (plate IVA) in dry quartz results in little change in the appearance from the initial cold worked state. On the electron microscope scale, the initially tangled dislocations remain tangled even after annealing for 15 h at 15 kbar and 1,200°C (McLaren and Retchford, 1966). There is no tendency to form networks, This contrasts with single crystals in a hydrous environment where the tangled dislocations straighten out and form networks in a very short time, Thus, removal of stored strain energy by recovery appears to be a very slow process in dry quartz even at temperatures close to the melting point. If, as indicated in the discussion above, nucleation is part of the recovery process in structurally suitable environments, then the observed slowness of recovery accounts for the lack of nucleation in dry quartz. Electron microscope observations indicate that in quartz (as in metals) recovery involves the climb of dislocations to form stable arrays. Adopting a model similar to that of Griggs’ (1967, fig.15) for the glide of an edge dislocation, for a dislocation to climb in dry quartz, we would expect to haveto break Si-0 bonds, requiring an activation energy of perhaps 100 kcal/g-mol. If Si-0-Si bridges are hydrolysed (Griggs and Blacic, 1965), then the dislocations can climb by breaking Si-OH bonds, representing an order of magnitude decrease in activation energy. The rate controlling process would presumably be the diffusion of (OH) through the quartz structure to the dislocation sites. Influence of stress and strain on the pattern of Preferred orientation during syntectonic recrystallization One of the central problems in structural geology is to determine the influence of stress and/or strain on the patterns of preferred orientation that develop during syntectonic recrystallization. Theoretical aspects of the problem have been considered by Verhoogen (1951) MacDonald (1957,196O) and Kamb (1959, 1961). In the models discussed by these authors, one grain is considered to grow at the expense of differently oriented neighbours, the driving force being a difference in free energy resulting from different elastic energy densities in adjacent grains. However, these models take no account of grain boundary effects such as structure and interfacial energy and moreover, they implicitly or explicitly assume that both nucleationand the subsequent growth of nuclei driven by the stored energy of deformation do not play roles in the recrystallization process. The experiments described here indicate that in either or both of the nucleation and growth processes, the host orientation exerts a considerable influence on the orientation of new grains that form and ultimately grow. On the evidence available, it appears that this may be true independently of whether recrystallization takes place during hydrostatic annealing, stress annealing or syntectonic recrystallization experiments.

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Geological implications of host control Although the observations reported here have been only for the recrystallization of single crystals of quartzi analogy with metals and with calcite suggests that the same results may be true for the recrystallization of polycrystalline aggregates of quartz. Thus, the reviews by Wassermann and Grewen (I962), Dillamore and Roberts (1965) and by Beck and Hu (1966) show that the presence or absence of grain boundaries has little effect on the patterns of preferred orientation which develop during the primary recrystallization of metals. Again, in both hydrostatic annealing and syntectonic recrystallization of Yule Marble (Griggs et al. 1960a; 1960b and Ferreira and Turner, 1964) new grains of calcite that nucleate both within an old grain and within a grain boundary, exhibit identical orientation relationships with the immediately adjacent host grain. Future experiments will indicate how far the results obtained here for single crystals can be applied to the primary rec~stall~ation of polycrystal~ne quartz aggregates. If the conclusion that the host orientation has a significant influence on the orientations of newly recrystallized grains is true, then this means that in order for a preferred lattice orientation to develop during recrystallization of a polycrystalline aggregate, the host grains must develop a preferred orientation during deformation and prior to or during the formation of new grains or that nucleation takes place preferentially in host grains of a certain orientation. This prediction should be readily tested in natural tectonites. The final pattern of preferred orientation that develops may be duplicated or substantially altered by anisotropic growth during later grain growth or secondary recrystallization.

CONCLUSIONS

(1) Single crystals of quartz will not recrystallize unless trace amounts (greater than about 1,000 p.p.m. OH/Si) of (OH) are present in the str .mture. (2) In both hydrostatic annealing and stress annealing experiments nucleation of strain free grains occurs at kink boundaric The orientations of new grains is consistent with nuclei being pre-formed kinked regions of the lattice which have suffered slight reorientations during recovery. Growth of new grains appears to be anisotropic so that grains in which c is within loo of the host c axis remain small; grains in which c lies at 20° - 40° to the host c axis grow fastest. (3) During syntectonic recrystallization of single crystals of quartz new grains appear to develop from subgrains in which c has developed a differentorientation from that of the host crystal. Nucleation from sub-microscopic regions does not occur. Subgrains which differ greatly in orientation from the neighbouring subgrains grow into the adjacent matrix in a manner resembling strain induced grain boundary migration. (4) The patterns of preferred orientation that develop during syntectonic recrystallization are similar to those that develop during hydrostatic annealing in that the c axes of new grains tend to lie at 30° - 50° to the adjacent host c. There may be a control exerted by the stress in that new c axes tend to lie at 50° to UI_ Further experiments are necessary to

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elucidate the relative importance of host control and stress on the orientations of newly recrystallized grains in syntectonic experiments. (5) Coesite has been nucleated and grown well below the hydrostatically determined quartz-coesite phase boundary. It is possible that the stability field of coesite is influenced by stored strain energy.

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