Textures Formed due to Deformation -- Twinning, Lattice bending, Kinking, Brecciation and cataclasis

Many ores contain evidence of deformation which ranges from minor pressure-induced twinning to complete cataclasis.

Deformation in minerals is of two types - DUCTILE FAILURE (slip, twinning, bending and kinking) and BRITTLE FAILURE (shear cleavage, irregular fracture).

The degree of response of minerals and preservation of deformational effects depends upon a) the mineral, b) the rate of strain, c) nature of deformation, d) the associated minerals, e) temperature at the time of deformation and f) post-depositional history.

The response threshold of minerals appears to be a primary function of hardness.

Thus native metals, sulphosalts, copper and silver sulfides deform most readily.

Cu-Fe sulfides and monosulfides less readily.

And disulfides, oxides and arsenides least readily.

Therefore in polymetallic ores deformation textures are often evident only in some minerals (the softer ones) but these also recrystallize most easily so that the deformational textures are obliterated.

Specific deformation textures commonly observed include:

 

TWINNING:

Deformation twins commonly occur as uniformly thick lamellae commonly associated with bending, cataclasis and incipient recrystallization.

Defromation twin lamellae commonly pass through adjacent grains.

  NOTE: rough treatment of specimens, eg hammering, grinding and careless polishing may induce local twinning in some soft minerals.

Another effect of deformation is the curvature or offset of linear or planar features eg cleavage, fractures, twins, crystal faces, mineral layering, veins, or exsolution lamellae.

 

LATTICE BENDING:

Deformation may not always be as simple and regular as that reflected in twinning.

Heterogeneous straining manifests in the form of bent cleavages, undulose pleochroism and undulose extinction.

 

KINKING:

 When a specimen is uniformly compressed it may yield by kinking rather than by twinning.

Such kinking occurrs when the plane of weakness (slip or twin) is only at a small angle to the axis of compression.

The kinking develops as a well defined band, the margins of which are usually nearly, but not quite parallel, and whose length is at a high angle (nearly 90o) to the axis of compression.

 

CRITERIA FOR DIFFERENTIATING DIFFERENT TYPES OF TWINNING

Twinning may occur in minerals during initial growth,during structural inversion on cooling (see transformation textures) or as a result of deformation.  Though quantitative studies are yet to be made, the following criteria is suggested for distinguishing between these types:

Growth: Occurrs as lamellar twins of irregular width that    are unevenly distributed, present only in some grains and may be strongly interwoven. 

Inversion: Commonly occurs as spindle-shaped and intergrow networks not parallel throughout grains.

Deformation: Occurrs as uniformly thick lamellae, commonly associated with bending, cataclasis, and incipient recrystallization (regions of very small equant grains) lamellae often pass through adjacent grains.

BRECCIATION & CATACLASIS:

Brittle hard minerals, at particularly lower temperatures, offer resistance to slip and have a greater tendency to yield by brittle fracture rather than ductile failure by slip.

A similar tendency develops with increase in the amount of impurity.

The amount of brecciation may depend both on the degree of deformation and the mineralogy of the ore.

Moderate deformation may result in complete brecciation of massive pyrite, chromite or magnetite.

On the other hand pyrite with pyrrhotite or chalcopyrite may suffer little even under extreme deformation.

Minor brecciation grades into complex cataclasis leading to complete randomness of fragment orientation and development of "Ball Textures" in which fragments of foliated gangue are rounded into balls.

Injection of softer minerals into fractures and cleavages in moer brittle ore and gangue minerals are other common effects of deformation.

 

TEXTURES FORMED DUE TO ANNEALING:

These textures include effects of slow cooling or slow heating of ores during metamorphism.

Since cooling and metamorphism are both prolonged annealing processes, the effects discussed here must include those discussed under transformation textures.

TEXTURES FORMED DUE TO DEFORMATION

 

- Large areas of metamorphosed terrain, eg. the Precambrian shields, contain cross-cutting and stratiform deposits in regionally metamorphosed rocks.

- It is logical that at least some of these existed prior to metamorphism, even though they now appear to be normal replacements in foliated rocks.

- The ease with which high temperature sulfide phases revert to low temperature phases on cooling, confirm that metamorphism is not reflected in sulfide mineralogical assemblages.

- Nevertheless, Many ores contain evidence of deformation which ranges from minor pressure-induced twinning to complete cataclasis.

- Deformation in minerals is of two types - DUCTILE FAILURE (slip, twinning, bending and kinking) and BRITTLE FAILURE (shear cleavage, irregular fracture).

- The degree to which individual minerals respond to and preserve deformational effects depends upon:

1. The nature of mineral

2. The rate of strain

3. The nature of deformation

4. The associated minerals

5. Temperature at the time of deformation

6. Post-deformational history

- The response threshold of minerals appears to be a primary function of hardness, hence:

a) Minerals such as native metals, sulfosalts, Cu- and Ag-sulfides deform most readily

b) Cu-Fe sulfides deform less readily, and

c) Disulfides, oxides and arsenides deform least easily.

- Accordingly, in polymetallic aggregates, deformation textures are seen in only some minerals.

- The softer minerals deform most easily, but they also recrystallize most easily, so that deformational effects are obliterated before those in the more refractory ones.

- Specific deformational textures commonly observed include:

1. Twinning, kinkbanding, and pressure lamellae

2. Curvature or offset of linear features

3. Schlieren

4. Brecciation or cataclasis

1. TWINNING:

- These features occur in ores subjected to deformation, or artificially induced in some softer minerals by rough treatment.

- Deformational twinning occurrs as uniformly thick lamellae, commonly associated with bending, cataclasis and incipient recrystallization. The lamellae pass through adjacent grains.

- This is different from growth and inversion twinning.

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CRITERIA FOR DIFFERENTIATING DIFFERENT TYPES OF TWINNING

Twinning may occur in minerals during initial growth,

during structural inversion on cooling (see transformation

textures) or as a result of deformation. Though

quantitative studies are yet to be made, the following

criteria is suggested for distinguishing between these

types:

Growth: Occurrs as lamellar twins of irregular width that

are unevenly distributed, present only in some grains

and may be strongly interwoven.

Inversion: Commonly occurs as spindle-shaped and intergrow

networks not parallel throughout grains.

Deformation: Occurrs as uniformly thick lamellae, commonly

associated with bending, cataclasis, and incipient

recrystallization (regions of very small equant grains)

lamellae often pass through adjacent grains.

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2. KINK BANDING:

- Deformation in pyrrhotite may be as kinkbands (but subparallel lamellae each of which show undulose extinction or twinning).

- At < 2 Kb kinkbands predominate below *p 247* 300oC. Above this temperature both twinning and kinkbanding are common.

- In some minerals, particularly stibnite and pyrargyrite, deformation may give rise to two or more sets of parallel, spindle shaped twin lamellae.

- These twin lamellae are inclined to one another, and to the long axis of the deformed crystal. These exhibit either undulatory extinction or slightly different extinction positions and are called PRESSURE LAMELLAE.

 

2. LATTICE BENDING:

- Deformation of ores is often evidenced by the curvature or offset of linear or planar features, eg:

a) Crystal faces b) Cleavages

c) Fractures d) Twin Lamellae

e) Exsolution lamellae f) Primary mineral layering or veining

- Deformation induced twin lamellae in pyrrhotite, ilmenite, chalcopyrite etc exhibit such curvature which extends across several grains.

- Exsolution intergrowths of cubanite in chalcopyrite, ilmenite in hematite, chalcopyrite in sphalerite, pentlandite in pyrrhotite and bornite in chalcopyrite are often bent during deformation.

- Folding or offset of primary mineral banding is common on micro and macro scales.

- Some hard minerals yield by brittle fracture during folding. Such fractures are often filled by later generations of ore or gangue minerals.

4. BRECCIATION AND CATACLASIS:

- Deformed ores often contain zones along which shearing has occurred.

- In such zones known as "schlieren" the ore minerals may be pulverized and smeared out parallel to the direction of shearing movement.

- Schlieren are usually planar features in which the ores are very fine grained relative to the surrounding rocks.

- Deformation is often evidenced by fracturing and brecciation of harder and more brittle minerals such as pyrite, chromite, magnetite, etc.

- The a,ount of brecciation depends upon the degree of deformation and mineralogy of the ores.

- Thus moderate deformation will result in considerable brecciation of pyrite, magnetite or chromite.

- In contrast, pyrite admixed with pyrrhotite or chalcopyrite suffers little even under extreme deformation because the strain is taken up by the softer sulfides.

- Minor brecciation grades into complete cataclasis with an increasing degree of fragmentation and disorientation.

- This penetrative deformation has been termed "durchbewegung" (= move through).

- In fault zones and in ores that have suffered penetrative high grade matamorphism there is:

a) Pulverization of ore and gangue minerals

b) Complete randomness of fragment orientation

c) Development of "ball textures" in which fragments of foliated gangue are rolled into balls

d) Injection of softer ore minerals into fractures and cleavages in more brittle ore and gangue minerals.

 

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