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Shock deformation of quartz sand
Int. J. Rock Mech. Min. Sci. Vol. 3, pp. 81-88. PergamonPress Ltd. 1966. Printed in Great Britain
SHOCK DEFORMATION OF QUARTZ SAND C. C. FRYER Postgraduate School of Mining, University of Sheffield (Received 30 September 1965)
Abstract--Using a new method of deformation in a simple piece of apparatus, a quartz sand has been deformed in such a way that the grains collectively show all the features of brittle and plastic deformation previously ascribed to quartz. With the identification of coesite in some specimens, light is cast on the low-temperature boundary in the quartz-coesite stability field. The geological and engineering significance of the results is discussed. 1. INTRODUCTION
IT HAS only recently become widely accepted that quartz may deform in a ductile manner. Although the mechanism of this deformation is not well understood, it has been known for some time that strained quartz, both natural and laboratory-simulated, shows strain effects in the form of undulatory extinction and deformation bands. In a recent work, CARTER et al. [1] have statically deformed loose quartz sand, quartzite, single quartz crystals and flint in a simple squeezer, a cubic apparatus, and also in a variable strain-rate apparatus. Deformation features thus produced include fracturing, undulatory extinction, deformation (B6hm) lamellae, deformation bands and the formation of coesite. In a companion paper to the aforementioned, the same authors [2] have shown how certain of these features can be ascribed to basal slip in the quartz crystals. In the present range of experiments the mode of deformation is quite different to the above: the quartz sand, which is disaggregated, is deformed by shock waves at a low temperature, the entire sample being recoverable for examination. Two series of experiments were run concurrently, firstly, with a completely dry sand, and secondly, with a completely water-saturated sand. 2. APPARATUS AND EXPERIMENTAL METHODS
2.1 General Ten experiments were carried out in the same apparatus but under different conditions. Five experiments were performed using, with each, an increasing and known amount of Polar Gelignite. These experiments were carried out with dry Kelloways (Jurassic) sand obtained from a natural exposure in East Yorkshire. Approximately 5 g of sand was used in each experiment. Concurrently a parallel series of experiments was carried out using a saturated sand, approximately 3.5 cm3 of water being needed to completely fill the intergranular spaces. 2.2 The pressure cell 2.2(a) Construction. This consists of two circular plates 3~- in. in diameter one plate (the bottom) being ] in. thick, the top plate being ¼ in. thick. In the centre of the bottom 81 F
82
c . c . FRYER
plate is a 1½ in. diameter depression, ~ in. deep in which the sample is placed. The top plate is then bolted on with six ~g ~ in. hard steel Allen bolts. Figure 3 shows the cell both assembled and open. In use, the gelignite is placed on top of the cell, in the centre between the Allen bolt heads, and held in position with Plasticine. Detonation is by low-voltage electric detonators. Each cell is of sufficiently strong construction to withstand the forces generated during three or four experiments, after which time surface grinding may become necessary to again ensure a perfect fit between the faces. 2.2(b) Principles of wave transmission. The detonation wave generated in the high explosive is capable of exerting extreme pressure on an adjacent surface, and detonation pressures of the order of 300 kbars are common to industrial explosives. It is not anticipated that such pressures will be experienced under these experimental conditions, since the energy transfer will be partly dependent upon the interfacial fit, and the degree of confinement of the sample, and also partly dependent upon the conditions given by the mismatch equations derived by GORANSON [3]. In this sense the explosive cell is being used as a transmitting medium in the transfer of explosive energy to the enclosed sand specimen. 2.3 Preparation of material Prior to deformation the sand is in the form of a loose, sieved aggregate, ranging in size between 350 and 500/~. After deformation in most cases the grains suffer a reduction in size to what may be loosely described as a powder. In order to prepare a petrological thin section from such material, impregnation is necessary and is carried out by the method previously described by the author elsewhere [4]. Standard thin sections are then prepared in the normal manner. 3. PETROGRAPHIC STUDIES
3.1 Undeformed sand The mineralogical constitution of the sand is about 85 % quartz, 10 % plagioclase felspar, and about 5 % clay, mica and ferruginous material, by volume. The quartz grains are of medium (0.4-0.6) sphericity, and medium (0.25-0.40) roundness, according to the scale advocated by PETT1JOIaN[5]. The majority of grains are strain-free, with only a small amount of undulatory extinction and fracturing. All discrete grains are in simple optic and hence crystallographic continuity, i.e. there are no complex grains present. Figures 4 and 5 show the nature of the undeformed sand. 3.2 Deformed sand 3.2(a) Formation ofcoesite. The dense polymorph of silica now known as coesite was one of the first new phases discovered in investigations of silicate equilibria at high pressures and temperatures. Since the first synthesis by CoEs JR. [6] there has been much interest both in determining the properties and stability of coesite and also in the possibility of finding natural occurrences of the phase. In 1960, CHAO et aL [7] described such an occurrence of coesite in Meteor Crater, Arizona. The quartz--coesite stability field has been evaluated by BOYD and ENGLAND [8] at high temperatures (see Fig. 1) but so far there have been no indications as to whether the natural occurrence of coesite results from high temperature or low. With the production of coesite in this series of experiments (carried out at low temperatures) some modifications may be made to the parameters which have been extrapolated
SHOCK DEFORMATION
OF QUARTZ
SAND
83
for the quartz-coesite stability field. Geological implications of this will be described more fully elsewhere. The material produced in the experiments was analysed both by standard optical and X-ray techniques. Because of the large difference in refractive indices between quartz and coesite (mean indices 1.550 and 1.595 respectively) it has been found relatively easy to
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20 Pressure,
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FIG. 1. The quartz-coesite stability field.
distinguish the coesite optically. Figure 6 shows the coesite developed as a mass growing into a larger grain of quartz in what was probably an area of high stress distribution. 3.2(b) Fracturing. In the undeformed rock fracturing is very low. According to the method outlined by BORG et aL [9] the fracture index is of the order 130-140. The fractures are generally curved, being planar for only short distances and the amount of fracturing in any one grain is variable, but never exceeds two fractures. Table 1 below gives a comparison between the fracture indices of dry deformed sand and the H20 saturated deformed sand. It will be noticed that despite the greater stress concentration in the 'wet' samples fracturing is not increased significantly over the fracturing intensity in 'dry' samples. The reduction in fracturing intensity in the samples subjected to maximum stress (4 oz charge) can be explained by fracturing resulting in disaggregation of the grains (and thus reduction
84
C. C. FRYER TABLE1 Specimen number
Charge ( o z )
Fracture index dry
C0t~ C I tL Clh C~t ~-'2h C'Att C3h C4~ C4~
Undeformed
130-140 160
I 1
Fracture index wet
155
2 2 3 3 4 4
197 200
240
230
225
150
in the grain size) in the dry specimen, and the increasing importance of recrystallization as a means of accommodating the stress in the 'wet' saturated specimen. A statistical analysis was performed on the sample showing the highest degree of fracturing (sample C~a) to determine the angular relationship between the fracture plane and the optic axis in 100 grains. The results of the analysis are shown in the form of a histogram in Fig. 2. A slight preference is observed for fracturing to take place at right angles to C,,, or along the basal plane. 7~,
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.
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Angle of frQcture from C, FIG.
2
3.2(c) Undulatory extinction. This phenomenon results from bending of the optic axis possibly occurring as a cumulative effect of lattice dislocations. Undulatory extinction has been known for many years as a common occurrence in igneous and metamorphic rocks, although not in clastic quartz. The first definite indication of plastic flow was given by BA]LE¥ et al. [10] who showed that Laiie (X-ray) photographs of quartz displaying undulatory extinction commonly showed asterism and polygonization as compared with the sharp Laiie figures produced from non-undulose quartz. This indicates that undulatory extinction is the optical result of the effects of plastic deformation. In terms of dislocation theory, it has been postulated that the crystallites created in a deformed quartz by bend gliding and polygonization are inclined to each other at small angles. This must result in a relative displacement of the optic axes of the sub-grains, and
FIG. 3. The cell displayed in open form, and ready for detonation.
FIG. 4. The undeformed
facing page 84 R.M.
sand, plane polarized light.
FE. 5. The undeformed
sand. crossed nicols.
FIG. 6. Coesite, developing inwards from the extremity of a quartz crystal, crossed nicols.
FIG. 7. Quartz grain showing undulatory extinction : crossed nicols.
FIG. 8. Deformation
bands, crossed polarized light.
FIG..9. Deformation
lamellae, reflected light.
FIG. 10. BBhm lamellae, crossed nicols.
FIG. 1I. Recrystallization,
forming larger grain than original, crossed nicols.
SHOCK DEFORMATION OF QUARTZ SAND
85
the extinction position of the host crystal will therefore vary from one area to another, which will account for the wavy extinction noticed under the microscope. On the rotation of the microscope stage this extinction will sweep across the field of view if the quartz grain is orientated with the axis of bending vertical. For other orientations the sweeping effect will be noticeably less, and the variation in extinction angles will be smaller. Since the displacement of the crystallites is small, the bands of undulatory extinction will therefore be sub-parallel to each other. Departure from this relationship, however, will increase with increasing deformation. As the deformation increases in intensity the extinction bands become regular, narrow and pronounced. This has been considered to be a consequence of the greater concentration of the dislocations into walls normal to the bent glide planes, creating elongate polygonized zones, or crystallites. The phenomenon of undulatory extinction, as observed in the deformed specimens in these experiments is in line with that observed by other workers, such as FA1RBAIRN[11], INGERSONand TURTLE[12] and BAILEYet al. [10] who have noted that it occurs in rocks which are deformed but not recrystallized. Recrystallization which is more abundant in the saturated shocked specimens, will tend to form new, strain-free, quartz, which will only show undulatory extinction after a later phase of deformation. In recent experimental work, CARTER et al. [1] have differentiated two forms of undulatory extinction--undulatory extinction and deformation bands. This distinction is based on the radius of bending of the basal planes as revealed by variation in the extinction position in the boundary region between the reoriented zones. This is an arbitrary distinction, however, as there is a complete gradation between the features. Figures 7 and 8 show two extreme cases: in the former, undulatory extinction is widespread across one crystal; in the latter, deformation bands are developed in two sets, trending about 60 ° to each other. Deformation bands were only observed in the specimens here at the upper end of the shock-loading scale, but undulatory extinction was observed in most specimens. 3.2(d) Deformation lamellae. Many workers studying the deformation of quartz at high temperatures have remarked upon the occurrence of deformation lamellae. These are very thin, nearly planar lamellae in parallel sets. Unlike deformation bands they are not distinguishable from the surrounding host grain by virtue of variation in extinction position but they have a different refractive index to that of quartz in the host material. Thus, when in focus in plane-polarized light, they are noticeable by their brightness. Extinction in crosspolarized light is simultaneous with the host material. In some cases rows of inclusions may be observed which are sub-linear in direction. These are, in fact, lamellae containing many minute brownish inclusions--the so-called BOhm lamellae. The nature of the inclusions has never been determined optically due to the / smallness in size, but INGERSONand TUTTLE[12] have identified some as liquid, others may be gaseous. In the experiments carried out in the present work, deformation lamellae, (including B~hm lamellae) were not commonly observed. In the undeformed rock, not one grain was observed displaying these features. In the deformed specimens, only samples 3a, 4a and 4b (i.e. those subject to the highest degree of shock-loading) displayed deformation lamellae. The percentage of grains in these cases was 2, 4 and 3 respectively. Figures 9 and 10 show the development of simple deformation lamellae and BShm deformation lamellae in specimen 4a. The origin of deformation lamellae is uncertain, but CHRISTIEet aL [2] have shown that
86
c . c . FR'LER
deformation features produced at high temperature and confining pressure with high shear stress on the basal plane are parallel to slip zones in quartz. These are also parallel to the basal plane. 3.2(d) Recrystallization. Syntectonic recrystallization has been produced in only one sample, 4b, deformed at the highest pressure and under H20 saturation. Figure 11 shows a typical development of a recrystallized grain. Some of the grains have crystallized entirely to a mosaic of small new grains, others are only partially recrystallized. In the former case, the mosaic is larger in size than the original undeformed material. Due to the smallness of the sample, however, sieve analysis on the deformed rock was not possible. The mechanism of recrystallization is not clear. There is little evidence of serrated edges to the crystals, a feature which is usually taken as the first sign of recrystallization. This would tend to dispose of the theory of diffusion which in any case partly depends upon the juxtaposition of crystals with differing lattice orientations [13]. Recrystallization does not tend to develop along any obvious preferential crystallographic plane. Thus for the time being, no explanation can be proffered for the mechanism. 4. CONCLUSION. DISCUSSION AND SIGNIFICANCE OF RESULTS The purpose of the work is primarily to evaluate the response of rocks to stresses which may affect them in the earth's crust, and, more particularly in engineering situations. Thus, more specifically, if the response of individual sand grains is known, then these results may be extrapolated to include consolidated sands which are frequently encountered as sandstones in engineering situations (and whose mechanisms of deformation and fracture are still largely unknown). The results may be tabulated briefly and are discussed below.
4.1 Formation of coesite The stability field of this comparatively rare polymorph of silica is not well known, especially in the low temperature (<500°C) field relevant to these experiments. Consequently, the phenomenon of its occurrence at low temperatures has important significance both to its artificial synthesis and its occurrence in natural environments such as meteoric craters. 4.2 Fracturing As might be expected, because of the relative freedom of individual grains, fracturing is very low in all the samples. Instead, grain rotation probably occurs, although it is impossible to distinguish this microscopically.l~Where fracturing is observed, there is a strong possibility that such grains were in close contact to the cell walls. No difference is observed in the fracturing intensity of 'wet' and 'dry' samples. In all possibility the grain packing is not dense enough to transmit the increased stress which occurs in the saturated samples. Where fracturing is observed, slight crystallographic control appears to be exerted indicating a preference for the basal plane. This is in contrast to findings of other workers who have noted a tendency for fractures to run along the prismatic direction. 4.3 Undulatory extinction This is the most pronounced new feature of the deformed sand. It has been shown that with increasing deformation undulatory extinction increases in intensity until narrow bands appear. This is the ultimate product seen of plastic deformation in quartz. A theory of
SHOCK DEFORMATION OF QUARTZ SAND
87
dislocation has been put forward to account for the phenomenon. As could be expected, in samples which showed extensive recrystallization, undulatory extinction is absent, the very high stress concentration which in itself has caused undulatory extinction, has also caused a migration and reorganization of grain boundaries, possibly by a process akin to diffusion between adjacent crystallites to give rise to new, strain-free, quartz. It is significant to note that the intensity of undulatory extinction observed in the deformed sand is greater than any so far observed in natural sandstones and quartzites. Once again this must be attributed to recrystallization: in naturally occurring deformed rocks longterm 'creep' effects will tend to reduce the observed values of undulatory extinction, whilst in the experimentally deformed samples a 'strain-hardening' type of effect will increase the observed values. 4.4 Deformation lamellae The optical expression of basal slip in quartz crystals has not been frequently observed. From the results obtained by CARTER et aL [1] it is known that such deformation characteristics are most commonly produced at high temperatures (500-800°C), which, nevertheless, are still within the metamorphic environment. The present work has shown that at rapid loading rates at low temperature it is still possible to produce such features in isolated grains. 4.5 Recrystallization CARTER et al. [1] have noticed that the temperature at which quartz starts to recrystallize is dependent upon the rate of deformation. RALEIGH[14] has found that recrystallization will take place in flint at 500°C in experiments carried out for several days, whereas short (1 hr) experiments will not produce recrystallization at less than 900°C. Contrary to this are the results of the present work, where, with a very high loading rate, quartz has begun to recrystallize in the presence of water at very low temperatures. (The increase of temperature of the cell during deformation is negligible.) 5. SUMMARY OF CONCLUSIONS The major importance of this work is that all present-known deformation features in quartz have been produced in the same sample at very low temperature using a rapid strainrate shock-loading device. Important light is cast on the environments of deformation and recrystallization of quartz, and new parameters are indicated for the quartz-coesite stability field. The inferences of these results are important in geological and engineering environments. Acknowledgements--This work was carried out in the Postgraduate Schoolof Mining, Universityof Sheffield' under the guidance of the Director, Dr. A. ROBERTS. Valuable discussionhas taken place with D. HASLAM,who has also assisted in the experimental work. REFERENCES 1. CARTER N. L., CHRISTIE J. M. and GRIGGS D. T. Experimental deformation and recrystallization of quartz, J. Geol. 72, 687-733 (1964). 2. CHRISTIEJ. M., GRIGGS D, T. and CARTERN. L. Experimental evidence of basal slip in quartz, Jr. Geol. 72, 734-756 (1964). 3. GORANSONR. W. in COOK M. A. The Science o f High Explosives, p. 111, Reinhold, N.Y. (1958). 4. FRYER C. C. Impregnation of loose and poorly consolidated material, Trans. BrL Ceram. Soe. 65 103109 (1966).
88
C . C . FRYER
5. PETTIJOHNF. J. The Sedimentary Rocks, Harper, N.Y. (1949). 6. COESL. JR. A new dense crystalline silica, Science 118, 131-132 (1953). 7. CHAO E. C. T., SHOEMAKERE. M. and MADSENB. M. First natural occurrence of coesite, Science 132, 220-222 (1960). 8. BOYD F. R. and ENGLANDJ. L. The quartz--coesite transition, J. geophys. Res. 65, 749-756 (1960). 9. BORG I., FRIEDMAN M., ]-IANDINJ. and H1GGS D. V. Experimental deformation of St. Peter Sand: study of cataclastic flow, Geological Society of America, Memo. 79, Chap. VI, 133-191 (1960). 10. BAILEYS. W., BELL R. A. and PENG C. J. Plastic deformation of quartz in nature, Bull. geol. Soc. Am. 69, 1443-1466 (1958). 11. FAIRBAIRNH. W. Deformation lamellae in quartz from the Ajibik formation, Michigan, Bull. geol. Soc. Am. 52, 1265-1278 (1941). 12. INGERSONE. and TUTTLEO. F. Relations of lamellae and crystallography of quartz and fabric directions of some deformed rocks, Trans. Am. geophys. Un. 26, 95-105 (1945). 13. COTTRELLA. H. Dislocations andPlastic Flow in Crystals, O.U.P. (1953). 14. RALEIGHC. B. Crystallization and recrystallization of quartz in a simple piston-cylinder device, J. Geol. 73, 369-377 (1965).
SHOCK DEFORMATION OF QUARTZ SAND C. C. FRYER Postgraduate School of Mining, University of Sheffield (Received 30 September 1965)
Abstract--Using a new method of deformation in a simple piece of apparatus, a quartz sand has been deformed in such a way that the grains collectively show all the features of brittle and plastic deformation previously ascribed to quartz. With the identification of coesite in some specimens, light is cast on the low-temperature boundary in the quartz-coesite stability field. The geological and engineering significance of the results is discussed. 1. INTRODUCTION
IT HAS only recently become widely accepted that quartz may deform in a ductile manner. Although the mechanism of this deformation is not well understood, it has been known for some time that strained quartz, both natural and laboratory-simulated, shows strain effects in the form of undulatory extinction and deformation bands. In a recent work, CARTER et al. [1] have statically deformed loose quartz sand, quartzite, single quartz crystals and flint in a simple squeezer, a cubic apparatus, and also in a variable strain-rate apparatus. Deformation features thus produced include fracturing, undulatory extinction, deformation (B6hm) lamellae, deformation bands and the formation of coesite. In a companion paper to the aforementioned, the same authors [2] have shown how certain of these features can be ascribed to basal slip in the quartz crystals. In the present range of experiments the mode of deformation is quite different to the above: the quartz sand, which is disaggregated, is deformed by shock waves at a low temperature, the entire sample being recoverable for examination. Two series of experiments were run concurrently, firstly, with a completely dry sand, and secondly, with a completely water-saturated sand. 2. APPARATUS AND EXPERIMENTAL METHODS
2.1 General Ten experiments were carried out in the same apparatus but under different conditions. Five experiments were performed using, with each, an increasing and known amount of Polar Gelignite. These experiments were carried out with dry Kelloways (Jurassic) sand obtained from a natural exposure in East Yorkshire. Approximately 5 g of sand was used in each experiment. Concurrently a parallel series of experiments was carried out using a saturated sand, approximately 3.5 cm3 of water being needed to completely fill the intergranular spaces. 2.2 The pressure cell 2.2(a) Construction. This consists of two circular plates 3~- in. in diameter one plate (the bottom) being ] in. thick, the top plate being ¼ in. thick. In the centre of the bottom 81 F
82
c . c . FRYER
plate is a 1½ in. diameter depression, ~ in. deep in which the sample is placed. The top plate is then bolted on with six ~g ~ in. hard steel Allen bolts. Figure 3 shows the cell both assembled and open. In use, the gelignite is placed on top of the cell, in the centre between the Allen bolt heads, and held in position with Plasticine. Detonation is by low-voltage electric detonators. Each cell is of sufficiently strong construction to withstand the forces generated during three or four experiments, after which time surface grinding may become necessary to again ensure a perfect fit between the faces. 2.2(b) Principles of wave transmission. The detonation wave generated in the high explosive is capable of exerting extreme pressure on an adjacent surface, and detonation pressures of the order of 300 kbars are common to industrial explosives. It is not anticipated that such pressures will be experienced under these experimental conditions, since the energy transfer will be partly dependent upon the interfacial fit, and the degree of confinement of the sample, and also partly dependent upon the conditions given by the mismatch equations derived by GORANSON [3]. In this sense the explosive cell is being used as a transmitting medium in the transfer of explosive energy to the enclosed sand specimen. 2.3 Preparation of material Prior to deformation the sand is in the form of a loose, sieved aggregate, ranging in size between 350 and 500/~. After deformation in most cases the grains suffer a reduction in size to what may be loosely described as a powder. In order to prepare a petrological thin section from such material, impregnation is necessary and is carried out by the method previously described by the author elsewhere [4]. Standard thin sections are then prepared in the normal manner. 3. PETROGRAPHIC STUDIES
3.1 Undeformed sand The mineralogical constitution of the sand is about 85 % quartz, 10 % plagioclase felspar, and about 5 % clay, mica and ferruginous material, by volume. The quartz grains are of medium (0.4-0.6) sphericity, and medium (0.25-0.40) roundness, according to the scale advocated by PETT1JOIaN[5]. The majority of grains are strain-free, with only a small amount of undulatory extinction and fracturing. All discrete grains are in simple optic and hence crystallographic continuity, i.e. there are no complex grains present. Figures 4 and 5 show the nature of the undeformed sand. 3.2 Deformed sand 3.2(a) Formation ofcoesite. The dense polymorph of silica now known as coesite was one of the first new phases discovered in investigations of silicate equilibria at high pressures and temperatures. Since the first synthesis by CoEs JR. [6] there has been much interest both in determining the properties and stability of coesite and also in the possibility of finding natural occurrences of the phase. In 1960, CHAO et aL [7] described such an occurrence of coesite in Meteor Crater, Arizona. The quartz--coesite stability field has been evaluated by BOYD and ENGLAND [8] at high temperatures (see Fig. 1) but so far there have been no indications as to whether the natural occurrence of coesite results from high temperature or low. With the production of coesite in this series of experiments (carried out at low temperatures) some modifications may be made to the parameters which have been extrapolated
SHOCK DEFORMATION
OF QUARTZ
SAND
83
for the quartz-coesite stability field. Geological implications of this will be described more fully elsewhere. The material produced in the experiments was analysed both by standard optical and X-ray techniques. Because of the large difference in refractive indices between quartz and coesite (mean indices 1.550 and 1.595 respectively) it has been found relatively easy to
I
L I i ~ I [ I ~.1 ~ I i ~._ Liquid /~. I 1750 ~ _ / _ ~ ' "
J r ~lJ
I I
I '
I
t
I t
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/I
;/
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/
////
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Coesite
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/
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//'i ;/ / /
//' I
250 -
III ill
I0
20 Pressure,
30
Jl~[tl,I 40
50
Jl
f
k bars
FIG. 1. The quartz-coesite stability field.
distinguish the coesite optically. Figure 6 shows the coesite developed as a mass growing into a larger grain of quartz in what was probably an area of high stress distribution. 3.2(b) Fracturing. In the undeformed rock fracturing is very low. According to the method outlined by BORG et aL [9] the fracture index is of the order 130-140. The fractures are generally curved, being planar for only short distances and the amount of fracturing in any one grain is variable, but never exceeds two fractures. Table 1 below gives a comparison between the fracture indices of dry deformed sand and the H20 saturated deformed sand. It will be noticed that despite the greater stress concentration in the 'wet' samples fracturing is not increased significantly over the fracturing intensity in 'dry' samples. The reduction in fracturing intensity in the samples subjected to maximum stress (4 oz charge) can be explained by fracturing resulting in disaggregation of the grains (and thus reduction
84
C. C. FRYER TABLE1 Specimen number
Charge ( o z )
Fracture index dry
C0t~ C I tL Clh C~t ~-'2h C'Att C3h C4~ C4~
Undeformed
130-140 160
I 1
Fracture index wet
155
2 2 3 3 4 4
197 200
240
230
225
150
in the grain size) in the dry specimen, and the increasing importance of recrystallization as a means of accommodating the stress in the 'wet' saturated specimen. A statistical analysis was performed on the sample showing the highest degree of fracturing (sample C~a) to determine the angular relationship between the fracture plane and the optic axis in 100 grains. The results of the analysis are shown in the form of a histogram in Fig. 2. A slight preference is observed for fracturing to take place at right angles to C,,, or along the basal plane. 7~,
c
u
.
o_ .
©
]
L
r
]
J--]
I0
Angle of frQcture from C, FIG.
2
3.2(c) Undulatory extinction. This phenomenon results from bending of the optic axis possibly occurring as a cumulative effect of lattice dislocations. Undulatory extinction has been known for many years as a common occurrence in igneous and metamorphic rocks, although not in clastic quartz. The first definite indication of plastic flow was given by BA]LE¥ et al. [10] who showed that Laiie (X-ray) photographs of quartz displaying undulatory extinction commonly showed asterism and polygonization as compared with the sharp Laiie figures produced from non-undulose quartz. This indicates that undulatory extinction is the optical result of the effects of plastic deformation. In terms of dislocation theory, it has been postulated that the crystallites created in a deformed quartz by bend gliding and polygonization are inclined to each other at small angles. This must result in a relative displacement of the optic axes of the sub-grains, and
FIG. 3. The cell displayed in open form, and ready for detonation.
FIG. 4. The undeformed
facing page 84 R.M.
sand, plane polarized light.
FE. 5. The undeformed
sand. crossed nicols.
FIG. 6. Coesite, developing inwards from the extremity of a quartz crystal, crossed nicols.
FIG. 7. Quartz grain showing undulatory extinction : crossed nicols.
FIG. 8. Deformation
bands, crossed polarized light.
FIG..9. Deformation
lamellae, reflected light.
FIG. 10. BBhm lamellae, crossed nicols.
FIG. 1I. Recrystallization,
forming larger grain than original, crossed nicols.
SHOCK DEFORMATION OF QUARTZ SAND
85
the extinction position of the host crystal will therefore vary from one area to another, which will account for the wavy extinction noticed under the microscope. On the rotation of the microscope stage this extinction will sweep across the field of view if the quartz grain is orientated with the axis of bending vertical. For other orientations the sweeping effect will be noticeably less, and the variation in extinction angles will be smaller. Since the displacement of the crystallites is small, the bands of undulatory extinction will therefore be sub-parallel to each other. Departure from this relationship, however, will increase with increasing deformation. As the deformation increases in intensity the extinction bands become regular, narrow and pronounced. This has been considered to be a consequence of the greater concentration of the dislocations into walls normal to the bent glide planes, creating elongate polygonized zones, or crystallites. The phenomenon of undulatory extinction, as observed in the deformed specimens in these experiments is in line with that observed by other workers, such as FA1RBAIRN[11], INGERSONand TURTLE[12] and BAILEYet al. [10] who have noted that it occurs in rocks which are deformed but not recrystallized. Recrystallization which is more abundant in the saturated shocked specimens, will tend to form new, strain-free, quartz, which will only show undulatory extinction after a later phase of deformation. In recent experimental work, CARTER et al. [1] have differentiated two forms of undulatory extinction--undulatory extinction and deformation bands. This distinction is based on the radius of bending of the basal planes as revealed by variation in the extinction position in the boundary region between the reoriented zones. This is an arbitrary distinction, however, as there is a complete gradation between the features. Figures 7 and 8 show two extreme cases: in the former, undulatory extinction is widespread across one crystal; in the latter, deformation bands are developed in two sets, trending about 60 ° to each other. Deformation bands were only observed in the specimens here at the upper end of the shock-loading scale, but undulatory extinction was observed in most specimens. 3.2(d) Deformation lamellae. Many workers studying the deformation of quartz at high temperatures have remarked upon the occurrence of deformation lamellae. These are very thin, nearly planar lamellae in parallel sets. Unlike deformation bands they are not distinguishable from the surrounding host grain by virtue of variation in extinction position but they have a different refractive index to that of quartz in the host material. Thus, when in focus in plane-polarized light, they are noticeable by their brightness. Extinction in crosspolarized light is simultaneous with the host material. In some cases rows of inclusions may be observed which are sub-linear in direction. These are, in fact, lamellae containing many minute brownish inclusions--the so-called BOhm lamellae. The nature of the inclusions has never been determined optically due to the / smallness in size, but INGERSONand TUTTLE[12] have identified some as liquid, others may be gaseous. In the experiments carried out in the present work, deformation lamellae, (including B~hm lamellae) were not commonly observed. In the undeformed rock, not one grain was observed displaying these features. In the deformed specimens, only samples 3a, 4a and 4b (i.e. those subject to the highest degree of shock-loading) displayed deformation lamellae. The percentage of grains in these cases was 2, 4 and 3 respectively. Figures 9 and 10 show the development of simple deformation lamellae and BShm deformation lamellae in specimen 4a. The origin of deformation lamellae is uncertain, but CHRISTIEet aL [2] have shown that
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deformation features produced at high temperature and confining pressure with high shear stress on the basal plane are parallel to slip zones in quartz. These are also parallel to the basal plane. 3.2(d) Recrystallization. Syntectonic recrystallization has been produced in only one sample, 4b, deformed at the highest pressure and under H20 saturation. Figure 11 shows a typical development of a recrystallized grain. Some of the grains have crystallized entirely to a mosaic of small new grains, others are only partially recrystallized. In the former case, the mosaic is larger in size than the original undeformed material. Due to the smallness of the sample, however, sieve analysis on the deformed rock was not possible. The mechanism of recrystallization is not clear. There is little evidence of serrated edges to the crystals, a feature which is usually taken as the first sign of recrystallization. This would tend to dispose of the theory of diffusion which in any case partly depends upon the juxtaposition of crystals with differing lattice orientations [13]. Recrystallization does not tend to develop along any obvious preferential crystallographic plane. Thus for the time being, no explanation can be proffered for the mechanism. 4. CONCLUSION. DISCUSSION AND SIGNIFICANCE OF RESULTS The purpose of the work is primarily to evaluate the response of rocks to stresses which may affect them in the earth's crust, and, more particularly in engineering situations. Thus, more specifically, if the response of individual sand grains is known, then these results may be extrapolated to include consolidated sands which are frequently encountered as sandstones in engineering situations (and whose mechanisms of deformation and fracture are still largely unknown). The results may be tabulated briefly and are discussed below.
4.1 Formation of coesite The stability field of this comparatively rare polymorph of silica is not well known, especially in the low temperature (<500°C) field relevant to these experiments. Consequently, the phenomenon of its occurrence at low temperatures has important significance both to its artificial synthesis and its occurrence in natural environments such as meteoric craters. 4.2 Fracturing As might be expected, because of the relative freedom of individual grains, fracturing is very low in all the samples. Instead, grain rotation probably occurs, although it is impossible to distinguish this microscopically.l~Where fracturing is observed, there is a strong possibility that such grains were in close contact to the cell walls. No difference is observed in the fracturing intensity of 'wet' and 'dry' samples. In all possibility the grain packing is not dense enough to transmit the increased stress which occurs in the saturated samples. Where fracturing is observed, slight crystallographic control appears to be exerted indicating a preference for the basal plane. This is in contrast to findings of other workers who have noted a tendency for fractures to run along the prismatic direction. 4.3 Undulatory extinction This is the most pronounced new feature of the deformed sand. It has been shown that with increasing deformation undulatory extinction increases in intensity until narrow bands appear. This is the ultimate product seen of plastic deformation in quartz. A theory of
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dislocation has been put forward to account for the phenomenon. As could be expected, in samples which showed extensive recrystallization, undulatory extinction is absent, the very high stress concentration which in itself has caused undulatory extinction, has also caused a migration and reorganization of grain boundaries, possibly by a process akin to diffusion between adjacent crystallites to give rise to new, strain-free, quartz. It is significant to note that the intensity of undulatory extinction observed in the deformed sand is greater than any so far observed in natural sandstones and quartzites. Once again this must be attributed to recrystallization: in naturally occurring deformed rocks longterm 'creep' effects will tend to reduce the observed values of undulatory extinction, whilst in the experimentally deformed samples a 'strain-hardening' type of effect will increase the observed values. 4.4 Deformation lamellae The optical expression of basal slip in quartz crystals has not been frequently observed. From the results obtained by CARTER et aL [1] it is known that such deformation characteristics are most commonly produced at high temperatures (500-800°C), which, nevertheless, are still within the metamorphic environment. The present work has shown that at rapid loading rates at low temperature it is still possible to produce such features in isolated grains. 4.5 Recrystallization CARTER et al. [1] have noticed that the temperature at which quartz starts to recrystallize is dependent upon the rate of deformation. RALEIGH[14] has found that recrystallization will take place in flint at 500°C in experiments carried out for several days, whereas short (1 hr) experiments will not produce recrystallization at less than 900°C. Contrary to this are the results of the present work, where, with a very high loading rate, quartz has begun to recrystallize in the presence of water at very low temperatures. (The increase of temperature of the cell during deformation is negligible.) 5. SUMMARY OF CONCLUSIONS The major importance of this work is that all present-known deformation features in quartz have been produced in the same sample at very low temperature using a rapid strainrate shock-loading device. Important light is cast on the environments of deformation and recrystallization of quartz, and new parameters are indicated for the quartz-coesite stability field. The inferences of these results are important in geological and engineering environments. Acknowledgements--This work was carried out in the Postgraduate Schoolof Mining, Universityof Sheffield' under the guidance of the Director, Dr. A. ROBERTS. Valuable discussionhas taken place with D. HASLAM,who has also assisted in the experimental work. REFERENCES 1. CARTER N. L., CHRISTIE J. M. and GRIGGS D. T. Experimental deformation and recrystallization of quartz, J. Geol. 72, 687-733 (1964). 2. CHRISTIEJ. M., GRIGGS D, T. and CARTERN. L. Experimental evidence of basal slip in quartz, Jr. Geol. 72, 734-756 (1964). 3. GORANSONR. W. in COOK M. A. The Science o f High Explosives, p. 111, Reinhold, N.Y. (1958). 4. FRYER C. C. Impregnation of loose and poorly consolidated material, Trans. BrL Ceram. Soe. 65 103109 (1966).
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5. PETTIJOHNF. J. The Sedimentary Rocks, Harper, N.Y. (1949). 6. COESL. JR. A new dense crystalline silica, Science 118, 131-132 (1953). 7. CHAO E. C. T., SHOEMAKERE. M. and MADSENB. M. First natural occurrence of coesite, Science 132, 220-222 (1960). 8. BOYD F. R. and ENGLANDJ. L. The quartz--coesite transition, J. geophys. Res. 65, 749-756 (1960). 9. BORG I., FRIEDMAN M., ]-IANDINJ. and H1GGS D. V. Experimental deformation of St. Peter Sand: study of cataclastic flow, Geological Society of America, Memo. 79, Chap. VI, 133-191 (1960). 10. BAILEYS. W., BELL R. A. and PENG C. J. Plastic deformation of quartz in nature, Bull. geol. Soc. Am. 69, 1443-1466 (1958). 11. FAIRBAIRNH. W. Deformation lamellae in quartz from the Ajibik formation, Michigan, Bull. geol. Soc. Am. 52, 1265-1278 (1941). 12. INGERSONE. and TUTTLEO. F. Relations of lamellae and crystallography of quartz and fabric directions of some deformed rocks, Trans. Am. geophys. Un. 26, 95-105 (1945). 13. COTTRELLA. H. Dislocations andPlastic Flow in Crystals, O.U.P. (1953). 14. RALEIGHC. B. Crystallization and recrystallization of quartz in a simple piston-cylinder device, J. Geol. 73, 369-377 (1965).