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An understanding of the thermodynamics of triple-chain silicate phases can offer insight into their formation in nature. Because the more common Mg triple-chain silicates have not yet been synthesized, this study focused on the Na-analogue to clinojimthompsonite (Na-cjt), which forms readily in a laboratory setting. Synthetic Na-cjt was characterized using electron microscopy and microanalysis, thermogravimetric analysis, infrared spectroscopy, and loss on ignition to determine its water content. The results of these studies indicated a formula for Na-cjt of [Na.sub.3.74][Mg.sub.8.13][Si.sub.12][O.sub.32.00][(OH).sub.4]*1.40 [H.sub.2]O, although there was substantial uncertainty ([+ or -] 13%) associated with the total water (OH + [H.sub.2]O) content. A series of experimental reversals was performed over a temperature range of 350-650[degrees]C and a pressure range of 0.1-1 GPa for the reaction between Na-cjt and a double-chain silicate. In the pressure range of 0.2-0.5 GPa, this reaction has the stoichiometry [Na.sub.3.74] [Mg.sub.8.13][Si.sub.12][O.sub.32.00][(OH).sub.4]*1.40[H.sub.2]O(Na-cjt) = 1.5 [Na.sub.2.49][Mg.sub.5.42][Si.sub.8][O.sub.21.34][(OH).sub.2.66] (Na-Mg amph) + 1.40 [H.sub.2]O. The equilibrium boundary reaches a maximum temperature near 550[degrees]C at 0.4 GPa, continuing to lower temperatures above this pressure. The third-law entropy and enthalpy of formation of both Na-cjt and the associated Na-Mg amphibole were measured using adiabatic and oxide-melt solution calorimetry, respectively. Calculating the equilibrium boundary for the reaction using the calorimetrically determined values does not produce the same boundary found through the experimental reversals. The discrepancy was attributed to water variability in Na-cjt with pressure. This variability and its effect on Na-cjt's thermodynamic values can be expressed by the following equations that, for simplicity, indicate the total water content (OH + [H.sub.2]O) as moles of [H.sub.2]O in Na-cjt over the range of 0.1 to 1.0 GPa: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] where the entropy does not include any contributions from configurational mixing. If one includes configurational entropy contributions from cation and vacancy disorder in the triple-chain silicate and amphibole, one obtains [S.sup.o] and [DELTA][H.sup.o.sub.f] for Na-cjt of 919.2 J/(K*mol) and-18417.6 kJ/mol, respectively, and [S.sup.o] and [DELTA][H.sup.o.sub.f] for Na-Mg amphibole of 650.9 J/(K*mol) and -11 919.7 kJ/mol, respectively. Keywords: Triple-chain silicate, Na-clinojimthompsonite, thermochemistry, calorimetry

A series of experiments with and without sample retreatment was performed on a starting mixture of dolomite plus quartz at 0.5 GPa and 600 [degrees]C in a C[O.sub.2]-[H.sub.2]O fluid with a mole fraction of C[O.sub.2] of 0.2 for durations up to 582 h. The initial reaction of dolomite and quartz led to rapid formation of talc and calcite instead of tremolite and calcite by the intended reaction: 5 dolomite + 8 quartz + [H.sub.2]O = tremolite + 3 calcite + 7 C[O.sub.2]. With continued treatment, the talc+ calcite + quartz assemblage gradually reacts to form increasing amounts of tremolite and diopside with the eventual loss of quartz and nearly complete loss of talc. The detailed structure of the amphibole and the nature of the Mg enrichment were revealed using AEM analysis of individual amphibole crystals. The most abundant defects are triple-chain chain multiplicity faults (CMFs), which appear as isolated lamellae with single periodicity in short-duration experiments and increasingly as groups of lamellae with variable multiplicity and periodicity in longer duration experiments and especially in a long-duration experiment without retreatment. In this latter experiment, a calcian clinojimthompsonite domain extending 15 unit cells along the b axis was observed. The sample after 582 h and five retreatments shows, on average, 3.5% true solid solution with Mg expressed as the Mg-cummingtonite (MC) component after correction for the presence of CMFs. This sample is thought to most closely approach the equilibrium composition for the amphibole. The sample after 250 h with three retreatments has about 10%, whereas that after 582 h without retreatment has 0.6% MC component, the latter having a relatively high density of CMFs. This study affirms the importante that precursor non-amphibole biopyriboles play in the formation of tremolitic amphibole. Keywords: Tremolite, HRTEM, analytical electron microscopy, mineral growth, calcian clinojimthompsonite

A series of low-Ca (1.8-1.65 atoms Ca per formula unit) actinolites were synthesized in the system CaO-FeO-MgO-Si[O.sub.2]-[H.sub.2]O with bulk compositions of 0, 10, 20, and 30 mol% ferro-actinolite when projected onto the tremolite--ferro-actinolite join for the purpose of determining the cation site occupancies as a function of bulk composition. Syntheses were done by multiple treatments of the oxide-metal mixtures in internally heated gas vessels at 600-800 [degrees]C, 6 kbar, f[o.sub.2] near the Co-CoO buffer, which is sufficiently low to prevent the appearance of [Fe.sup.3+] in this study, and for total durations up to 774 hours. Bulk compositions of the synthesis products were assessed by electron microprobe analysis (EMPA), analytical transmission electron microscopy (AEM), and by using unit-cell dimensions (CD method), whereas the site occupancies were assessed using powder X-ray diffraction (XRD) Rietveld refinements, Fourier-transform infrared (FTIR) spectroscopy, and Mossbauer spectroscopy. Selected-area electron-diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images were obtained to study the stRucture and defects of the samples. A maximum of about 2% non-amphibole chain-multiplicity faults (CMFs) were observed in the sample with 10 mol% ferroactinolite component, indicating the strong reduction in CMFs that occurs with only a small addition of ferro-actinolite to tremolitic amphiboles. This study, as with earlier studies, found no significant preferential partitioning of Fe and Mg relative to the bulk Fe content of the sample at the M1 and M3 sites, weak partitioning of Mg relative to Fe into the [M.sub.2] site, and strong partitioning of Fe relative to Mg at the M4 site. No ferric iron was detected. The partitioning of Fe and Mg at the M4 site was modeled by a simple ideal-activity, two-site exchange reaction using the actinolites from this study and those of previous studies with bulk Fe/(Fe + Mg) [less than or equal to] 0.3 and with low Al, Mn, and Na contents using 1 bar and 298K values of [DELTA]G[degrees] = 62 kJ and [DELTA]S[degrees] = 54 J/K. The FFIR data were analyzed for the presence of short-range order of Fe and Mg at the M1 and M3 sites but none was found.

The formation mechanisms of tremolite in a time series of hydrothermal experiments at 850 [degrees]C and 6.l 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 l 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.

High-resolution transmission-electron microscopy (HRTEM) and infrared spectroscopy (FTIR) analyses have been done on amphiboles near the join tremolite [[Ca.sub.2][Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.]2 = TR]-pargasite [Na[Ca.sub.2][Mg.sub.4][Al.sub.3][Si.sub.6][O.sub.22] [(OH).sub.2] = PG] in the ternary system tremolite-pargasite-cummingtonite [[Mg.sub.7][Si.sub.][O.sub.22] [(OH).sub.2] = MC] that were synthesized previously by Sharma and Jenkins (1999). Representative samples across the join were examined in detail by HRTEM to document the presence and concentration of chain multiplicity defects (CMFs). There was relatively little change in the defect density with composition, with the tremolitic sample (TREM 23-13) having the highest defect concentration (6%) and the more PG-rich samples having slightly lower CMF concentrations (4-5%). CMFs with multiplicities of 1,3, 4, 5, and 6 were observed, usually as isolated chains, with the most common being triple-chain slabs. Correction of the bulk composition of the tremolitic amphibole for the presence of these Mg-rich, wide-chain defects reduces the MC content from an apparent value of 8.5 to 4-7.5 mol% MC, depending on which composition is used for the triple-chain defect. The entire amphibole join was examined by FTIR spectroscopy in the OH-stretching region (3000-3800 [cm.sup.-1]) for the purpose of determining the presence of short-range order. A total of 10 component bands were fitted to the spectra across the join. These bands were assigned to specific cation configurations on the basis of earlier studies of the FTIR spectra of chemically simplified amphibole joins pertinent to this study. The extent of short-range order was qualitatively determined by comparing the observed intensities for groups of related bands, corrected for differences in their molar absorptivities, to their calculated intensifies based on random-mixing probabilities. From this exercise, it is observed that the intensities of sodic amphibole configurations are consistently high, tremolite is lowest near the middle of the join, and aluminous amphibole configurations cross over from being higher (at low A1 contents) to being lower (at high A1 contents) than expected near the middle of the join. These differences between observed and predicted band intensities may reflect the presence of deviations in the thermodynamic activities of amphibole components from those predicted on the basis of random-mixing models.

A critical aspect of the debate about the origin and conditions of metamorphism of the Alpe Arami (AA) peridotite is the disagreement over how much ilmenite is contained in the older generation of olivine and therefore how much Ti[O.sub.2] might have been dissolved at high pressure and temperature. We have now determined quantitatively the 3-dimensional distribution of ilmenite in AA olivine by confocal laser scanning microscopy (CLSM). The CLSM measurements show an average concentration of 0.31 vol% of ilmenite in olivine, with individual grains containing up to 1.2 vol%. This translates into average and maximum concentrations of 0.23 and 0.9 wt% Ti[O.sub.2] in olivine, respectively, and confirms the original estimation of maximum concentration of ~1 vol% Ti[O.sub.2]. The vast majority of ilmenite in AA olivine is distributed randomly (although topotactically oriented) and, in all cases, is accompanied by chromite in a ratio of ~4:1. These observations are consistent with an origin of the ilmenite (and chromite) by exsolution from an olivine solid solution at P = 9-12 GPa and temperatures above the stability field of titanian clinohumite, but are not consistent with suggested breakdown of titanian clinohumite. Combining these results with other recent findings suggests that exsolution followed deformation under relatively high fugacity of [H.sub.2]O, and that the high solubility of Ti[O.sub.2] is probably explained by pressure-induced accommodation of Ti in the tetrahedral site of silicates.

A transmission electron microscopy study reveals the microstructure of polysynthetically twinned, near end-member grunerite from Rockport, Massachusetts. A small percentage of Pnma ferroanthophyllite of similar composition is present as thin slabs intergrown parallel to (100) of grunerite and as individual [micro]-size crystals. HRTEM imaging shows that grunerite is free of chain-multiplicity faults. The observed microstructures are interpreted as a result of partial transformation of ferroanthophyllite to grunerite, a mechanism that is also supported by microstructures observed in the Pnmn protoferroanthophyllite from Cheyenne, Colorado, with similar composition. This study supports the possibility that grunerite, ferroanthophyllite, and protoferroanthophyllite may all possess true stability fields.

A series of experiments with and without sample retreatment was performed on a starting mixture of dolomite plus quartz at 0.5 GPa and 600 °C in a CO2-H2O fluid with a mole fraction of CO2 of 0.2 for durations up to 582 h. The initial reaction of dolomite and quartz led to rapid formation of talc and calcite instead of tremolite and calcite by the intended reaction: 5 dolomite + 8 quartz + H2O = tremolite + 3 calcite + 7 CO2. With continued treatment, the talc + calcite + quartz assemblage gradually reacts to form increasing amounts of tremolite and diopside with the eventual loss of quartz and nearly complete loss of talc. The detailed structure of the amphibole and the nature of the Mg enrichment were revealed using AEM analysis of individual amphibole crystals. The most abundant defects are triple-chain chain multiplicity faults (CMFs), which appear as isolated lamellae with single periodicity in short-duration experiments and increasingly as groups of lamellae with variable multiplicity and periodicity in longer duration experiments and especially in a long-duration experiment without retreatment. In this latter experiment, a calcian clinojimthompsonite domain extending 15 unit cells along the b axis was observed. The sample after 582 h and five retreatments shows, on average, 3.5% true solid solution with Mg expressed as the Mg-cummingtonite (MC) component after correction for the presence of CMFs. This sample is thought to most closely approach the equilibrium composition for the amphibole. The sample after 250 h with three retreatments has about 10%, whereas that after 582 h without retreatment has 0.6% MC component, the latter having a relatively high density of CMFs. This study affirms the importance that precursor non-amphibole biopyriboles play in the formation of tremolitic amphibole. [ABSTRACT FROM AUTHOR]