Biaxial calcite: Occurrence,optics, and associated minor strain phenomena

Biaxiality in calcite (2 V10°, less commonly 15°) is widespread in metamorphic rocks of all grades and in carbonatites. It is a minor effect of late strain, in many cases not directly related to visible {01¯12} twinning, and partially independent of lattice bending. For values of 2V up to about 15° the optic axial plane coincides, within limits of error of measurement, with {10¯10}. For those rare crystals with 2V in the range 25–50°, there is a real departure—never greater than 10°—from this orientation; and biaxiality is certainly in some, probably in all such cases, confined to domains of overlap of single thin twin lamellae.Microscopic procedure is described for determining the orientation of the optical indicatrix relative to crystallographic coordinates.     

Biaxiality in relation to visible twinning in experimentally deformed calcite

Moderate to strong biaxiality (2V = 10 °–45 °) in experimentally deformed calcite (in single crystals and in marble) is attributed to overlap between one or two thin {01¯12} twin lamellae and the enclosing host. A perfectly centered conoscopic figure (section normal to [0001]) is perceptibly asymmetric about the trace of the optic axial plane. This asymmetry is pronouned in thick sections (> 0.04 mm) and completely distrupts the biaxial configuration of the figure if the overlapping lamella exceeds about 0.0025 mm in thickness. In sections somewhat oblique to [0001] and cut at 20 ° or less to the plane of twinning the conoscopic figure may appear to be perfectly biaxial-expecially in thin sections ( 0.02 mm) enclosing thin ( 0.001 mm) but still visible twins.Similar values of 2V recorded for natural calcite likewise are attributed to twinning on a visible scale.     

Deformation twinning in calcite,dolomite, and other rhombohedral carbonates

Transmission electron microscope (tem) observations of single and multiple twins in calcite and dolomite are presented, and the results are analysed by means of selected area diffraction and trace analysis. Simple twinning in rhodochrosite and kutnahorite is also analysed. It is shown that the ordered carbonates, such as dolomite, have a common twinning plane {01 \(\bar 1\) 2} and this appears to be their only mode of deformation twinning. The carbonates with higher symmetry, such as calcite, have {01 \(\bar 1\) 8} as the primary twinning plane but calcite itself has other twinning mechanisms, of which the most important is illustrated. Crossing and stopping twins are also discussed. It is shown that twinning in calcite, which occurs predominantly at low temperatures, is characterized by the generation of large numbers of glide dislocations.     

Mechanical twinning in calcite considered with the concept of ferroelasticity

The mechanical twinning of calcite is compared with ferroelastic behavior. For calcite a paraelastic prototype phase does not exist, therefore a virtual prototype is defined with cubic m3m symmetry. Using this condition the strain tensors of all domain states of mechanical twinning are calculated. With the use of the strain tensors, application of Sapriel's strain compatibility law gives the crystallographic orientations of all possible twin planes between different domain states. The findings indicate that the twin walls are {1 0 0} and {1 1 0} planes with respect to the morphological rhombohedral lattice. These twin plane orientations from the equated strain tensors are in accordance with the r-and e-twin systems commonly observed in calcite. Received: 17 December 1998 / Revised, accepted: 24 April 1999     

Deformation twinning and residual stress in calcite studied with synchrotron polychromatic X-ray microdiffraction

Microstructures of deformed calcite in marble from the Bergell Alps are studied by using a microfocused polychromatic synchrotron X-ray beam. The high spatial resolution, together with orientation and strain resolutions, reveals twin plane orientation, multiple twin lamellae, and strain distributions associated with the twins. Single and multiple mechanical twins on e = { 01[`1] 8 } e = \left\{ {01\overline{1} 8} \right\} systems are confirmed. Residual stresses are derived from the strain tensor that is derived from Laue diffraction patterns. Average lattice strains from several hundred to over one thousand microstrains are detected in a deformed marble from the Bergell Alps. Such strains suggest 60–120 MPa residual stresses. A detailed study of strain components shows that shear stresses on twin planes are completely released.     

The effect of Dauphiné twinning on plastic strain in quartz

We present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies (T = 300–400°C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphiné twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal to the compression direction (z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction (r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r} planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphiné twins induced a switch in the dominant active slip systems during plastic deformation, from basal <a> (regions without twinning) to {π} and {π′} <a> (pervasively twinned regions). Dynamic recrystallization is localized along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphiné twinning could have a potential use as palaeopiezometer in quartz-bearing rocks.     

Verformung von Hornblende-Einkristallen bei Drucken bis 21kb

Zusammenfassung Hornblende-Einkristallzylinder (Fundort: Luckow, Böhmen) verschiedener Orientierung wurden in einer Stempel-Zylinder Hochdruck-Apparatur bei Drucken bis 21 kb und 20° C verformt. Die Versuche haben gezeigt, daß an Hornblenden durch Anwendung von Drucken >10 kb kristallographisch indizierbare Deformationsstrukturen erzeugt werden können. Bei Druck [001] und [h0l] beobachtet man mechanische, polysynthetische Zwillingsbildung nach (¯101) (Aufstellung (C2/m). Ab ca. 20 kb bilden sich daneben Scherbrüche nach {141}. Druck [010] bewirkt ab ca. 15 kb eine {120}-Spaltbarkeit, die jedoch hinter der nach {110} stark zurückbleibt.

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Deformation of hornblende single crystals at pressures up to 21 kbs

Cylinders drilled from hornblende single crystals were deformed at 20° C in a pistoncylinder high-pressure apparatus in the following orientations: [001], [h0l], normal to (110), normal to (100) and [010]. Pyrophyllite was used for transmitting the confining pressure. Experiments with cylinder axis [001] and [h0l] revealed lamellar twinning on (¯101) at pressures above l0 kbs. The twinning elements are: K ₁=(¯101), ₁ = [¯10¯1], K ₂=(100), ₂=[001], s=0.56 (space group setting C2/m). At pressures around 20 kbs, {141}-shear fracture was observed. This means that the SiO₄-ribbons parallel to [001] are broken. Crystals compressed parallel to [010] show {120} cleavage at pressures above 15 kbs.

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Der überwiegende Teil der experimentellen Arbeit wurde im Mineralogischen Institut der Technischen Hochschule Darmstadt durchgeführt. Der Deutschen Forschungsgemeinschaft sei für materielle Hilfe gedankt.     

Aragonite crystallization from strained calcite at reduced pressures and its bearing on aragonite in low-grade metamorphism

Aragonite, the dense form of CaCO₃, grew hydrothermally at 100–300° C and dry at 300–400° C at very low pressures from calcite strained by grinding. Nearly complete inversion to aragonite occurred in some runs with Ca-Mg chloride solutions at 0–2.4 kb and 100–200° C on strained calcite having a (10¯14) reflection with a half-width of 0.48° 2 Cu K. A little aragonite grew dry at one atm. from the ground calcite at 300–400° C in a few hrs. Simultaneous shear during recrystallization of calcite in a rotating squeezer resulted in significant aragonite at 300–400° C several kb. below the stability field. No inversion occurred in any ground calcite when previously annealed in CO₂ at 500° C for a few hrs. Thermochemical data show that at least 200 cal/mole of strain energy can be produced in calcite by mild deformation. This much stored energy would lower the pressure requirements of aragonite, relative to the strained calcite by more than 3 kb, and our observation that aragonite growth was faster than strain recovery of calcite indicates that aragonite can grow in nature at reduced pressures from strained calcite.Some experiments were also carried out on highly magnesian calcites with the thought that aragonite might also form at the expense of this metastable material. No aragonite was produced, but the possibility that this mechanism could be operative in nature cannot be discounted.The microtexture of aragonitic deformed marbles from NW Washington (prehnite-pumpellyite facies rocks, courtesy of J. A. Vance) as well as electron probe microanalysis of these rocks indicates that aragonite selectively replaced highly strained calcite. The calcite-aragonite transition is thus a questionable indicator of high-pressure in certain metamorphic rocks.     

Electron microscopy study of domain structure due to phase transitions in natural perovskite

Transmission electron microscopy on natural calcium metatitanate perovskite (dysanalyte) reveals the following twin laws in the orthorhombic (space group Pbnm) phase: reflection twins on the {110} and {112} planes, and 90° rotation twins about the [001] axis (referred to as [001]_90° twin). Single crystals that were heattreated and quenched from above 1585 K exhibit a dramatic change in domain structure compared with the starting material and specimens quenched from T < 1470=" k.=" mutually=" perpendicular=" {110}=" and=">_90° twins are observed throughout the crystal, forming a cross-hatched domain texture. 1/2[001] antiphase domains, which are very rarely observed in the starting material, also become dominant in the crystal. This change in domain structure is interpreted as due to a structural phase transition in perovskite at a temperature below 1585 K. From the point symmetry elements that describe the twin laws and the translational elements that relate the antiphase domains, the most likely phase near 1585 K is tetragonal with space group P4/mbm. These results are consistent with high-temperature powder X-ray diffraction study. On the other hand, density of the {112} twins is increased significantly in the crystal quenched from 1673 K. Twin domains are either bound by mutually perpendicular {110} and (001) walls, or by {112} walls with {110} twin domains within the polygonal {112} domains. Both twin density variation and domain morphology suggest that the crystal may be cubic at this temperature. Microstructure of a single crystal deformed at 1273 K and 3.5 GPa (within the orthorhombic stability field) is morphologically quite distinct from that of the heat-treated specimens. Dislocations dominate the microstructure and often interact with twin domain boundaries.A National Science Foundation Science and Technology Center     

The influence of disordered,non-equilibrium dolomites on the Mg-solubility in calcite in the system CaCO3-MgCO3

The influence of different degrees of disorder of dolomites on the solubility of MgCO₃ in calcite has been studied under isothermal and isobaric conditions. At 900° C, 4kb and 1000° C, 5 and 7kb, varied smoothly as function of the particular structural and cationic disorder of coexisting dolomite. Higher degrees of disorder of dolomite, estimated by the d _00.6/d _11.0 values and the peak height ratio I _01.5/I_00.6, lead to greater solubility of MgCO₃ in calcite. The run time for all experiments was 96 h, much longer than in previous work. The influence of disorder of dolomite on appears to be larger than that of temperature, as shown by the large range of (0.12–0.30) in calcite at 900° C 4 kb, found in this study. The state of order of dolomite seems to control the solubility limits in this system, and may explain discrepancies found in previous experimental work.     

Mg-cordierite: Si/Al ordering,optical properties,and distortion

Flux-grown crystals of Mg-cordierite, Mg_1.93 Al_3.95 Si_5.07 O₁₈ synthesized by Lee and Pentecost (1976) appear biaxial (2V _x=10°–25°) under the polarizing microscope whereas their distortion index =0°. Between crossed polars, (001) sections display lamellar and cyclic twinning on {110} and, less frequently, {310}. As duration of annealing at 1,300° C, increased, 2V _x increased. Simultaneously, undulatory extinction and intragrain variations in 2V _x increased slightly up to 4 h annealing, then steadily decreased. For this Mg-cordierite, which lacks significant channel H₂O or CO₂, 2V _x and reach maxima of 88° (=589 nm) and 0.25° after 42 h of annealing but other sectors still display lesser values for 2V _x. Presumably, to the extent 2V _x is less than 88°, these sectors represent intergrown submicros copically twinned orthorhombic domains and thus possess shortrange but not long-range order. Annealing at 1,300° C likely increased long-range order by promoting growth of larger domains at the expense of smaller ones. Ultimately, two differently oriented domains, growing toward each other by annexation (and re-orientation) of smaller domains, meet in a twin boundary that, with time, tends to become straight.The cause of intermediate values for , whether compositional or from submicroscopic {110} or {310} twinning, may be revealed by single crystal X-ray photographs. Streaking of diffraction spots along a^* or b^* (but not c^*) will indicate such twinning as the sole or major cause.     

Preferred orientation in experimentally deformed limestone

Over sixty syntectonic deformation experiments in uniaxial compression have been done on fine-grained limestones in the stability fields of calcite I, calcite II and aragonite. X-ray techniques and spherical harmonic analysis of the data were used to determine preferred orientation quantitatively, and inverse pole-figures were derived for these axially symmetric specimens. They display in most cases strong preferred orientation which varies as a function of the experimental conditions, mainly temperature and pressure. At temperatures below 350° C recrystallization is lacking and flattened grains indicate that translation, twin gliding and kinking have been the dominant deformation mechanisms. The inverse pole-figure shows a maximum at c with a shoulder towards or a second maximum at e. This is in agreement with preferred orientation observed in experimentally deformed Yule marble and can be explained as the product of dominant twin gliding on e and translation gliding on r (Turner et al., 1956). At high temperatures (900–1000° C) strong grain growth (from 4 to 50 microns) indicates that the fabric recrystallized. Grains are equidimensional and clear with a marble-like texture. The inverse pole-figure shows a single maximum at r, and c-axes are oriented in a small circle around the axis of compression, ₁. Such a pattern of preferred orientation would be expected on thermodynamic grounds assuming that recrystallized grains will be oriented in such a way that the strain energy is a maximum (e.g. MacDonald, 1960). Decrease in confining pressure caused a decrease of the maximum at c and the formation of a secondary maximum at highangle positive rhombs in the inverse pole-figure. This can be interpreted as r translation dominating over e twinning. In all deformation experiments an equilibrium in preferred orientation was reached after 20 percent shortening. The strength of preferred orientation decreased with increasing temperature. Aragonite was produced within its hydrostatic stability field at temperatures above 500° C. Close to the phase boundary, coarse-grained textures showed preferred orientation with poles to (010) parallel to ₁. At higher pressures the fabric is fine-grained and [001] is aligned parallel to ₁. Evidence is given that the phase change from calcite to aragonite in these deformation experiments is a diffusive and not a martensitic transformation.Publication No. 1043, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California.     

How tightly does calcite e-twin constrain stress?

Mechanical twinning along calcite e-planes has been used for paleostress analyses. Since the twinning has a critical resolved shear stress at ∼10 MPa, not only principal stress axes but also differential stress can be determined from the twins. In this article, five-dimensional stress space used in plasticity theory was introduced to describe the yield loci of calcite e-twinning. The constraints to paleostress from twin and untwin data and from calcite grains twinned on 0, 1, 2 and 3 e-planes were quantified by using their information contents, which were defined in the stress space. The orientations of twinned and untwinned e-planes are known to constrain not only stress axes but also differential stress, D, but they loose the resolution of D if the twin lamellae were formed at D greater than 50–100 MPa. On the other hand, it is difficult to observe twin lamellae subparallel to a thin section. The stochastic modeling of this effect showed that 20–25% of twin lamellae can be overlooked. The degradation of the constraints by this sampling bias can be serious especially for the determination of D.     

The solubility of calcite in water at 6–16 kbar and 500–800 °C

The solubility of calcite in H₂O was measured at 6–16 kbar, 500–800 °C, using a piston-cylinder apparatus. The solubility was determined by the weight loss of a single crystal and by direct analysis of the quench fluid. Calcite dissolves congruently in the pressure (P) and temperature (T) range of this study. At 10 kbar, calcite solubility increases with increasing temperature from 0.016±0.005 molal at 500 °C to 0.057±0.022 molal at 750 °C. The experiments reveal evidence for hydrous melting of calcite between 750 and 800 °C. Solubilities show only a slight increase with increasing P over the range investigated. Comparison with work at low P demonstrates that the P dependence of calcite solubility is large between 1 and 6 kbar, increasing at 500 °C from 1.8×10^–5 molal at 1 kbar to 6.4×10^–3 molal at 6 kbar. The experimental results are described by:

Shock induced planar deformation structures in quartz from the Ries crater,Germany

Crystalline rocks from breccias of the Ries basin, Germany, contain highly deformed quartz. Various planar deformation structures could be observed and classified into five different types: (1) Decorated planar elements, (2) Non-decorated planar elements, (3) Homogeneous lamellae, (4) Filled lamellae, (5) Planar fractures. All these structures are parallel to crystallographic planes: {10¯13}, {10¯12}, {10¯11}, {0001},{11¯21}, {11¯22}, {21¯31}, {51¯61}, {10¯10}. The most typical and most abundant planar structures are decorated and nondecorated planar elements parallel to {10¯13} and {10¯12}. Planar fractures are parallel to {0001} and {10¯11} and form at lower stress levels, probably earlier than the planar elements.Quartz containing planar elements, especially of the non-decorated type, has lower density, index of refraction and birefringence than normal quartz. This quartz is apparently a mixture of an amorphous phase and crystalline quartz, the amount of which can be calculated using average density or refractive index.Comparison of planar quartz structures found in tectonites and those produced artificially under static or dynamic high pressure conditions demonstrates that Ries quartz closely resembles deformed quartz recovered from shock wave experiments. The planar structures found in Ries quartz have been formed by shock wave actions with peak pressures in the 100–400 kbar range.Planar elements are explained to be traces of gliding processes during shock loading visible due to the fact that a high pressure phase (stishovite and/or a stishovite-like glass phase) has been produced along the glide planes. Upon pressure release most of the high pressure phase was transformed into an SiO₂-glass (diaplectic glass).In comparison with experimental data the amount of residual crystalline quartz as well as type and orientation of planar structures in the quartz grains are clues to estimate the peak pressures responsible for these deformations. Shock waves with peak pressures exceeding about 400 kbar completely transform quartz into diaplectic SiO₂-glass.     

Influence of lysozyme on the precipitation of calcium carbonate: a kinetic and morphologic study

Several mechanisms have been proposed to explain the interactions between proteins and mineral surfaces, among them a combination of electrostatic, stereochemical interactions and molecular recognition between the protein and the crystal surface. To identify the mechanisms of interaction in the lysozyme-calcium carbonate model system, the effect of this protein on the precipitation kinetics and morphology of calcite crystals was examined. The solution chemistry and morphology of the solid were monitored over time in a set of time-series free-drift experiments in which CaCO₃ was precipitated from solution in a closed system at 25°C and 1 atm total pressure, in the presence and absence of lysozyme. The precipitation of calcite was preceded by the precipitation of a metastable phase that later dissolved and gave rise to calcite as the sole phase. With increasing lysozyme concentration, the nucleation of both the metastable phase and calcite occurred at lower Ω_calcite, indicating that lysozyme favored the nucleation of both phases. Calcite growth rate was not affected by the presence of lysozyme, at least at protein concentrations ranging from 0 mg/mL to 10 mg/mL.Lysozyme modified the habit of calcite crystals. The degree of habit modification changed with protein concentration. At lower concentrations of lysozyme, the typical rhombohedral habit of calcite crystals was modified by the expression of {110} faces, which resulted from the preferential adsorption of protein on these faces. With increasing lysozyme concentration, the growth of {110}, {100}, and finally {001} faces was sequentially inhibited. This adsorption sequence may be explained by an electrostatic interaction between lysozyme and calcite, in which the inhibition of the growth of {110}, {100}, and {001} faces could be explained by a combined effect of the density of carbonate groups in the calcite face and the specific orientation (perpendicular) of these carbonate groups with respect to the calcite surface. Overgrowth of calcite in the presence of lysozyme demonstrated that the protein favored and controlled the nucleation on the calcite substrate. Overgrowth crystals nucleated epitaxially in lines which run diagonal to rhombohedral {104} faces.     

Über Beanspruchung und Verformung von Gesteinen

A new real triaxial deformation apparatus for p ₁p ₂p ₃ tests and temperatures up to 700° C has been developed. In five creep tests over 8 hours the strain-time curves at 650° C for axial and orthorhombic symmetry of stress are plotted in the three main directions and compared with the petrofabric diagrams of the deformed material. In all tests the symmetry of the petrofabrics can be correlated with the symmetry of the external loading. From the strain-time curves it can be deduced that the intermediate loading p ₂ is of great influence in the stress-strain behavior.Syntectonic recrystallization is found to be common for all tests in the form of broad lensoid and irregular defined {01¯12} lamellae oriented favorably for twinning. Measurements of c-axes in newly recrystallized grains show preferential alingnement subparallel to the axis of maximum principal loading.

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Der Deutschen Forschungsgemeinschaft sei für die finanzielle Unterstützung der Untersuchungen besonders gedankt.     

Twinning induced by the rhombohedral to orthorhombic phase transition in lanthanum gallate (LaGaO3)

Phase-transformation-induced twins in pressureless-sintered lanthanum gallate (LaGaO₃) ceramics have been analysed using the transmission electron microscopy (TEM). Twins are induced by solid state phase transformation upon cooling from the rhombohedral to orthorhombic (o, Pnma) symmetry at ∼145°C. Three types of transformation twins {101} _o , {121} _o , and {123} _o were found in grains containing multiple domains that represent orientation variants. Three orthorhombic orientation variants were distinguished from the transformation domains converged into a triple junction. These twins are the reflection type as confirmed by tilting experiment in the microscope. Although not related by group–subgroup relation, the transformation twins generated by phase transition from rhombohedral to orthorhombic are consistent with those derived from taking cubic aristotype of the lowest common supergroup symmetry as an intermediate metastable structure. The r→ o phase transition of first order in nature may have occurred by a diffusionless, martensitic-type or discontinuous nucleation and growth mechanism.     

A case study of the amount and distribution of heat and fluid during metamorphism

The volume of fluid and amount of heat involved in a portion of a metamorphic event around three synmetamorphic granitic stocks has been quantitatively estimated using mineral composition and modal data from carbonate rocks. Values of volumetric fluid-rock ratios range, with respect to a reference zoisite isograd, from 0.001 to 0.434. Amounts of heat involved range from –25 to 134 cal/cm³ rock. Contours of constant fluid-rock ratio and of constant amount of heat are generally concentric about the granitic stocks indicating that the stocks are foci of high heat and fluid fluxes during metamorphism. In addition, contours of fluid-rock ratios and amount of heat outline NE-SW-trending channelways of high fluid and heat fluxes that alternate with regions of lower fluid and heat fluxes. The NE-SW-trending vertical bedding and schistosity in the area — of premetamorphic origin — probably constrained fluid and heat transfer to occur preferentially in NE-SW directions. Large values of heat involved in metamorphism are strongly correlated with large fluid-rock ratios, suggesting that fluids are an important carrier of heat during metamorphism. Configurations of mapped isograds in the area mimic the patterns of contours of constant fluid-rock ratio and of heat content, indicating that configurations of isograds may contain useful information about regional patterns of heat and fluid transfer during metamorphism.Notation T Last temperature recorded by metacarbonate rocks (°C) - P Lithostatic pressure (bars) - Pi Partial pressure of component i (bars) - of last fluid in equilibrium with carbonate rocks during metamorphism - R 1.987 cal/bar-degree - K _s Activity constant for an assemblage of solid mineral phases - In Natural logarithm - c _v Volumetric heat capacity (cal/cm³-degree) - Q Heat added to or subtracted from a rock during metamorphism in the zoisite zone (kcal/100 cm³ rock; cal/cm³ rock) - Q{ibrxn} Heat added to or subtracted from a rock due to mineral reactions during metamorphism in the zoisite zone (kcal/100 cm³ rock; cal/cm³ rock) - Std. Dev. Standard Deviation - Average of fluid in equilibrium with carbonate rocks during their metamorphism in the zoisite zone - of fluid in equilibrium with carbonate rocks at the zoisite isograd - T Temperature at the zoisite isograd (°C) - X _i,j Mole fraction of component i in phase j - H _i Molar enthalpy of reaction i at 0 bars pressure - ¯V _i Change of molar volume due to reaction _ii - _i Measure of progress of reaction i - V Change in rock volume due to fluid-rock reactions - iV Initial rock volume before metamorphism within the zoisite zone - ¯V _s,i Change in molar volume of solid minerals due to reaction i Component notation an CaAl₂Si₂O₈ Phase notation Pl Plagioclase - Am Amphibole - Cc Calcite - Qz Quartz - Di Diopside - Zo Zoisite - Ga Garnet - Bi Biotite - Kf Microcline - Mu Muscovite     

Mechanical twinning of plagioclase in a deformed meta-anorthosite — the production of M-twinning

Mechanical twins on both the Albite and Pericline laws are well developed on an optical scale in basic plagioclase from the Harris meta-anorthosite where the rock is affected by closely spaced shear fractures associated with pseudotachylite formation. In some cases the twinning is accompanied by kinking or intracrystalline fractures. The twins may be periodic and form a ladder structure and sometimes appear to intersect on an optical scale showing structures which very closely resemble those seen in microcline.Both Albite and Pericline twins are lenticular as observed by TEM, but isolated twin tips were rarely seen. No dislocations are associated with the twin tips. The twins in the optically observed ladder structure (when thicker than 2m), have themselves a very fine secondary ladder structure, consisting of periodic (200–600 nm) lenticular twins which impinge on the perpendicular twin wall. Intersecting twins were also observed and where two sets of equally thick twins intersect, M-twinning develops as shown by selected-area diffraction.The twin formation is analyzed in terms of the plagioclase structure and a coherent model for twin nucleation proposed. True twins can develop in plagioclase only if the Al/Si distribution is unchanged or nearly so after deformation. Mechanical twinning is quite easy in low plagio-clases between An₁₀₀ and about An₃₀ which have P¯1 or I¯1 lattices or domains with I¯1-type structures. Only one kind of nucleus can develop coherently in a single crystal, but it may grow to give either an Albite or a Pericline twin. Periodic nucleation and growth give complex textures. The qualitative variation of the twin energy as a function of the obliquity is given for different twin shapes and degrees of Al/Si order. M-twinning arises mechanically by coherent growth and interference of the strain fields.