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Talc is widely distributed over the Earth's surface and is predicted to be formed in various tectonic settings. Talc is a very soft and anisotropic sheet silicate showing very low friction behavior. Therefore, the formation of talc is expected to weaken the strength of talc-bearing rocks and may be associated with the initiation of subduction, and with a decrease in the coupling coefficient resulting in aseismic movements along faults and shear zones within subduction zones. For these reasons, understanding the crystallographic preferred orientation (CPO) of talc is important to quantify the anisotropy and physical properties of the host rock. However, it is difficult to measure a significant number of talc crystal orientations and to evaluate the accuracy of the measurements using electron-backscattered diffraction (EBSD). Therefore, talc CPO has not been reported, and there is uncertainty regarding the estimation of the strength of deformed talc-bearing rocks. Using methods developed for antigorite, we report the first successful EBSD measurements of talc CPO from a talc schist formed due to Simetasomatism of ultramafic rocks by subduction zone fluids. We used a combination of W-SEM and FE-SEM measurements to examine domains of various grain sizes of talc. In addition, we used TEM measurements to evaluate the accuracy of the EBSD measurements and discuss the results of talc CPO analysis. Talc CPO in the present study shows a strong concentration of the pole to the (001) plane normal to the foliation. The strongest concentration of the [100] direction is parallel to the lineation. The talc schist produces similar S-wave splitting and P- and S-wave anisotropy as antigorite schist in deeper domains, thus identifying talc-rich layers in subduction zones may require a combination of geophysical surveys, seismic observations, and anisotropy modeling. The presence of strong talc CPO in rocks comprising the slab–mantle interface boundary may promote spatial expansion of the slip area during earthquakes along the base of the mantle wedge.

Large deposits of unusual black talc interstratified with dolostone layers of the late Neoproterozoic Dengying Formation were discovered in Guangfeng County, Jiangxi Province, southeastern China. The black talc ore exhibits primarily oolitic structures and consists mainly of talc (30–70 wt%), dolomite, and quartz, with trace amounts of pyrite and apatite. The ooids are composed of nearly pure black talc crystals, most of which consist of ultrafine nano-plates. The black talc contains small amounts of carbon, which causes the black coloring. Raman spectra and X-ray photoelectron spectroscopy (XPS) results indicate structural disorder and chemical impurities within bonds (e.g., sp 3 hybridized carbon and C–O bonds) in the carbonaceous material, instead of perfectly structured graphite or graphene. Isolated graphene-like carbon interlayers are present in the talc nano-crystals, as shown by Z-contrast transmission electron microscope (TEM) imaging. Based on previous studies on Mg-silicate precipitation from surface water, we propose a sedimentary formation mechanism for the black talc, in which tetrahedral–octahedral–tetrahedral (T-O-T) layers of Mg-silicates, 1~2 unit-cells thick, co-precipitated with abundant organic matter derived from microorganisms thriving in locally Al-depleted sea water with high concentrations of Mg 2+ and SiO 2 (aq), in a shallow marine or lagoonal environment. The involvement of organic matter may have facilitated the precipitation of Mg-silicate. Further diagenesis and re-crystalliztion of the biomass-coated precursor resulted in the formation of graphene-like layers between neighboring talc nano-crystals with same orientation.

The synthesis of 10 A phase via the reaction of talc plus water at 8 GPa and 500 °C was studied by in situ Raman spectroscopy using a diamond-anvil cell. The initial fast (2 h) incorporation of interlayer H2O molecules into the talc structure is traced by gradual growth of new OH stretching bands at 3592 and 3621 cm−1 and the shift of several framework bands. Further monitoring at HP-HT conditions over 7 h reveals gradual weakening of the 3592 cm−1 band, which can probably be related to the onset of the formation of “long-run” 10 A phase through the appearance of silanol groups following the model proposed by Pawley et al. (2010), influencing the interlayer hydrogen bonding.

For the first time, this work reports concentration maps of asbestos minerals in contaminated mine tailings drawn using the results of Rietveld quantitative phase analysis (QPA). The investigated sites are located in the Valle d’Aosta region (Italy): Cretaz, the most important Italian magnetite mine, active until 1979 and Emarese, one the most important chrysotile asbestos mines in Italy, active until 1968. The results of the study permit to draw the spatial distribution of the asbestos (chrysotile and tremolite in this specific case) concentration, useful to plan reclamation of the sites, with priority given to the areas with the highest asbestos concentration. Because of the complexity of the mineral assemblage, which includes, among the others, antigorite, chlorite, talc, and tremolite, the concentration of chrysotile was cross-checked using different experimental techniques such as X-ray powder diffraction (XRPD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), polarized light optical microscopy (PCOM), and differential thermal analysis (DTA). The accuracy of the results was validated by analyzing standard samples with known concentrations of chrysotile and tremolite. The comparison allowed to point out the advantages and disadvantages of each experimental method. At Cretaz, chrysotile ranges from 4.4 to 22.8 wt% and tremolite from 1.0 to 10.3 wt%, whereas at Emarese the concentration of chrysotile varies from 3.3 to 39.5 wt% and tremolite from 5.9 to 12.4 wt%. Antigorite and chlorite are the major accompanying phases with variable amounts of other accessory minerals including magnetite, carbonates, talc, olivine, pyroxene, talc, and brucite. The results of our study are of key importance for the local environmental policies as the knowledge of the spatial distribution of the asbestos concentration allows to plan a detailed reclamation agenda of the contaminated sites. The spots with the highest surface contamination of both chrysotile and tremolite were identified and classified as priority areas in the reclamation plan.

Since the first report of triple-chain silicates in 1977, there has been considerable uncertainty about their formation in nature, primarily concerning whether they are stable minerals or an intermediate step in the formation of other chain or sheet silicate minerals. The lack of thermochemical data and relevant experimental studies for any triple-chain silicate has impeded establishment of their thermodynamic stability relative to other minerals. Here we combine recently reported thermochemical data for a sodium-analog of the triple-chain silicate clinojimthompsonite (Na-cjt) with existing data for various common minerals (i.e., enstatite, talc, anthophyllite, and chrysotile). We also present new experimental data to show that Na-cjt should form stably at typical metamorphic conditions of 0.2 GPa and 450 °C. Ion-activity vs. pH diagrams calculated at 100–300 °C and 0.1 GPa show that Na-cjt could form by reaction between primary igneous minerals (enstatite, forsterite) in oceanic harzburgite and seawater at temperatures up to 300 °C. However, formation of Na-cjt from harzburgite that has been altered to talc and chrysotile is not expected to occur, except at the unusually high pH conditions observed for forearc serpentine mounds. Estimates of the thermochemical properties of the Na-free end-member clinojimthompsonite [Mg 10 Si 12 O 32 (OH) 4 ] suggest that it also should form from the primary mineralogy of harzburgite at temperatures and Si concentrations typically observed for various oceanic hydrothermal and geothermal systems, but probably not from ultramafic rock already hydrated to serpentine.

As shown by in situ infrared spectroscopy and analysis of quenched samples, phyllosilicates (muscovite, sericite, pyrophyllite, and talc) under dehydroxylation conditions lack the characteristic bands near 1600 cm–1 (bending) and 5200 cm–1 (combination) of H2O, and they contain virtually no H2O but an abundance of OH. This observation appears to be at variance with the formal description of dehydroxylation in bulk samples as 2(OH) -> H2O + O, whereas it is suggested that hydrogen diffuses in the form of (OH) – or/and H+ in dehydroxylation. The upper limit of H2O in the dehydroxlated bulk is likely to be at the parts per million level in phyllosilicates that contain structural OH ions equivalent to 4–5 wt% H2O. The observations suggest that H2O molecules are probably formed near the surface of the sample.

Acicular Mg amphibole in thermal-metamorphic serpentinites from Elba Island, Italy, is affected by polytypic disorder (with the possible coexistence of Pnmn , Pnma , and C 2/ m structures), chain-width defects, and retrograde hydration reactions. Based on HR-TEM nanostructures, we documented three main hydration stages: (1) formation of chain-multiplicity faults (CMFs); (2) formation of fibrous topotactic talc, elongated parallel to [001] Tlc and resulting from progressive polymerization of CMFs; and (3) pervasive serpentinization. Incipient serpentinization produces isolated inclusions within Mg amphibole, rhombic in shape and parallel to {210} Mg-Am , or elongated parallel to (100) Mg-Am ; the inclusions are filled by not-topotactic, curved (001) Srp layers or chrysotile partial fibers. As the Mg amphibole-to-serpentine replacement proceeds, the isolated serpentine inclusions merge together, producing a sponge-like texture, with interconnected fluid paths, from which serpentine can rapidly grow at the expense of enclosing amphibole. We suggest that whereas the first hydration reactions to CMFs and fibrous talc take place at relatively high T and low fluid/rock ratio (fluid circulation confined to microfractures, cleavage planes, and subgrain boundaries), subsequent serpentinization requires significant changes in physical conditions, with definitely lower T and pervasive fluid circulation.

Clay samples of hydrothermal origin from several oceanic spreading centers were studied using XRD, microprobe, infrared, and thermal analysis. They are talc, smectite, and mixed-layer talc-smectite (T-S) where the talc layers have several degrees of crystalline order. The smectite is trioctahedral in most cases, but there is also dioctahedral smectite both as mixed-layer and as a separate phase. All specimens contain Fe 3+ , some of them in moderate amounts (up to 17% Fe 2 O 3 ) distributed between the tetrahedral and octahedral sheets (maximum values: IV Fe = 0.32, VI Fe = 0.68, per O 10 [OH] 2 ). Octahedral Fe abundance correlates with the presence of molecular water that is lost in a well-defined dehydration event above 200 °C. This water does not cause layer expansion and is interpreted to be present within the pseudo-hexagonal cavity, next to Fe 3+ ions that generate a local charge imbalance. The presence of octahedral Fe 3+ is accompanied by vacancies in the octahedral sheet to balance the excess positive charge. An infrared band at ~790 cm −1 is assigned to OH bending in the group Fe-Mg-□-OH. Analysis of this band suggests a range of short-range Fe-□ distributions, from random to ordered. Our sequence talc, T-S, trioctahedral smectite is defined by an increasing Al for Si substitution in the tetrahedra and increasing crystal disorder. The presence of Fe also causes crystal defects. This mixed-layer series can be considered as a continuum generated by the combination of chemical and crystal defect variability. Kerolite was used to designate disordered, hydrated talc. We find that there is no clear line delimiting talc from kerolite as a single phase or in mixed-layer minerals and that it is better to use a descriptive term for the latter such as “disordered talc.” Dioctahedral smectite is also a possible end-member of the mixed-layer sequence, which implies an Al + □ for Mg substitution in the octahedral sheet. If T-S consists of polar TOT layers, the existence of dioctahedral smectite in T-S raises the question of the actual composition of the octahedral sheets within polar TOT layers.

X-ray diffraction measurements of natural talc plus water at combined pressures and temperatures of 0-15 GPa and 23-400 degrees C reveal the presence of a structural change that could be interpreted as a new high-pressure phase at 4.0 (+/- 0.5) GPa, and raise the possibility that the newly inferred phase transition takes place in cold subducting stabs as a precursor to appearance of the 10 angstrom phase of talc.

Ni-bearing magnesium phyllosilicates (garnierites) are significant Ni ores in Ni-laterites worldwide. The present paper reports a detailed TEM investigation of garnierites from the Falcondo Ni-laterite deposit (Dominican Republic). Different types of garnierites have been recognized, usually consisting of mixtures between serpentine and talc-like phases that display a wide range of textures at the nano meter scale. In particular, chrysotile tubes, polygonal serpentine, and lizardite lamellae are intergrown with less crystalline, talc-like lamellae. Samples consisting uniquely of talc-like and of sepiolite-falcondoite were also observed, occurring as distinctive thin lamellae and long ribbon-shaped fibers, respectively. HRTEM imaging indicates that serpentine is replaced by the talc-like phase, whereas TEM-AEM data show preferential concentration of Ni in the talc-like phase. We suggest, therefore, that the crystallization of Ni-bearing phyllosilicates is associated with an increase in the silica activity of the system, promoting the replacement of the Ni-poor serpentine by the Ni-enriched talc-like phase. These results have interesting implications in material science, as garnierites are natural analogs of Ni-bearing phyllosilicate-supported synthetic catalysts. Finally, SAED and HRTEM suggest that the Ni-bearing talc-like phase corresponds to a variety of talc with extra water, showing larger d(001) than talc (i.e., 9.2-9.7 angstrom), described as 'kerolite'-'pimelite' in clay mineral literature.

We present high-pressure infrared spectra of two geologically important hydrous minerals: talc, Mg3Si4O10(OH)2 and lawsonite, CaAl2Si2O7(OH)2·H2O,·at room temperature. For lawsonite, our data span the far infrared region from 150 to 550 cm−1 and extend to 25 GPa. We combine our new spectroscopic data with previously published high-pressure mid-infrared and Raman data to constrain the Gruneisen parameter and vibrational density of states under pressure. In the case of talc, we present high-pressure infrared data that span both the mid and far infrared from 150 to 3800 cm−1 covering lattice, silicate, and hydroxyl stretching vibrations to a maximum pressure of 30 GPa. Both phases show remarkable metastability well beyond their nominal maximum thermodynamic stability at simultaneous high-pressure and high-temperature conditions.

Chrysotile from Thetford, Quebec, was heated at constant temperature in quench furnaces from 200 to 1000 °C for 4 to 720 h, and the products were analyzed by XRD and optical microscopy. XRD patterns for chrysotile from Thedford, Jeffrey Quebec, and New Idria, California, heated for up to 8 h at constant temperatures from 400 to 800 °C in hydrothermal diamond-anvil cells, were obtained at 300 s intervals by using synchrotron radiation. The studies show that differences in the decomposition temperatures of chrysotile reported in the literature can be explained by differences in the temperatures, time at temperature, particle size, and partial pressure of water. Chrysotile heated for 30 days is destroyed between 475 and 500 °C, whereas chrysotile heated to 800 °C survives for only minutes. A prograde heating experiment, consistent with the literature, shows chrysotile destruction beginning at 600 °C and completed at 750 °C, demonstrating that such experiments should not be used to establish temperatures of thermal stability. Forsterite forms readily at 600 °C when water is retained transiently in the decomposition products, but does not form after 4 h of heating samples of small fiber size and high surface area from which water is readily lost. When water is retained in the sample chamber during heating to 800 °C over 4 h, a rapid destruction of chrysotile and formation of forsterite was observed, followed by the appearance and later destruction of the intermediate reaction products talc, a tridymite-like phase, and anthophyllite; enstatite appearance coincides with the destruction of the tridymite-like phase and talc. In quench experiments, a 10+ A phase is observed between 500 and 600 °C, and talc is observed between 587 and 800 °C. The optical properties of the run products between 600 and 900 °C are highly variable, with a range in the index of refraction perpendicular to length of up to 0.06 and a minimum index of refraction 0.03 below the starting material in fibers in the 650 °C run product; the variability reflects the inhomogeneity in products and reactants in this temperature range. In quench experiments, the intensity of the XRD patterns of chrysotile decreased on heating between 200 and 400 °C, and increased at 450 °C, demonstrating recrystallization of chrysotile. A gradual heating experiment showed a similar pattern with the lowest intensities at 250 °C and maximum intensities at 550 °C. The two types of chrysotile reported in the literature may reflect simply the presence of chrysotile of different particle sizes. Chrysotile may also possess a spectrum of stabilities due to variable strain energies of curvature. The presence of a tridymite-like phase supports the formation of a Si-rich dehydroxylate II. The absence of forsterite and chrysotile in brake-wear debris reflects the high temperature and disaggregated fiber bundles resulting from friction.

29 Si MAS NMR spectroscopy on samples of 10-A phase synthesized from oxides (6.0 GPa/600 °C/400 h) and from partial transformation of talc (6.5 GPa/650 °C/12.5 h) reveals that this phase contains Q 2 -type Si sites in a ratio Q 3 :Q 2 of 5.33:1. It is proposed that the Q 2 arise from adjacent vacancies in the tetrahedral sheets for which charge balance is most likely achieved by hydroxylation via a hydrogarnet-like substitution involving the formation of Q 2 silanol groups. Variable-contact-time 29 Si { 1 H} CP/MAS NMR spectra of the talc/10-A phase product support the assignment of Q 2 Si to the proposed SiO 3 (OH) groups. Electron microprobe analysis, including oxygen, gives the following empirical formula normalized to three Mg apfu and inferring a hydrogarnet component Si → 4H associated with Si vacancies: Mg 3 Si 3.83(8) O 9.32 (OH) 2.68. 1.1(4)H 2 O. The observed Mg:Si indicates a significant Si deficiency relative to talc. Comparison of the 29 Si MAS NMR and microprobe data indicates that Si vacancies likely occur as single isolated entities, rather than as pairs or clusters, and that between 1 in 18 and 1 in 23 Si sites is vacant. The results suggest new and intriguing possibilities for the incorporation of excess H into the 10-A phase and, potentially, other phyllosilicates under upper-mantle conditions.

Syntheses have been done at 2.5 GPa and 750 °C to examine the effects of varying the water content and composition of the solid starting mixture on the formation of glaucophane [Na2Mg3Al2Si8O22(OH)2]. Amphiboles of apparently ideal glaucophane composition, conÞ rmed by electron microprobe analyses, were obtained by treating oxide or oxide-hydroxide mixtures with bulk water contents of 4–5 wt% for durations of about 400 h in multiple treatments with intermittent grindings. This water content is necessary to permit growth of amphibole while preventing the intervention of a sheet silicate, which was identiÞ ed in this study as vermiculite. To maximize amphibole yields, multiple treatments with intermittent grinding are needed on account of the low water contents that reduce the reaction rate of the synthesis process. Calculation of the solubility curves of talc, jadeite, and glaucophane in log[aMg(OH) + /aH + ] vs. log[(aNa + )(aAlSiO4 – )] space at 2.5 GPa and 750 °C shows that glaucophane has a very narrow Þ eld of stability without coexisting jadeite or talc. This same diagram can be used to illustrate that the growth of vermiculite relative to glaucophane is not an issue of metastability but of the activity of water.

The reaction 2 talc + 2 forsterite = 5 enstatite + 2 H2O has been investigated between 1 and 3.5 GPa. Here, we report the results from 2 sets of experiments: (1) “conventional” static quenching experiments performed several years ago in non-end-loaded and end-loaded piston-cylinder apparatus, and (2) a new set of experiments conducted in a novel type of apparatus, a “rocking” piston cylinder. This new technique was employed for the reinvestigation to overcome segregation problems encountered during static experiments. The run-products of the latter experiments are homogenous throughout the entire capsule. The reaction was bracketed to 680–690 °C at 1 GPa, to 660–680 °C at 1.7 GPa, to 650–660 °C at 2.5 GPa, to 600–640 °C at 3 GPa, and to 550–600 °C at 3.5 GPa. The location of the reaction in P - T -space is consistent with low-pressure experimental data of Chernosky et al. (1985), and is in reasonable agreement with the “conventional” high-pressure experiments of this study. The new experimental results were used to refine the existing thermodynamic properties of talc within the framework of the Holland and Powell (1998) database. The standard state enthalpy and entropy amount to H of298 = 5912.358 kJ/mol and S o298 = 239.1 J/(K·mol). The refined standard entropy is about 9% lower than the entropy obtained calorimetrically. When the clinoenstatite-orthoenstatite transition is taken into account, S o298 of talc improves by ~6 J/K. We assume that a further increase of the standard state entropy of talc can be achieved by varying the activity of H2O in the fluid as a function of pressure and temperature.

The high-temperature dehydroxylation and structural change of talc, Mg3(Si2O5)2(OH)2, has been investigated in detail using infrared (IR) spectroscopy. The data (in the region of 20–12000 cm–1) on quenched samples show that absorptions from structurally incorporated OH and OD, as well as NH4-like species, have similar temperature dependences in dehydroxylation. The OH species exhibit weak variation in frequency on heating, which is inconsistent with thermally induced weakening of O-H bonds. Dehydroxylation in talc is a complex process that involves proton migrations and formation of new OH species. Additional fundamental OH bands near 3665 and 3745 cm–1 became detectable near 900 °C. On further heating the former disappear near 1200 °C, whereas the latter became undetectable near 1350 °C. The occurrence of CO2 is observed in samples quenched between 600 and 1250 °C. The phonon spectrum (20–1500 cm–1) of the dehydroxylate (obtained by annealing the sample at 1000 °C) gives features significantly different from that of talc, indicating the loss of the original layer structure. The IR data imply that the talc dehydroxylate consists of disordered SiO2 and enstatite (MgSiO3). Although MgSiO3 exists dominantly in the form of orthoenstatite, the characteristic bands of clinoenstatite phase are found to coexist in the samples treated at 1000 °C. The IR data from in situ measurements show that protons become mobile at temperatures below the dehydroxylation and an extra OH species near 3500 cm–1 develops on heating. This new species is not quenchable, and it decreases intensity on cooling and disappears at room temperature. The in situ results also indicate external carbon-related substances can diffuse into talc during dehydroxylation.

The first report of water/hydrolytic weakening of silicate minerals was based on the observation that the strength of quartz decreases significantly in the presence of the water released from talc, the confining medium used in some of high-pressure experiments (Blacic and Griggs 1965). Subsequently, a number of researchers have studied this important phenomenon in several different minerals. Publications examining the influence of water or protons on the creep behavior of nominally anhydrous silicate minerals are listed in Table 1⇓. Initial studies treated the water-weakening phenomenon as an on-off process; that is, minerals and rocks are weak under hydrous conditions but strong under anhydrous conditions. Further investigations, however, demonstrated that the strengths of nominally anhydrous minerals (NAMs) and rocks decrease systematically with increasing hydrogen concentration (Kronenberg and Tullis 1984; Kohlstedt et al. 1995; Post et al. 1996; Mei and Kohlstedt 2000a,b; Karato and Jung 2003). View this table: Table 1. Investigations of the effect of water/protons on the strength of nominally anhydrous silicate minerals. Two quite different approaches have been used in analyzing the effect of water or protons on the strength of nominally anhydrous silicate minerals. In the first model, a mechanism is envisioned in which water hydrolyzes strong Si-O bonds via the reaction Si-O-Si + H2O → Si-OH·OH-Si (Griggs 1967), thus the term water or hydrolytic weakening. As a result, the glide of dislocations becomes easier in wet quartz than in dry quartz since Si-O bonds do not need to be broken if water is present. In this case of water/hydrolytic weakening, the rate limiting step is the propagation of kinks along dislocations, which is facilitated by diffusion of HOH along dislocation cores. The resulting dislocation velocity is assumed to be proportional to the HOH concentration (Griggs 1974). In effect, this analysis implies …

Wonesite, an interlayer-deficient trioctahedral sodium mica, has been investigated mainly by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) using a Gandolficamera. The true structure of wonesite is triclinic with a large layer offset, i.e., displacement between the two tetrahedral sheets across the interlayer region, which is partially occupied by sodium ions. The direction and amount of the layer offset is approximately [110] and 1.25 A respectively, with unit-cell dimensions of a = 5.31, b = 9.18, and c = 9.75 A, α = 96.2, β = 96.5, γ = 89.9°. This interlayer structure, which is similar to that in talc, explains well the exsolution lamellae of talc in wonesite with continuous 2:1 layers. The volume ratio of wonesite and exsolved talc is estimated to be 4:1 to 5:1 from the XRD patterns. Disorder in the directions of the layer offset is occasionally observed in TEM. The interlayer in sodium micas (preiswerkite, paragonite, aspidolite, wonesite, etc.) possesses various amounts of layer offset, depending on the cavity space in the tetrahedral sheet that is primarily determined by the ditrigonal rotation angle.

New melting experiments in a 0.5 inch piston-cylinder apparatus from 0.5 to 3.0 GPa on compositions along the diopside-wollastonite join show that the eutectic liquid composition, which varies with pressure, does not change with time in experiments with barium carbonate or with talc/pyrex pressure media. Mapping out the trace of the diopside-wollastonite eutectic curve in P-T space with both piston-in and piston-out experiments reveals high-temperature pressure intensification in piston-in experiments with barium carbonate or talc/pyrex assemblies, in contrast to frictional loss of pressure at lower temperatures. A new pressure correction algorithm, derived by multiple regression analysis, incorporates both effects. When applied to a barium carbonate experiment at the intersection of the CMAS spinel-lherzolite to garnet-lherzolite transition (SGT) with the model peridotite solidus, the model pressure increases from 2.25 to 2.82 GPa. When a similar type of correction is applied to piston-out experiments run in talc/pyrex assemblies, the pressure of the intersection of the SGT and the CMAS solidus decreases from 3.0 to ~2.9 GPa, in agreement with the barium carbonate experiments from this study and with experiments from other studies run with salt/pyrex assemblies.

We determined details of the layer stackings in lizardite (variety: baltimorite) and chlorite in a splintery sample from a massive serpentinite. Based on high-resolution transmission electron microscopy (HRTEM) and image simulations, we localized the positions of projected columns of tetrahedral (T) and octahedral (O) cations that allow determination of the nature of the transformation of lizardite to chlorite and the structural formula of the chlorite. The lizardite stacking is of an unexpected type because adjacent layers are in non-hydrogen-bonded placements, a configuration that permits easy, strain-free transformation to chlorite. We conclude that the lizardite-chlorite transformation was isochemical and at constant volume. A consequence is that the product chlorite consists of neutral brucite- and talc-type layers and so can be regarded as a 1:1 interstratification of brucite and talc. Both the T and the O sheets in talc-type layers contain M 3+ (M = Al, Fe, Cr, ...) cations. An implication of our results combined with high-pressure studies of chlorite and TO silicates is that lizardite stacking sequences can indicate whether a given crystal has been subjected to high pressures. Lizardite in non-hydrogen-bonded sequences presumably has this configuration because of its prior exposure to elevated pressures and can thus be added to the list of indicators of past high pressure. Calibration of this new geobarometer remains for the future.

The crystal-chemistry of fifteen samples of talc from various localities and origins having a relatively wide range of Fe, Al, and F contents, have been studied mainly by Fourier-transform infrared (FTIR) spectroscopy, in both the near infrared (NIR) and middle infrared (MIR) regions, and by Mossbauer spectroscopy. For the first time, assignments have been made for the 2νOH bands in talc, and Mg/Al substitutions were revealed using NIR. Less than 0.01 atom of Fe2+ or Al per half unit cell can be detected from the 2νOH region, whereas the sensitivity is about half in the νOH region. The amounts of F also can be quantified by NIR measurement, using the F inductive effect on the width of the 2 νMg3OH band. The NIR data show that Fe2+ and Mg are distributed randomly in their octahedral sheet. Actual structural formulae of natural talcs can be obtained from NIR and Mossbauer spectroscopies only, even for impure talc samples. NIR spectroscopy is simple to use and is a very powerful tool to study the crystal chemistry of talc.

The formation mechanisms of tremolite in a time series of hydrothermal experiments at 850 °C and 6.1 kbar were studied using transmission electron microscopy (TEM), analytical TEM, and X- ray diffraction. The starting materials were unseeded mixtures of reagent-grade oxides. After the first 8 min of reaction, the synthesis products were diopside, enstatite, clinoenstatite, quartz, and minor C -centered and primitive monoclinic amphibole (13 wt% total). The compositions of the first- formed amphiboles vary considerably, from essentially pure cummingtonite (Ca/Mg = 0) to nearly ideal tremolite (Ca/Mg = 0.4). Three groupings of amphiboles with successively higher Ca/Mg ratios were identified, respectively 0.1, 0.22, and 0.35. Over a period of 97 h, the proportions of Mg-rich amphibole gradually decrease, and the average Ca/Mg ratios of all amphiboles steadily increase from about 0.22 to 0.35. Other changes include:(1) a decrease in the aspect ratio of amphibole crystals with increasing time; (2) a decrease in the number of non-amphibole chain multiplicity faults (CMFs), from early A′(2) values as low as 0.81 to a final value of 0.94; and (3) a distinct change in the rate of amphibole growth after 1 h of synthesis. These observations suggest that two mechanisms of amphibole formation are dominant:(1) formation of pyroxene modules with subsequent hydration and arrangement into the double-chain amphibole structure and (2) direct nucleation and growth of amphibole from hydrothermal solution. A third mechanism involving formation of talc-like modules, followed by their breakdown into double-chain modules, incorporation of Ca, and ordering into the amphibole structure, may also occur but probably only at the lamellar or unit-cell scale, because these syntheses were performed outside the stability field of talc.

The compressibilities of five sheet silicates-three chlorite compositions, pyrophyllite, and talc-have been measured in situ using energy-dispersive powder diffraction in a multi-anvil apparatus at the synchrotron radiation source, Daresbury Laboratory, U.K. Some high-temperature data were also collected. The three chlorite samples were a natural Mg-rich chlorite [Mg 4 . 4 9 Fe 0 . 2 8 Cr 0 . 1 2 Al 2 . 1 0 Si 2 . 9 5 O 1 0 (OH) 8 ], a natural Fe-rich chlorite [Mg 0 . 2 8 Fe 4 . 1 1 Al 2 . 7 1 Si 2 . 7 7 O 1 0 (OH) 8 ], and a synthetic clinochlore [Mg 5 Al 2 Si 3 O 1 0 (OH) 8 ]. The compressibility data for all three samples, measured up to between 3 and 6 GPa, can be fit to a single equation of state with a bulk modulus of 86.9(16) GPa (using K' = 4). The thermal expansivity measured for the natural Mg-chlorite [α 0 = 2.5(6) x 10 - 5 /K] is similar to previous measurements on various chlorite compositions, suggesting that the thermal expansivity of chlorite also can be described by a single equation, independent of composition. The bulk modulus of the natural pyrophyllite [A[Si 4 O 1 0 (OH) 2 ], measured up to 6.3 GPa, is 37(3) GPa, with K' = 10(1). The few high-temperature points agree with previous measurements. The natural talc [Mg 3 Si 4 O 1 0 (OH) 2 ], measured up to 6.2 GPa, has a bulk modulus of 41(4) GPa, with K' = 6(2). Our bulk modulus of chlorite is much higher than previously determined using X-ray diffraction. This finding explains why calculations of chlorite reactions using the previous data considerably overestimate its stability when compared to experimentally determined reaction positions. The pyrophyllite compressibility data should be useful in calculations of metamorphic reactions involving this mineral. The talc data are essentially identical to previous measurements.

Dissolution rates of a well-characterized sample of powdered talc were measured in solvents that mimic fluids found in the human lung. These experiments found that talc dissolution rates were the same for pH controlled aqueous solvents, phosphate buffered saline solution, and modified Gamble’s solutions. Variation of solvent chemistry, including the addition of organic chelators and proteins at intercellular fluid concentrations, does not markedly affect the measured dissolution rate of talc at 37 °C. The data further indicate that the dissolution mechanism for talc in aqueous solutions is independent of pH over the range of 2 to 8. The dissolution rate at 37 °C, determined by measuring the silicon release rate per unit surface area of talc in a mixed-flow reactor system, is 1.4 (±1.0) × 10−11 mol Si/m2·s. Application of a geometric shrinking particle model using this dissolution rate results in an estimated lifetime (upper limit) of approximately 8 years for a 1 μm talc particle under pulmonary conditions. Talc dissolves considerably faster than quartz, but slower than chrysotile and olivine in the body. These data can be used to place constraints on the role of particle dissolution in the disease models associated with airborne respirable mineral particles.

Samples of phengite-3 T , (K0.93Na0.01)(Al1.44Mg0.56Ti0.02)(Si3.54Al0.46)O10(OH1.93F0.07), which formed at about 3 GPa and 1000 K in ultrahigh-pressure metamorphic rocks of the Dora-Maira Massif (Western Alps, Italy), were investigated by HRTEM, AEM, EMPA, and SEM. The matrix of phengite-3 T , which is almost free of stacking faults, contains single-crystal α-quartz platelets 100–700 A thick that are confined by (001) planes of phengite. In the vicinity of quartz, the 30 A period of the phengite-3 T matrix is faulted by short sequences with about 19 A periodicity, interpreted as talc-2 M . The occurrence of these phases is not connected with any defects in the host phengite nor is their spatial distribution homogeneous on the TEM scale. The shape control exerted by phengite on the quartz crystals, the absence of deformation around them, and the nearby presence of an interlayered 19 A phase, suggest that quartz and talc may have been produced within mica by a reaction of the type: ![Formula][1] which leads to a less celadonite-rich phengite during decompression, after the rock had left the coesite stability field. In addition, examination by optical microscopy along [001] of “thick” sections of the Dora-Maira phengite and of phengite samples from other high-pressure terranes (Monte Mucrone and Sesia Zone in the Italian Western Alps, Dabie Mountains in central China), revealed the presence of micrometer-wide, amoeboid platelets of quartz interlayered at various depths parallel to (001) of micas. In spite of the different observation scale, these are interpreted as the same reaction product as identified by HRTEM. These new observations show that high-pressure white micas may not be homogeneous and should be examined more carefully. Some consequences for thermobarometry of such heterogeneity and intracrystalline re-organization during decompression are considered; they depend on the resolution of the analytical method employed. Implications for thermochronometry still have to be evaluated. [1]: /embed/tex-math-1.gif

The stability field of MgMgAl-pumpellyite, Mg 5Al5Si6O21(OH)7, was determined in the system MgO-Al2O3-SiO2-H2O in reversal experiments at pressures between 34 and 100 kbar and temperatures in the range of 597 to 1050 8C. Brackets were obtained on five breakdown reactions (in order of increasing pressure): 8Mg Al Si O (OH) 55 6 21 7 MgMgAl-pumpellyite 5 19MgAl SiO (OH) 1 Al SiO 1 7Mg Si O (OH) 25 2 2 5 3 4 10 2 chloritoid kyanite talc

The reaction talc+forsterite = enstatite+H 2 O has been investigated between 6 and 20 kbar. Previous high-pressure experimental studies suggested various reaction positions, mostly with positive P-T slopes. The new results show that the reaction has a negative slope in the pressure range studied. It was bracketed between 640 and 680 degrees C at 10 kbar, between 620 and 640 degrees C at 15 kbar, and between 580 and 600 degrees C at 20 kbar. The growth of antigorite at high pressures prevented the reaction from being determined above 20 kbar. The reaction position is consistent with the previous low-pressure data of Chernosky et al. (1985), is in very good agreement with the position calculated using the thermodynamic database of Berman (1988), and is in reasonable agreement with the calculation using the thermodynamic database of Holland and Powell (1990). The high reaction temperatures suggested by previous high-pressure experiments may reflect metastable or quench growth of talc in those experiments. Talc in equilibrium with forsterite is stable over a wide P-T range in hydrothermally altered and metamorphosed ultramafic rocks and may be important for carrying H 2 O into subduction zones. However, talc dehydration occurs at too shallow a depth for the H 2 O to contribute directly to subduction zone volcanism.

For the first time, chemically pure, nearly single-phase antigorite, Mg 48 Si 34 O 85 (OH) 62 , was synthesized directly without using seeds. Starting material was a stoichiometric mixture of previously synthesized talc and brucite. Synthesis conditions were 50 kbar, 500 degrees C, and 120 h. TEM studies show that the dominant wavelength for the structural modulation of antigorite is about 4.5 nm, which corresponds to m = 17 in the general antigorite formula M (sub 3m-1) T 2m O 5m (OH) (sub 4m-6) and thus to the structure of most natural antigorites. Selected-area electron diffraction patterns of single crystals exhibit the hkO reciprocal net diagnostic of antigorite.

The pressure-temperature conditions of the reactions governing the high-pressure stability of talc were investigated in experiments on the bulk composition Mg3Si40IO(OH)2 + H20 at 2.9-6.8 GPa, 650-820 °C, using piston-cylinder and multianvil apparatus. The reaction talc = enstatite + coesite + vapor was bracketed between 800 and 820°C at 2.90-2.95 GPa and between 770 and 780°C at 3.77-4.02 GPa. The lower-pressure bracket, which is just above the quartz-coesite phase transition, is consistent with some of the previous brackets on the reaction talc = enstatite + quartz + vapor and with the position of the talc dehydration reaction calculated using THERMOCALCv2.4 (Powell and Holland, 1988; Holland and Powell, 1990; Holland, personal communication). This revised version of THERMOCALC incorporates new compressibility and thermal expansivity data for talc (Pawley, Redfern, and Wood, in preparation). Agreement between experimental and calculated curves continues up to 4 GPa, but at higher pressures the talc dehydration reaction occurs at lower temperatures than calculated, so that by 4.6 GPa the thermal stability of talc is at