Simple-shear Deformation Experiments Research Articles

Crystallographic Preferred Orientation of Phase D at High Pressure and Temperature: Implications for Seismic Anisotropy in the Mid‐Mantle

AbstractSeismic anisotropy has been widely observed in the lower mantle transition zone and the uppermost lower mantle near several subducting slabs, which is typically attributed to the crystallographic preferred orientation (CPO) of constituent minerals. In this study, well‐controlled uniaxial and simple shear deformation experiments were conducted on Mg‐endmember phase D and Al‐bearing phase D aggregates at 20 GPa and 500–1,000°C. Strong (0001) fabrics were observed in the uniaxially compressed samples. The CPO from all experiments, including extension and simple shear, indicated that the most active slip systems in phase D were <110>(0001) and <100>(0001), with a dominance of <110>(0001). The calculated seismic velocity suggests that the deformed phase D generates positive radial seismic anisotropy (VSH > VSV) in both subhorizontal shearing and subvertical compression, causing significant shear‐wave splitting time. Our results suggest that the observed seismic anisotropy in the mid‐mantle in several cold subduction zones can be well explained by the presence of deformed phase D.

Effects of hydrothermal alteration on shear localization and weakening in the mantle lithosphere

Hydrothermal alteration occurs in upper-mantle fault zones and significantly modifies the rheological properties of ultramafic rocks. To understand how hydrothermal alteration affects shear localization and the strength of the fault zones, we conducted simple-shear deformation experiments on water-saturated harzburgite (70 wt% olivine +30 wt% orthopyroxene) gouges sandwiched between three types of shear pistons (tungsten, yttria-stabilized zirconia, and corundum) using two deformation-DIA apparatuses at confining pressures of 1–4 GPa, temperatures of 500–580 °C, average shear strain rates of 6.1 × 10−6 s−1 to 1.0 × 10−4 s−1, and water contents of 4–30 wt%. The harzburgitic gouges with water contents of 10 and 30 wt% exhibited steady-state sliding or strain-hardening behavior, and the shear strength of the latter sample was less than one-third of that of the former sample, possibly reflecting the difference in pore fluid pressure. Olivine/orthopyroxene grains exhibit distributed microfracturing and grain rotation towards P foliation, indicating the operation of cataclastic flow. Shear displacement is also accommodated by the development of serpentine- or talc-bearing multiple shear zones including B, R1, and/or Y shears, where intense cataclasis-related grain size reduction and dissolution/precipitation occur. For experimental runs using tungsten and zirconia shear pistons, the serpentine or talc blades that wrap finer-grained and dissolved olivine/orthopyroxene grains form an interconnected network, indicative of frictional sliding or dislocation glide on their basal planes as a dominant deformation mechanism. Raman spectra of the serpentine minerals indicate that they represent antigorite with low crystallinity. These findings suggest that when deep lithospheric fault zones undergo hydrothermal alteration, deformation is controlled by competition between cataclastic flow in olivine/orthopyroxene matrix and slip or glide of antigorite or talc in semi-brittle fault zones, the predominance of which depends on the amount of the hydrous phyllosilicates and the magnitude of pore fluid pressure.

Application of Automated Microscopy Equipment for Rock Analog Material Experiments: Static Grain Growth and Simple Shear Deformation Experiments Using Norcamphor

Application of Automated Microscopy Equipment for Rock Analog Material Experiments: Static Grain Growth and Simple Shear Deformation Experiments Using Norcamphor

Mantle dynamics inferred from the crystallographic preferred orientation of bridgmanite.

Seismic shear wave anisotropy is observed in Earth's uppermost lower mantle around several subducted slabs. The anisotropy caused by the deformation-induced crystallographic preferred orientation (CPO) of bridgmanite (perovskite-structured (Mg,Fe)SiO3) is the most plausible explanation for these seismic observations. However, the rheological properties of bridgmanite are largely unknown. Uniaxial deformation experiments have been carried out to determine the deformation texture of bridgmanite, but the dominant slip system (the slip direction and plane) has not been determined. Here we report the CPO pattern and dominant slip system of bridgmanite under conditions that correspond to the uppermost lower mantle (25 gigapascals and 1,873 kelvin) obtained through simple shear deformation experiments using the Kawai-type deformation-DIA apparatus. The fabrics obtained are characterized by [100] perpendicular to the shear plane and [001] parallel to the shear direction, implying that the dominant slip system of bridgmanite is [001](100). The observed seismic shear- wave anisotropies near several subducted slabs (Tonga-Kermadec, Kurile, Peru and Java) can be explained in terms of the CPO of bridgmanite as induced by mantle flow parallel to the direction of subduction.

In situ observation of crystallographic preferred orientation of deforming olivine at high pressure and high temperature

Simple-shear deformation experiments on polycrystalline olivine and olivine single-crystal were conducted at pressures of 1.3–3.8GPa and temperatures of 1223–1573K to understand the achievement of steady-state fabric strength and the process of dynamic recrystallization. Development of crystallographic preferred orientation (CPO) of olivine was evaluated from two-dimensional X-ray diffraction patterns, and shear strain was measured from X-ray radiographs. The steady-state fabric strength of the A-type fabric was achieved within total shear strain of γ=2. At strains higher than γ=1, an increase in concentration of the [010] axes mainly contributes to an increase in fabric strength. At strains higher than γ=2, the magnitude of VSH/VSV (i.e., ratio of horizontally and vertically polarized shear wave velocities) scarcely increased in most of the runs. The VSH/VSV of peridotite (70vol.% olivine+30vol.% minor phases) having the steady-state A-type olivine fabric coincides with that of recent global one-dimensional models under the assumption of horizontal flow, suggesting that the seismic anisotropy observed in the shallow upper mantle is mostly explained by the development of A-type olivine fabric. Experimental results on the deformation of single-crystal olivine showed that the CPO of olivine is influenced by the initial orientation of the starting single crystal because strain is concentrated in the recrystallized areas and the relic of the starting single crystal remains. In the upper mantle, the old CPO of olivine developed in the past may affect the olivine CPO developed in the present.

Crystallographic preferred orientation of wadsleyite and ringwoodite: Effects of phase transformation and water on seismic anisotropy in the mantle transition zone

Simple-shear deformation experiments on wadsleyite and ringwoodite aggregates were performed at 15–18 GPa and 1473–1873 K to investigate the effect of water on the development of the crystallographic preferred orientation (CPO) of wadsleyite and ringwoodite. The [001] axes of wadsleyite are preferentially sub-parallel to the shear direction and the [010] axes of wadsleyite concentrate in the direction of the shear-plane normal for water content less than 9000 ppm H/Si (i.e., ∼540 wt. ppm) in wadsleyite. At higher water content in wadsleyite (≥9000 ppm H/Si), the concentration of the [100] axes of wadsleyite becomes stronger than that of the [010] axes in the direction of the shear-plane normal. The fabric strength of wadsleyite having low water content (<3000 ppm H/Si) was much stronger than that having water content higher than 3000 ppm H/Si. The magnitude of VSH/VSV (the ratio of horizontally and vertically polarized shear wave velocities) in the upper transition zone is well explained by the flow of wadsleyite aggregates having water content higher than 3000 ppm H/Si. The back transformation from ringwoodite to wadsleyite may help to suppress the increase in fabric strength of wadsleyite during the deformation. In contrast to wadsleyite, the fabric strength of ringwoodite CPOs was not sufficient to cause robust seismic anisotropy even though the deformation of ringwoodite was controlled by dislocation creep. Thus, the lower transition zone is expected to be largely isotropic.

Crystallographic preferred orientation of olivine in the Earth’s deep upper mantle

Simple-shear deformation experiments of olivine were conducted at pressures of 11.1–12.7GPa and temperatures of 1670–1770K. C-type olivine fabric (developed by the (100) [001] slip system) and similar fabrics were dominantly developed in a wide range of water content (COH=49–19,242ppm H/Si in olivine). Because the seismic fast a-axes are perpendicular to the shear plane only for C-type fabric, a positive–negative transition of VSH–VSV (difference in velocity between horizontally and vertically polarized shear waves) inevitably occurs when C-type fabric is developed by a pressure-induced fabric transition. The positive–negative transition of VSH–VSV at ∼200km depth reported in global one-dimensional models is known to be well explained by the horizontal flow of olivine with a low water content (<750ppm H/Si in olivine). Our results demonstrate that the presence of a moderate or high water content in olivine (⩾750ppm H/Si) results in a drastic increase of the boundary depth for the VSH–VSV positive–negative transition (300–370km depth), namely, “wet” olivine flows cannot account for global one-dimensional models, suggesting that the whole upper mantle is mostly occupied by low-water content regions and the distribution of moderate- or high-water content regions is limited to a small part of the upper mantle.

Development of A-type olivine fabric in water-rich deep upper mantle

Water controls the activity of slip systems in olivine resulting in various types of olivine crystallographic preferred orientation (i.e., fabric) in mantle rocks. The A-type olivine fabric is the most commonly observed olivine fabric in natural peridotites. Development of A-type olivine fabric (developed by the (010)[100] slip system) is known to be limited to the water-poor conditions of the shallow upper mantle (<200km depth). We have performed simple-shear deformation experiments of olivine at 7.2–11.1GPa and 1400–1770 K. Here we show that A-type olivine fabric was developed under water-rich conditions (>2130ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) was observed under moderately wet conditions (750–2130ppm H/Si). Developments of C-type (by the (100)[001] slip system) fabric was limited to water-poor conditions (<220ppm H/Si). We found that monotonic decrease in the seismic anisotropy VSH/VSV (the ratio of horizontally and vertically polarized shear waves) with depth in the global one-dimensional models is well explained by the olivine fabrics developed in the horizontal flow of a water-poor mantle. Only A-type olivine fabric can explain the vertical mantle flow which associates the seismic anisotropy of VSH/VSV<1 in the deep upper mantle (>200km depth). A strong anomaly of VSH/VSV<1 observed in the deep upper mantle beneath the East Pacific Rise is well explained by the distribution of water-rich regions (in which A-type olivine fabric is dominantly developed) in the deep upper mantle and upwelling mantle flows.

Deformation experiments of bubble‐ and crystal‐bearing magmas: Rheological and microstructural analysis

Simple shear deformation experiments on three‐phase, hydrous, haplogranitic magmas, composed of quartz crystals (24–65 vol.%), CO2‐rich gas bubbles (9–12 vol.%) and melt in different proportions, were performed with a Paterson‐type rock deformation apparatus. Strain rates from 5 · 10−6 s−1 to 4 · 10−3 s−1 were applied at temperatures between 723 and 1023 K and at pressure of 200 MPa. The results show that the three‐phase suspension rheology is strongly strain rate dependent (non‐Newtonian behavior). Two non‐Newtonian regimes were observed: shear thinning (viscosity decreases with increasing strain rate) and shear thickening (viscosity increases with increasing strain rate). Shear thinning occurs in crystal‐rich magmas (55–65 vol.% crystals; 9–10 vol.% bubbles) as a result of crystal size reduction and shear zoning. Shear thickening prevails in dilute suspensions (24 vol.% crystals; 12 vol.% bubbles), where bubble coalescence and outgassing dominate. At intermediate crystallinity (44 vol.% crystals; 12 vol.% bubbles) both shear thickening and thinning occur. Based on the microstructural observations using synchrotron radiation X‐ray tomographic microscopy, bubbles can develop two different shapes: oblate at low temperature (&lt;873 K) and prolate at high temperature (&gt;873 K). These differences in shape are caused by different conditions of flow: unsteady flow, where the relaxation time of the bubbles is much longer than the timescale of deformation (oblate shapes); steady flow, where bubbles are in their equilibrium deformation state (prolate shapes). Three‐phase magmas are characterized by a rheological behavior that is substantially different with respect to suspensions containing only crystals or only gas bubbles.

High-temperature magnetic fabric development from plastically deformed magnetite in experimental shear zones

Magnetic fabric development has been studied in synthetic magnetite-silicate aggregates in a series of high-temperature simple shear deformation experiments. Samples composed of magnetite grains with a nominal size of 20-40 µm dispersed at 3 wt % in a matrix of plagioclase were deformed between 1000 and 1200 °C with a confining pressure of 300 MPa, shear stresses in the range 10-130 MPa, and shear strains up to γ = 3. We determined that both magnetite and plagioclase were deformed plastically at these conditions. An oblate magnetic fabric ellipsoid develops subparallel to the shear plane and the degree of AMS rapidly increases with strain up to a value of 2.5. Theoretical strain response models of magnetic fabric development were applied to the data to obtain estimated magnetite strains. The shape-preferred orientation of magnetite grains after deformation, determined from reflected light image analysis, was used to calculate independent magnetite strain estimates. These results were then compared with strains estimated from theoretical magnetic anisotropy development. Both strain estimation methods indicate strain partitioning between magnetite and the plagioclase matrix and the results are used to determine approximate viscosity ratios between the two phases at the experimental conditions.

Change of olivine a-axis alignment induced by water: Origin of seismic anisotropy in subduction zones

The effects of water on the crystallographic preferred orientation (CPO) of olivine aggregates were investigated through simple-shear deformation experiments under asthenospheric upper mantle conditions (pressure=2.1–5.2GPa, temperature=1490–1830K) using a deformation-DIA apparatus. Formation of the A-type olivine fabric (developed by the (010)[100] slip system) was observed under water-depleted conditions (COH<650ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) or a B-type-like fabric (by the (010)[h0l] slip system) were predominantly formed under water-rich conditions (>1000ppm H/Si). In comparison with fabrics of anhydrous olivine (≤111ppm H/Si), those of olivine having higher water contents (≥625ppm H/Si in olivine) showed stronger anisotropic properties (e.g., P-wave anisotropy, S-wave polarization anisotropy, and the ratio of horizontally and vertically polarized shear waves). The water-induced olivine CPO transition from A-type to B-type(-like) fabric accompanies a change in the alignment of the seismic fast a-axes, resulting in flow-parallel and flow-perpendicular shear wave splitting under water-depleted and water-rich conditions, respectively. The rotation of the fast direction of shear-wave splitting across an arc, which is observed in many subduction zones, is well explained by the likely bimodal water distribution along the mantle wedge (i.e., water-rich in fore-arc and water-depleted in back-arc regions) and the developments of two different types of olivine fabrics (i.e., B-type(-like) fabric in fore-arc and A-type fabric in back-arc regions).

High pressure and temperature fabric transitions in olivine and variations in upper mantle seismic anisotropy

The effects of pressure on crystallographic preferred orientation (CPO) of olivine aggregates were investigated through simple-shear deformation experiments at pressures between 2.1 and 7.6 GPa and temperatures of 1493–1673 K under dry conditions using a deformation-DIA apparatus, and the variations in seismic anisotropy were evaluated under the Earth's upper mantle conditions. We found that the monotonic decrease in seismic anisotropy with depth is caused by the pressure-dependency of the seismic properties of A-type (developed by the (010) [ 100 ] slip system) olivine fabric, while the rapid decrease is caused by the fabric transition from A-type to B/C-type (by the (hk0) [ 001 ] slip systems) at 7.6 GPa and 1673 K. Moreover, an alternative transition, from A-type fabric to B-type-like fabric (by the (010) [ 001 ] slip system), occurs at 7.6 GPa and lower temperature. These two temperature-dependent fabric transitions occurring at 7.6 GPa result in low seismic anisotropy with V SH / V SV (the ratio of horizontally and vertically polarized shear waves) > 1 at low temperatures (i.e., old-continental mantle conditions) and V SH / V SV < 1 at high temperatures (i.e., oceanic mantle conditions) at greater depths, consistent with seismological observations. Thus, the variations of CPO with pressure and temperature in olivine under dry conditions can explain the seismic anisotropy signatures observed in the upper mantle, without invoking other mechanisms.

Technical development of simple shear deformation experiments using a deformation-DIA apparatus

Technical developments for simple shear deformation experiments at high pressures were made. The newly designed cell assembly can be compressed by deformation-DIA apparatuses with the MA 6-6 system, which consists of six second-stage tungsten carbide anvils (with a truncated edge length of 5 mm) and the anvil guide. Deformation of samples was barely observed during the compression process, showing that the shear strain of the deformed samples can be measured by the rotation of a strain marker. Simple shear deformation experiments on anhydrous and hydrous olivine aggregates were conducted under upper mantle conditions (pressures of 5.2–7.6 GPa and temperatures of 1 473–1 573 K), and sample deformation with a shear strain of γ=0.8−1.2 was successfully achieved at a shear strain rate of 4.0×10−5−7.5×10−5 s−1. The present study extended the pressure range of simple shear deformation experiments in the deformation-DIA apparatus from 3 GPa in an early study to 7.6 GPa at high temperatures.

Origin of seismic anisotropy in the D″ layer inferred from shear deformation experiments on post-perovskite phase

Seismic anisotropy is one of the significant features in the D″ layer of the Earth and is thought to be derived from the lattice preferred orientation (LPO) of constituent materials or shape preferred orientation (SPO) of heterogeneous materials such as melt and inclusions. Recent experimental and theoretical studies strongly suggest that the D″ layer consists mainly of a MgSiO 3 post-perovskite phase together with ferro-periclase. To understand the anisotropy in the D″ layer, we have conducted a series of simple shear deformation experiments at high temperature and pressure on polycrystalline CaIrO 3 as an analogue of MgSiO 3 and measured the LPO of the post-perovskite phase. Crystallographic orientation analysis of the deformed post-perovskite phase showed strong LPOs with the dominant slip system being [100](010). Calculation of the elastic wave velocities considering the effect of LPOs of post-perovskite and ferro-periclase showed as azimuthal and polarization anisotropies in the horizontal shear plane where the velocity of horizontally polarized S-wave is considerably faster than that of vertically polarized S-wave. Thus, the seismic anisotropy observed in the D″ layer can be reasonably explained by the LPO of the mixture of post-perovskite and ferro-periclase, where the LPO may result from the horizontal shear flow generated by the mantle convection.

Lattice preferred orientation in deformed polycrystalline (Mg,Fe)O and implications for seismic anisotropy in D″

Magnesiowüstite [(Mg,Fe)O] is an important constituent of the lower mantle, probably occupying about 20–25% of its volume. Laboratory and theoretical studies have shown this mineral to be highly elastically anisotropic at lower mantle pressures and temperatures. Thus, strain-induced formation of lattice preferred orientation (LPO) in magnesiowüstite is a candidate mechanism for the origin of anisotropic structure in D″. Although observations of seismic anisotropy within D″ are robust, both the occurrence and the style of that anisotropy are spatially variable. Two hypotheses have been offered to explain the observations of D″ anisotropy: LPO of intrinsically anisotropic minerals, or shape preferred orientation (SPO), perhaps in the form of horizontal layering or oriented inclusions. To investigate the first hypothesis, we performed confined simple shear deformation experiments in the dislocation creep regime using a gas-medium deformation apparatus over a range of compositions: the MgO and FeO endmembers and three intermediate compositions. Samples were deformed at 1273–1473 K, at a confining pressure of 300 MPa, to large shear strains ( γ = 3.5–4.5) using deformation pistons cut at 45°. After deformation, the LPO was measured by electron backscatter diffraction (EBSD). The LPO produced varied for differing compositions, indicating the activity on individual slip systems and/or the nature of grain boundary migration in (Mg,Fe)O are affected by composition and/or homologous temperature. We predicted seismic anisotropy from the measured LPOs and theoretically determined single-crystal elastic constants. Anisotropic behavior predicted from LPO agrees well with observations of D″ anisotropy, so the LPO hypothesis appears to satisfy the seismological constraints. Our calculated anisotropy patterns suggest that if D″ anisotropy is due to LPO of (Mg,Fe)O, then azimuthal variations in anisotropy in the horizontal plane should be present. Such azimuthal variations are not generally predicted for SPO-type hypotheses, and this may provide a means for distinguishing the cause of D″ anisotropy.

Comparison of Single Crystal Simple Shear Deformation Experiments with Crystal Plasticity Finite Element Simulations

The crystal plasticity finite element method is an ideal approach to simulate the mechanics, texture, and elastic‐plastic anisotropy of materials. Experimental validation of such simulations requires to measure these quantities without the disturbing influence of friction known for instance from channel‐die experiments. This work presents results obtained form simple shear deformation experiments of aluminium single crystals together with corresponding crystal plasticity finite element simulations.

New type of olivine fabric from deformation experiments at modest water content and low stress

Research Article| December 01, 2004 New type of olivine fabric from deformation experiments at modest water content and low stress Ikuo Katayama; Ikuo Katayama 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA Search for other works by this author on: GSW Google Scholar Haemyeong Jung; Haemyeong Jung 2Institute of Geophysics and Planetary Physics, University of California, Riverside, California 92521, USA Search for other works by this author on: GSW Google Scholar Shun-ichiro Karato Shun-ichiro Karato 3Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (12): 1045–1048. https://doi.org/10.1130/G20805.1 Article history received: 05 May 2004 rev-recd: 03 Sep 2004 accepted: 08 Sep 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ikuo Katayama, Haemyeong Jung, Shun-ichiro Karato; New type of olivine fabric from deformation experiments at modest water content and low stress. Geology 2004;; 32 (12): 1045–1048. doi: https://doi.org/10.1130/G20805.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A new type of olivine fabric was found by high-strain simple-shear deformation experiments in the presence of trace amounts of water at ∼0.5–2.2 GPa and ∼1470–1570 K. In this fabric, called E-type fabric, the olivine [100] axis is subparallel to the shear direction, and the (001) plane is parallel to the shear plane; this geometry suggests that the [100](001) slip system makes the dominant contribution to total strain. This fabric is dominant at a modest water content, 200 < COH < 1000 H/106Si at low stresses and high temperatures. Some mylonites from peridotite massifs show this type of olivine fabric, which suggests the presence of water during the shear localization. The seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric (A type), but the magnitudes of anisotropy are different between the two types: for horizontal flow, the amplitude of VSH/VSV anisotropy is weaker, but the amplitude of shear-wave splitting is stronger for the E-type fabric than for the A-type dry fabric. Seismic anisotropy in the oceanic upper mantle may be due to the olivine E-type fabric. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lattice preferred orientation of olivine aggregates deformed in simple shear

REGIONS of the Earth's upper mantle show significant seismic ani-sotropy due to the preferred crystallographic orientation ('lattice preferred orientation') adopted by its constituent minerals in response to deformation1–6. Seismic anisotropies thus provide clues to the flow and/or stress patterns in the upper mantle, but the use of lattice preferred orientation to infer such properties from seismic data has been hampered by the lack of experimental studies relating changes in crystallographic orientation to simple-shear deformation. (Simple shear is probably the dominant mode of deformation in the upper mantle, whereas most previous experiments have focused on the effects of uniaxial compression7–9.) Here we describe the results of simple-shear deformation experiments on olivine aggregates, conducted at high temperatures and pressures (~1,500 K and 300 MPa). For large strains (up to 150%), our experiments reproduce the lattice preferred orientation observed in highly deformed upper-mantle rocks, in which the olivine [100] axes lie nearly parallel to the flow direction. But for relatively small strains, the preferred orientation is rotated with respect to the flow direction, indicating that seismic anisotropy should also be sensitive to the sense of shear in the upper mantle.

The role of shear strain in the graphitization of a high-volatile bituminous and an anthracitic coal

A ‘soft’ carbon-based high-volatile bituminous ( R o max=0.68%) coal and a ‘hard’ carbon-based Pennsylvania anthracite ( R o max=5.27%) were deformed in the steady state at high temperatures and pressures in a series of coaxial and simple shear deformation experiments designed to constrain the role of shear strain and strain energy in the graphitization process. Tests were carried out in a Griggs-t type solid (NaCl) medium apparatus at T=400–900°C, constant displacement rates of 10 -5−10 -6 s −1, at confining pressures of 0.6 GPa (coaxial) or 0.8 and 1.0 GPa (simple shear). Coaxial samples were shortened up to 50%, whereas shear strains up to 4.9 were attained in simple shear tests. Experiments lasted up to 118 h. Deformed, high-volatile bituminous coal was extensively coked and no correlation between strain and R o max, bireflectance or coal texture was observed in any samples. With increasing temperature, R o max and bireflectance increase in highly anisotropic, coarse mosaic units, but remain essentially constant in the fine granular mosaic, which becomes more abundant at higher temperatures. Graphite-like reflectances are observed locally only in highly reactive macerals and in pyrolytic carbon veins. The degree of molecular ordering attained in deformed bituminous coal samples appears to be determined by the heating-pressurization path rather than by subsequent deformation. Graphitization did not occur in coaxially deformed anthracite. Nonetheless, dramatic molecular ordering occurs at T>700°C, with average bireflectance values increasing from 1.68% at 700°C to 6.36% at 900°C. Anisotropy is greatest in zones of high strain at all temperatures. In anthracite samples deformed in simple shear over the 600–900°C range at 1.0 GPa, the average R o max values increase up to 11.9%, whereas average bireflectance values increase up to 10.7%. Bireflectance increases with progressive bedding rotation and, thus, with increasing shear strain. Graphitization occurs in several anthracite samples deformed in simple shear at 900°C. X-ray diffraction and transmission electron microscopy of highly anisotropic material in one sample confirms the presence of graphite with d 002=0.3363 nm. These data strongly suggest that shear strain, through its tendency to align basic structural units, is the factor responsible for the natural transformation of anthracite to graphite at temperatures far below the 2200°C required in hydrostatic heating experiments at ambient pressure.