Kinks in mica: Role of dislocations and (001) cleavage

49

‘I~ctonoph_vsics. 127 (1986) 49-65 Eisevier

Science

Publishers

B.V.. Amsterdam

- Printed

in The Netherlands

KINKS IN MICA: ROLE OF DISLOCATIONS

(Received

June 24, 1985; revised version accepted

December

AND (001) CLEAVAGE

13, 1985)

ABSTRACT Bell. I.A.. Wilson, C.J.L., McLaren, and (001) cleavage. Kinking

in natural

transmission kink band

electron

(KBBs).

Gentle

where (001) planes

between

active slip planes together

dilation

occur

observed

deformed

biotite

and

(TEM). It is found that complex

process;

parallel

M.A., 1986. Kinks in mica: Role of dislocations

127: 49-65.

and experimentally microscopy

boundaries

A.C. and Etheridge,

Tectonophysirs,

bending

of (001) planes

are bent several degrees,

plays

KBBs which may be applicable

and

the KBB. A model

to other crystalline

has been

in complex

wails

to (001). Microfracturing

and

is presented

materials

by

role in the kinking

are observed

parallel

investigated

exist in the vicinity of

an important

edge dislocations

with broad bands of dislocations

to (001) planes

muscovite

mjcrostructures

to explain

the origin

of

with only one active slip plane.

INTRODUCTION

The purpose in the bending

of this paper is to examine and

kinking

of natural

the role of dislocations and experimentally

and (001) cleavage

deformed

biotite

and

muscovite, using trans~ssion electron microscope (TEM) observations. The samples examined in this study originated from large single crystals of biotite obtained from schists

adjacent

to the pegmatites

that occur at Mica Creek,

Mount

Isa, Australia

(Wilson, 1972). Kinking in micas is believed to be a result of slip parallel to (001). Slip directions parallel to [lOO] and (110) were predicted from “percussion” or “pressure” figures on (001) by Miigge (1898). The studies of Borg and Handin (1966) and Etheridge et al. (19’73), confirmed the slip directions to be [loo] and (110), assuming that kinking was caused by slip and that the direction of slip was normal to the kink axis. Crystallographic models to account for the angular characteristics of kink bands

* Present address:

Research

School of Earth Sciences, Australian

2600. Australia.

BOO-19~1/86/~03.50

8 1986 Elsevier Science Publishers

B.V.

National

University,

Canberra.

A.C.T.

50

(KBs)

micas

in

have been

(1983). These models ships across a sharp that sharp continuous

band

to deformation understood.

kink band boundary

(McLaren

mechanisms,

(1968) and

and involve

(KBB).

Baronnet

and

near coincidence

However,

Olives

relation-

our investigations

scale may be characterised

by a more

show gradual.

on the TEM scale and may be referred to as a and Etheridge, 1980). Despite its obvious importance the detailed structure of KBBs in micas is poorly

OBSERVATIONS

The as-collected are bounded

Fig;1.

by Starkey

ideal geometry

KBBs on the optical change in orientation

deformation

OPTICAL

proposed

assume

by regions

0pd& -graphs

Experimentally experimentally

natural

deformed deformed

by TEM. For explanation

micas contain

isolated

a few millimeters

of KBs cut normal biotite

sample

see text.

to (001). The KBBs

wide over which the (001) cleavage

to the KBB and (001) planes.

compressed

biotite single crystal,

KBBs normal

parallel

A. Natural

to [310]. C. Muscovite

planes

biotite.

inclusion

sample 830. Areas 1, 2 and 3 have been examined

B.

in an in detail

51

are curved range

boundary from

through

from

4-6”

10 to 40”.

and additional

undeformed

deformed 3kbar

areas

in the apparatus

and strain

microstructure

(Fig. The

1A). The angles largest

planar

rotation

boundaries

within

single

described

of rotation of (~1)

(Fig. 1B) was examined

across

occur at a. Cylindrical

biotite

crystals

by Paterson

rates of approximately

of (001) across

occurs

lop4

s-’

by

cores were taken

Etheridge

(1971)

(1970) at confining to produce

in detail by Etheridge

KBBs

the central and

pressures

of

kinks. The resulting

et al. (1973).

Plate-like muscovite inclusions intergrown with the natural biotite are common. The muscovite inclusion in an experimentally deformed sample (Fig. 1C) shows optically continuous bending at BI and a KBB the surrounding biotite. Bl is an anticlockwise anticlockwise bending (area 2), whereas KB2 surrounded by anticlockwise bending (area 3).

at KB2, mimicking the structure in (area 1) bend of (001) bounded by is a clockwise kink (26”) of (001) Bending of (001) near Bl tends to

broaden this boundary, whereas at KB2 the kinking and bending are in the opposite sense and there is a reduction of the width. In order to correlate the crystal defects of Bl

and

KB2 with

ion-beam

thinned

the optical

observations,

and re-examined

the sample

in the electron

has been

progressively

microscope.

TEM OBSERVATIONS

Three samples showing well-developed KBBs were selected and examined by TEM at 200 kV: a natural biotite and two experimental samples, 830 (shortened 20% parallel to (310) at 3OO’C) and 839 (shortened 20% parallel to (001) at 600°C). Muscovite inclusions have been found in the deformed sample 830 (Fig, 1C). Radiation damage in the electron beam occurs in both biotite and muscovite. However, it is significantly less in the muscovite and this has made it possible examine the crystal defects associated with the KBBs in this mineral.

to

Natural biotite The natural biotite has a 2M, structure and has been cut so that the few naturally induced KBBs were normal to the specimen foil. Several (001) foils were also prepared and observed in the electron microscope; to minimise generation of dislocations

during

preparation

of samples

by cleavage

techniques,

very thick foils

f - 100 pm) were cleaved and then ion-beam thinned. Rapid radiation damage in the microscope of the biotites precludes informative bright field (BF) or dark field (DF) micrographs

and most of the results have been deduced

from diffraction

photos

or

observations of shapes and sizes of KBBs. Similar microstructures exist both within and outside a kink band. There was a low to medium density (approximately 15 pm-‘) of stacking faults on (001) with the fault vectors: R 1.4 = i f]OIOl;

R,., = & 6[310] or R,,

= t- +[3iO]

Fig. 2. BF micrographs

of natural

section

dislocations.

showing

parallel

biotite.

A. An area within

a KB. B. KBB at h (Fig. 1A). C. A (001)

53

(Bell and

Wilson,

1977, 1981). No twins

occur outside

the KB, whereas within

were common

(Fig. 2A).

were observed.

the KBs dislocation

Occasional densities

dislocations

of p = 10’ cm-’

KBBs that are sharp on the scale of an optical micrograph (area b Fig. 1A) have a marked curvature of (001) on the TEM scale (Fig. 2B). The biotite has been divided into slabs (less than 0.3 pm wide) by cleavage along (001). Within of about curvature

the KBB, rotation

40” takes place across a region up to 0.2 pm wide, within which the is fairly constant. These regions of (001) rotation are V-shaped, with each

thin slab apparently having deformed and other areas are missing. Observations

of (001) sections

independently.

Parts of the KBB have cracked

(Fig. 2C) show two sets of pnrailel,

equally

spaced

(0.8 pm), dislocation images (p = 10’ cme2). Rapid degradation of the contrast by radiation damage made a Burgers vector analysis impossible. The majority of these dislocation

images are parallel

to the trace of the KBB which is [310]. that is, parallel

to the rotation axis of the kink. Assuming these dislocations are pure edge and are causing the bending of (001) adjacent to the KB shown in Fig. 1A. their Burgers vector is [llO]. The dislocations and may contribute

of the second

to the asymmetric

rotation

In areas between KBBs low densities of together with occasional partial ( p in Fig. 3A) similar to that in the as-collected biotites. bending or other deformation features in areas

set are approximately

15” from [310]

of (001) across a KBB.

stacking faults are found (Fig, 3A). and unit dislocations. Fault density is There are no dislocation networks, between KBs (Fig. 38).

BF micrographs (Fig. 3B, 4) of the KBBs generally show a gentle curvature of the (001) planes and V or Y-shaped areas. Diffraction patterns at KBBs (Figs. 3C-E) show intermediate orientations between the two extremes. Intermediate orientations are as small as 1’. They are closer to the KB orientation and

may not be observed

continuous

curvature

in the BF micrograph.

than the host orientation

Streaking

of the arcs is due to

of (001).

At most KBBs, tilting experiments show that the rotation axis of the kink boundary is contained in the (001) plane and is normal to the original compression direction. Occasionally, the kink is not a simple rotation of (001): as well as the rotation, a small (usually less than 5”) twist of (001) has occurred. Cleavage of (001) planes is associated with all KBBs (Fig. 4). This

cleavage

divides the biotite into curved slabs (of the order of 0.1 pm wide) and creates boomerang-shaped openings (Fig. 4A). These openings are commonly not symmetrical about the KBB and their curvature is not always continuous. In Fig. 4B, there are lineations at a high angle to (001) and stacking faults on either side of the KBB. These lineations may be walls of edge dislocations viewed edge-on. Fractures also occur parallel to KBBs (Fig. 4C) and are in part responsible for the Y-shaped

A

F

0.5 urn

Fig. 4. BF micrographs biotite.

of some common

A. (001) cleavage.

B. Lineations

features

normal

associated

with KBBs in experimentally

deformed

to (001) in the lower sectton of B. C. Cracking

parallel

to

the KBB.

(Fig. 4B). There is the possibility that some cleavage and fracturing occurred the sample preparation. Even so, these cleavages and fractures still highlight

regions

during

areas of crystal

weakness.

Experimentally deformed ~~~cou~~einclusions

These inclusions have the polytype structure (2M,) and the crystallographic orientation of the host biotite. The most common defects in the muscovites are dislocations, in association with stacking faults (Fig. 5) and cracks on (001) cleavage planes. Dark-field images and diffraction patterns (Fig. 5A-.D) recorded with increasing

tilt from the muscovite

inclusion

(Fig. 1C) show that the stacking

faults

and dislocations lie on the (001) plane (Fig. 5B-D). A series of micrographs taken under weak-beam conditions (Fig. 6) correspond with the features illustrated in Fig. 1C and are described

below.

Fig. 3. TEM microstructures within

associated

with experimentally

deformed

a KB. B. BF of KBBs (Bl and B2) with radii of curvature

patterns

from the host orientation

rows of 001 reflections, (corresponding

to the most

mediate

orientation

discrete

reflections,

reflections distinguished

spread

(H) and kink boundaries

indicating

three discrete

widely

is 19O from

an angle

due to streaking.

reflections)

exist between

In D, there are three

these are the host and

and an intermediate

KB orientations

orientation. respectively.

in the rest of the arc,

the host and KB orientations

area

Diffraction

The interIn E, many

the host and KB. These are approximatety

of 15”. However,

The angle between

A. DF of a typical

Bl and B2 respectively.

orientations;

the host and 4O from the KB orientations,

hence orientations, through

separated

biotite.

of less than 2 pm. C-E.

no reflections is 20’.

ten 008 can

he

hhl: ZM,

A

Fig. 5. DF micrographs muscovite.

Using

diffraction

patterns.

image widths

and diffraction

at various

tilts (T)

of an experimentally

are O”, l.S”, 6.5” and 23.5”, respectively.

A with inset of C. shows that the tilt axis is aligned parallel

for dislocations.

the same position

patterns

,q = 111 the tilts in A-D planar

defects

and faults increase

for increasing

in C and D. B. C and D have the same magnification.

deformed

A comparison

of

to the trace of (001). The tilt. Note that X indicates

5

S~~struc~~res in area 1 of Fig.

uniformly

distributed

6B there appear

IC.

Figure

6A shows

across the field of view, with a density

to have been individual

dislocations of 1

(001) planes containing

x

relatively

lo9 crne2. In Fig.

an active dislocation

source. On the active slip plane (S) the spacing between adjacent dislocations reasonably constant, suggesting that the dislocations cannot be piled up against object

outside

the field of view. In the active

slip plane

(X),

the dislocations

is an are

approximately parallel to the foil surface and are interacting strongly. The interaction can be seen as zig-zags of the dislocation Iine images and a network has developed.

This network,

Fig. 7. DF micrographs indicates

of dislocation

the same dislocation

some parallel

although

to U? = (310).

highly

structures

tilted,

is similar

in area 2 (see Fig. 1C). A. g=lll.

in B and C. Most of the dislocations A-C

and D-E

in appearance

are parallel

have the same magnification.

B-E,

to those

g = 331. Z

to U, = [IlO] but there are

59

networks

developed

slip planes slip planes, spacing

on (001) in cleaved samples

are separated

by 1 to 3 pm. Between

there is a dislocation

of dislocations

less well-defined 2 x 10” cm-‘.

In a nearby

1961). The active

the two most widely spaced

active

wall (W’,) at a high angle to (001) in which the

is approximately

dislocation

(Silk and Barnes,

wall (W,).

0.25 pm. About Dislocation

2 pm to the right there is a

density

in this micrograph

area closer to Bl (Fig. 6C) most of the dislocations

is are

associated with low-angle dislocation boundaries. A comparison of these micrographs clearly shows a transition from uniformly distributed dislocations to a more ordered dislocation structure comprising walls and active slip planes as the KBB is approached. Substructures at Bl and area 2 at Fig. IC. Figure 6D shows the dislocation structures found at the optically observed 10” bend boundary (Bl). The dislocation walls are well developed, but are no longer simple low-angle boundaries. For example, the walls labelled X are quite complex in that the dislocations pile-up against the wall. Most of the dislocations are either confined in walls or broad bands running normal to the walls, that is, parallel to (001). Figure 6E shows a broad KB containing

a high density

Further low-angle

across boundaries

of uniformly

the bend

(Fig. 7A-C)

distributed

dislocations.

the dislocation

and an increase

Fig. 8. A is a composite

of DF micrographs

There is a 9” rotation

walls

in dislocation

again

form

density

typical

is observed

cl

c

8

area& indicated.

boundary

of the Bl boundary.

B-D

of (001) across this boundary.

are diffraction

patterns

from the

60

between

the walls. Active

from the planar

faults are in contrast

wall (Fig. 7A). Figures but

at slightly

different

Fig. 9. A BF micrograph has cracked

(001) slip planes

are common.

Fringes

and can be seen to bend across the dislocation

7B and C are of the same area (note the disIocations tilts of the specimen.

of the KB 2 boundary

and the diffraction

1 to 2 pm apart

pattern

indicates

with

The dislocation

its corresponding

diffraction

that there are several discrete,

images

pattern. intermediate

at 2). at T are

The boundary orientations.

61

probably

parallel

to (310).

Just adjacent

tend to form walls (Figs. 7D-E). Figs. 6 and 7 indicate the reflecting

planes

axis is approximately

to KB 2, in area 2. the dislocations

The contrast

a misorientatioil

changes

across the dislocation

of the reflecting

are g = hhf where 1 varies between parallel

planes.

In these micrographs,

+ 3 and

to the trace of (001). The change

< 10’. Figure 8 is a mosaic across the bend boundary.

again walls in

- 3. Thus, the tilt of orientation

is

Bl. There are five well-devel-

oped dislocation walls giving rise to obvious contrast changes. The inset diffraction patterns are from the areas indicated and show that there is a 9” rotation of (001)

Fig. 10. A and B are DF micrographs u = (110).

of area 3. The dislocations

are scattered

and few dislocations

have

across the field of view (8 pm). Each dislocation about

2’. Similar boundaries

surrounding

biotite

grains.

wall produces

a misorientation

have been traced from the muscovite However,

no information

into the

on the dislocations

in the

biotites could be obtained due to rapid radiation damage. ~ubstru~tu~~ in KK? and area 3 uf Fig. t C. KB 2, which

is a distinct

boundary

is a 24” orientation

in the optical

study,

is illustrated

of

inclusion

in Fig. 9. There

planar

change of (001) and several discrete intermediate orientations in the corresponding diffraction pattern. The boundary is not planar but is off-set across cracks that are paraliel to the KBB and cleavages. Note the staggering of the lower part of the boundary. The associated diffraction patterns indicate a small twist of (001) has occurred at KB 2. Away from KB 2, in area 3, the dislocations tend to form walls (Fig. 10A). Dislocation images are only approximately parallel to [llO]. Some stacking

faults

and isolated

dislocations

are found

further

away from KB 2 (Fig.

10B). DISCUSSION

A general theory of kinking in crystalline material was developed by Frank and Stroh (1952) on the basis that a pair of KBBs is composed of simple walls of dislocations of opposite sign. Neither the natural nor experimental KBBs observed in the micas are simple dislocation walls or single arrays of dislocations as suggested by Starkey (1968). Nor do the KBBs involve simple coincidence lattice relationships as required by the model of Baronnet and Olives (1983). Instead, they are complex boundaries governed by varying (001) configurations and associated microfractures in a finite zone, which Etheridge et al. (1973) determined, from optical measurements,

to have widths of the order of 1 to 2 pm. Their observations

this TEM study. Other features (1) discrete

intermediate

(2) complex

dislocation

active slip planes containing

observed

orientations

are confirmed

by

include: within

walls of irregular

the KBs;

spacing

(Figs. 6-7)

between

which are

dislocations;

(3) higher dislocation densities within a KB than outside (Fig. 2A); (4) Y and boomerang-shaped openings between (001) planes produced cleavage; (5) regions of gentle (001) plane curvature related to the distribution cation walls and cracks subnormal to (001); (6) asymmetry of KBs related to the distribution of dislocations.

by (001) of dislo-

On the basis of these observations a model for the evolution of deformation bands or KBs in mica is suggested (Fig. 11). Initial bending of the lattice appears to involve slip solely on (001) with the generation of dislocations that are capable of producing dislocation walls. At low strains, relatively few slip planes are active and walls are rare, presumably due to the inhibition of climb in material with such a low stacking fault energy (Etheridge and Hobbs, 1974). The active slip planes divide the

63

1-I

l------K8

Fig. 11. A model

to explain

(001).

bending

A. Elastic

dislocation

density

dislocation

and

the origin

formation

walls and secondary

D. Creation

of dilatation

of TEM microstructures

of an elongated

crystal

of dislocation

parallel walls

that

associated to active

with shortening

slip plane

become

(001).

KB 1. C. Creation

KBs (KB 2) within the region of lattice misorientation

openings

along (001) cleavage

planes

parallel

that bound

to

B. increased of further

confined

to KB 1.

slabs with a constant

width

(d).

crystal

into

increases,

slabs

and

relieve

stress in the immediately

the elastic limit of material

between

adjacent

the slip planes is approached,

a necessity for more active slip planes and giving rise to dislocation Further bending is accommodated by creating more complicated walls between

areas.

the first formed walls that define the original

As strain creating

walls (Fig. 11 B). and additional

zone of lattice bending

(Fig. 11C). Arrays of active slip planes and tilt walls divide the crystal into blocks. As the relative rotation between these blocks increases, dislocation densities in the arrays increases. Eventually cleavages along (001) and, to a lesser extent, along KBBs will be energetically favoured over a further increase in dislocation density. This creates the boomerang-shaped dilatation openings at KBBs. It is clear from this study that what may be considered a simple KBB on an optical scale is in fact quite complex. These observations in mica are compatible with results described from other minerals where kinking is a consequence of slip on one glide plane. For instance, McLaren and Etheridge (1980) have demonstrated that optically distinct KBBs in orthopyroxene are also complex dislocation arrays. The presence of microfracturing in association with kinking has also been described in ice by Wilson et al. (1986), in other micas by Wilson and Bell (1979) and its occurrence

in orthopyroxene

is suggested

by the secondary

alteration

along

KBBs

64

(McLaren metals

and Etheridge,

(Fraser

1980). Where secondary

et al., 1973)

KBBs are produced

glide planes without

are activated,

e.g. in

the need to fracture

the

material. CONCLUSIONS

TEN observations of kinking in 2M, micas suggest that KBB morphologies do not conform to simple tilt boundaries involving the pile-up of dislocations belonging to a simple slip system, as in the models of McLaren and Etheridge (1980). nor do they fit the ideal geometrical models of Starkey (1968) or Baronnet and Ohves (1983). Instead, the KBB regions in these micas consist of a complex combination of (001) plane curvature, small highly deformed regions containing many dislocations and sometimes high stacking fault density, together with dilation by (001) cleavage. Fractures subparallel to KBBs also occur. Stacking faults are common in biotite, but few dislocation networks were observed. However, in the muscovites dislocation arrays occur in active slip planes and form tilt (with minor twist) walls. This confirms the conclusion of Bell and Wilson (1981) that there are significant differences in the way the two micas deform. Slip is restricted to (001) in both micas, but radiation damage prevents the direct observation of dislocations in the experimentally deformed biotite. In muscovite broad regions of bending (observed optically) consist of dislocation vvalls normal to the trace of (001) planes, with individual walls rotating (001) by up to 2”. Dislocation arrays also occur in active slip planes, where they range from widely-spaced, non-interacting configurations to well-developed networks. ACKNOWLEDGEMENTS

We wish to thank M.S. Paterson

for many stimulating

discussions

on the origin of

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