Volume 80, Issue 9-10

High-resolution B K-edge X-ray absorption near-edge structure (XANES) spectra of K 2 O-SiO 2 -B 2 O 3 -P 2 O 5 glasses are reported using synchrotron radiation. Two prominent features, peak a at about 194.0 eV and peak b at about 198.0 eV, are observed. On the basis of the qualitative MO diagrams of BO 3 3- and BO 4 3- clusters, peak a is assigned to the transition of B 1s electrons to the unoccupied B 2p z (π*) states for threefold-coordinated B ( [3] B),and peak b is assigned to the transition of B 1s electrons to the unoccupied B σ* states for fourfold-coordinated B ( [4] B).B K-edge XANES spectroscopy is established as a method for "fingerprinting" [3] B and [4] B in borate and borosilicate minerals, glasses, and melts. Also, the relative proportions of [4JBand [3]Bin the borosilicate glasses are determined from the integrated peak areas for [3] B and [4] B edge peaks and are shown to be generally in good agreement with recent 11 B MAS NMR measurements. However, the surface and near-surface structure of powder particles of per boric glasses containing P 2 O 5 appears to have a lower proportion of [3] B entities than the bulk.

Recently, correlations of 23 Na chemical shifts with structural parameters such as bond length, coordination number, and bond strength in inorganic compounds have been proposed. When considering only silicate materials we have found that some of these models lead to conflicting predictions. To test the applicability of these correlations to silicates, we collected 23 Na MAS NMR spectra of a pure, ordered albite (NaAlSi 3 O 8 ) from 25 to 352°C. Over this temperature range, the Na-O bond lengths increase, and the measured isotropic shifts relative to 1 M NaCI decrease linearly from -7.1 ppm at room temperature to -7.8 ppm at 352°C. This agrees with the observed trend in silicates of increasing bond length leading to more negative values of δ but is not consistent with the bond-valence model, which predicts a change of several parts per million in the opposite direction over this temperature range. Quadrupolar coupling constants were also seen to change, decreasing linearly with temperature from 2.69 MHz at 20°C to 2.30 MHz at 352°C. Static spectra measured from 22 to 1020 °C yielded values consistent with this in the low-temperature range and continued to decrease linearly to 1.50 MHz at the highest temperature. The 23 Na spin-lattice relaxation times (T 1 ) were collected between 25 and 1140 °C as well. T 1 decreases gradually up to about 500°C and then drops rapidly, indicating that there is a change in the relaxation mechanism at this temperature.

Cation ordering and structural changes in synthetic Mg 2 TiO 4 and Zn 2 TiO 4 spinels at temperatures across the polymorphic transition from the high-temperature cubic (Fd3m) to the low-temperature tetragonal (P4 1 22) structure are examined by 17 O magic-angle spinning (MAS) NMR (9.4 T) and Rietveld structure refinement of powder X-ray diffraction data. The 17 O NMR spectra of cubic Mg 2 TiO 4 and Zn 2 TiO 4 are similar, each showing one broad peak, positioned at 303 and 301 ppm, respectively. At the transition to the tetragonal phase, spectra of both Mg 2 TiO 4 and Zn 2 TiO 4 show significant narrowing because of the onset of long-range cation ordering in the tetragonal structure. The 17 O NMR spectrum of tetragonal Zn 2 TiO 4 shows two narrow peaks, at 301 and 273 ppm, corresponding to the two crystallographically distinct O sites in the tetragonally distorted spinel, showing that '70 chemical shift is sensitive to octahedral Zn- Ti substitution in Zn 2 TiO 4 . In contrast, the 17 O NMR spectrum of tetragonal Mg 2 TiO 4 shows only one peak, at 298 ppm. The structures of cubic and tetragonal Mg 2 TiO 4 and Zn 2 TiO 4 are compared. Tetragonal Zn 2 TiO 4 exhibits greater distortion than Mg 2 TiO 4 . at the M1, O1, and O2 sites. These subtle structural differences do not explain differences in the 17 O NMR spectra. The 17 O NMR spectra of the cubic Mg 2 TiO 4 and Zn 2 TiO 4 show no change with quench temperature above the transition to the cubic phase, suggesting that short-range ordering does not occur in cubic Mg 2 TiO 4 and Zn 2 TiO 4 . A two-phase region is observed for both Mg 2 TiO 4 and Zn 2 TiO 4 , below 664 and 561°C, respectively, where the cubic and tetragonal phases are shown to be at equilibrium. The 17 O peak position of MgTiO 3 is observed at 398 ppm. This chemical-shift displacement of 100 ppm to high frequency of Mg 2 TiO 4 is related to increased distortion in MgTiO 3 .

Transmission electron microscopy (TEM) revealed a commensurate modulated structure in a Na-rich alkali feldspar (Ab 66 Or 29 An 5 ) from a basalt from Anhui Province, China. High-resolution TEM images with wavelike (001) lattice fringes directly reveal one-dimensional modulation waves in the crystal. The average modulation period along the b axis is 14d 010 (≅180 Å), although both larger and smaller spacings have been observed. The modulated structure can be described as periodic stacking of (010) layer domains following the albite twin law. The relationship between the extended cell (supercell) of the modulated structure and the normal alkali feldspar unit cell (subcell) is as follows: a sup ≅ a sub , b sup ≅ 14b sub , c sup ≅ c sub , β sup ≅ β sub (subscripts "sup" and "sub" indicate the supercell and subcell, respectively). A possible space group for the modulated structure is Pm. In situ heating and cooling experiments suggest that the phase transition in this feldspar thin foil might be a weakly first-order and displacive transformation. The transition temperature is close to 500 °C for the crystal examined. The modulated structure formed during the phase transition of the feldspar from C2/m symmetry to C1̅ at a rapid cooling rate. A heuristic nonlinear dynamics model for the arrangement of triclinic domains in the feldspar at the phase transition from monoclinic to triclinic symmetry was developed on the basis of a Landau potential, a damping term accounting for the structural differences among neighboring units, and a periodic force in the crystal along the b axis. Numerical solutions indicate that periodic modulation arises during the monoclinic to triclinic transition only under special conditions relatively far from equilibrium, and a nonperiodic arrangement of triclinic domains along the b axis is the more general case. Therefore, according to this model, the modulated structure is a metastable and intermediate structure between monoclinic (C2/m) and triclinic (C1̅) phases. This type of modulated structure in Na-rich alkali feldspars may indicate high-temperature growth as a monoclinic crystal, followed by very rapid cooling into the triclinic stability field

Domain textures related to the I1̅-12/c displacive phase transition were observed by transmission electron microscopy (TEM) in synthetic samples from the Sr-rich region of the CaAl 2 Si 2 O 8 -SrAl 2 Si 2 O 8 join. Albite and, more rarely, pericline transformation twins are formed in samples with preexisting polysynthetic Carlsbad twins; they also cut preexisting b-type antiphase domains. Tweed textures associated with albite twin lamellae were observed in triclinic samples and, more rarely, in monoclinic samples close to the transition point. Progressive annealing of a monoclinic disordered precursor was performed to follow the monoclinic-triclinic transition at room temperature. Triclinic and monoclinic phases coexisted close to the transition, whereas tweed textures, rarely present in monoclinic grains, were retained in the triclinic phase after longer annealing. In situ high-temperature TEM experiments performed on a grain with pericline twins showed that the pericline twins disappeared at T c , but, during cooling, tweed textures appeared and then were replaced by albite twins at lower temperatures. The tweed textures are interpreted as originating from strain modulations induced by the displacive transition rather than modulation in Al-Si order, and they form close to the transition point as precursors of the albite and pericline twinning.

We report structure refinements of three ferromagnesian cummingtonite crystals with Mg/(Mg + Fe) ≈ 0.63. An unheated crystal with X M4 Mg = 0.051 has C2/m symmetry,but crystals heat treated at 600 and 700°C, with X M4 Mg = 0.167 and 0.237, respectively, have P2 1 /m symmetry. In P2 1 /m cummingtonite from this study and from Hirschmann et ai. (1994), the A silicate chain is S rotated and the B chain is 0 rotated. This differs from the structure of P2, /m manganoan cummingtonite, in which both A and Bchains are 0 rotated (Papike et aI., 1969). Documented in nature for the first time, the S-rotated silicate chains in amphibole suggest that, like the P2 1 /c to C2/c phase transition in clinopyroxene, the change of the A-chain configuration from S to 0 rotation is intrinsic to the P2 1 /m to C2/m transformation in ferromagnesian cummingtonite. Documentation of different space groups for magnesio-cummingtonite crystals that differ only in site occupancies confirms that the M4-site population controls the relative stabilities of the C2/m and P2 1 /m phases and places limits on the tolerance of the C2/m structure for Mg on the M4 site. At room temperature, ferromagnesian cummingtonite with X M4 Mg > 0.15 ± 0.02 has P2 1 /m symmetry. In the P2 1 /m structure, the separation between M4-O6A and M4-O6B and between M4-O5A and M4-O5B bond distances increases with increasing Mg content, as does the kinking angle (O5-O6-O5) of the A and B chains. Structural distortions away from C2/m symmetry, particularly in the configuration of the A chain, are greater for Fe-bearing cummingtonite than for Mn-bearing varieties at similar Mg concentration, indicating that for a given Mg content, ferromagnesian amphibole is more stable in the P2 1 /m structure than manganoan amphibole.

Transmission electron microscopy (TEM) of microstructures in the orthopyroxene of the Serra de Magé cumulate eucrite shows augite lamellae thicker than 500 Å, irregularly spaced within the orthopyroxene matrix and parallel to (100), with a* Opx ≡ a* Aug , [101]* Opx ≡ c* Aug , and b* Opx ≡ b* Aug . Streaking along a* owing to stacking disorder was observed in selected-area electron diffraction (SAD) patterns. Slabs of clinopyroxene were observed, but no Guinier-Preston zones were detected. Single-crystal X-ray diffraction (XRD) data were collected and structure refinements completed in space group Pbca, converging to R obs = 2.81 and 5.17% for the two samples. Because of the presence of some Okl forbidden reflections with k = 2n + 1 and l = 4n + 2, structure refinement was attempted in space group P2 1 ca, as suggested by previous studies, and in space groups P12 1 /c1 and P112 1 /a, as suggested by the Laue symmetry of the reflections, which is 21m rather than mmm. However, the high residual indices R and the anomalous values of many displacement parameters and bond distances did not support the lower-symmetry space groups. The violations of the extinction conditions of the Pbca space group and the monoclinic symmetry of the reflections are due to overlap of reflections from the C2/c augite lamellae. From the intensity of the reflections forbidden in Pbca space group, the calculated augite content ranges from approximately 4 to 12%.

Transmission electron microscope images of the zeolite ferrierite, prepared by ultramicrotomy as cross sections of the main-channel axis, revealed several types of crystallographic defects. Both shear faults and sigma transformations occur in the vug-filling bundles of ferrierite near Kamloops Lake, British Columbia. Sigma contraction {110} planes occur as discrete domains several unit cells thick. Shear faults involving displacements of (a ± b)/2 on {110} planes occur in lesser amounts. In addition, mordenite occurs in the Kamloops Lake sample, underscoring the frequent association of these zeolites, their crystal chemical and structural affinities, and the importance of TEM-scale observations in petrologic studies of fine-scale intergrowths of zeolites.

Single crystals of lead aluminosilicate hollandite, with composition Pb 0.8 Al 1.6 Si 2.4 O 8 , have been synthesized at 16.5 GPa and 1450 °C. Crystals are tetragonal [space group I4, Z = 2, a = 9.414(3), c = 2.750(3) Å, V = 243.7(3) Å 3 ]. Si and Al are disordered on the octahedral site. Pb is best modeled by two atom sites, one on the fourfold axis and the other split, lying off the fourfold axis. This feature was predicted by Post and Burnham (1986) but not previously observed.

Lattice parameters of calcite were measured at simultaneous high pressures and temperatures up to 10 kbar and 500°C. Samples of Solnhofen limestone and distilled, deionized water were loaded in a hydrothermal diamond-anvil cell. In situ energy-dispersive X-ray diffraction was used to determine lattice parameters along five H 2 O isochores with densities of 1.040, 0.964, 0.760, 0.670, and 0.595 g/cm 3 from 30 to 500°C; these densities correspond to the ice-melting temperature of -7.7 °C and the liquid-vapor homogenization temperatures 92.4,274.5, 318.9, and 344.6 °C, respectively. In addition, data along the P axis were collected by hydrostatic compression at room temperature and along the T axis by heating at atmospheric pressure. Our results show that anisotropic thermal expansion of calcite continues up to 10 kbar, therefore making it a good double internal X-ray standard. Both a and c lattice parameters are fitted to second-order polynomials of pressure, temperature, and a cross term.

Proton nuclear magnetic resonance (NMR) and visible and near-infrared (VNIR) reflectance spectral data and heating experiments indicate that the Mg-rich calcite (commonly referred to as high-Mg calcite and hereafter as HMC) that is produced by six different species of echinoids contains H 2 O with mobilities and H bond strengths ranging from those characteristic ofliquid H 2 O to those observed in hydrated magnesium carbonate minerals. Total H 2 O (inorganic OH - and H 2 O, i.e., that occurring in inorganic phases rather than organic compounds) contents of echinoid HMC range from 1.97 to 3.38 wt% and differ in amount from one species to another and, within one species, between different skeletal elements. The proportion of total H 2 O that is rigidly bound is comparatively constant, ~0.4 in all samples. Total H 2 O content shows no correlation with Mg/Ca ratio in these samples. Although echinoid ossicles are generally noted for their chemical homogeneity, the total H 2 O content of echinoid HMC shows considerable variation within individual ossicles over distances of tens to hundreds of micrometers. Echinoid HMC contains small amounts of OH - (a few tenths to hundredths of a percent by weight). Brucite does not occur, and OH - is present either in solid solution or in a hydrated magnesium carbonate mineral.

The kinetics of the transformation of synthetically prepared mackinawite to hexagonal pyrrhotite have been studied to clarify the relationship between these two phases. Kinetic analysis using the Johnson-Mehl equation is based on measurements of the fraction of hexagonal pyrrhotite obtained by heating mackinawite samples in the temperature range of 530-545 K. This gives isothermal rate constants from which an activation energy of 493 kllmol and a frequency factor of 3.7 × 10 45 min -1 have been established. The mechanism proposed for this transformation is that of a solid-state diffusion process. Kinetic data derived from this experiment may be used to show that persistence of mackinawite in nature can be explained by kinetic factors alone. Reactions at low temperatures, in which mackinawite is oxidized or further sulfidized, are therefore more important than the kinetically inhibited transformation to hexagonal pyrrhotite. The irreversible mackinawite to hexagonal pyrrhotite relation is compared with the reversible tetragonal FeSe to NiAs-type FeSe phase transformation. This contrast in stability behavior is examined in terms of the thermal stability of the respective tetragonal phases relative to the upper magnetic ordering temperatures of their corresponding hexagonal phases.

Microintergrowths of titaniferous spinel and ilmenite have been experimentally produced in the system Fe- Ti-Cr-O by annealing in vacuo at constant temperatures in the range 700-1100 °C of spinel + ilmenite assemblages previously synthesized at 1300 °C under controlled low f O₂. Depending on the original f O₂ of synthesisat 1300°C,annealing produces three types of microintergrowths: Type I (lowest f O₂ : 10 -11 ) has rims and trellislike lamellae of ilmenite as well as small blebs of metallic iron; type II (f O₂ : 10 -10 -10 -9) shows spinel with rims and a few lamellae of ilmenite; type III (highest f O₂ : 10 -8.5 ) displays both spinel with rims and lamellae of ilmenite and ilmenite with rims and lamellae of spinel. On the basis of microprobe analysis, including precise determination of the 0 contents, it is shown that the synthetic microintergrowths did not form by a redox process. Rather, the textures represent ex solution caused by vacancy relaxation in spinel (and, for the most reduced samples, in ilmenite), according to the substitution scheme Fe 2+ 2 Ti 4+ -1 ⃞ -1 (1) The presence of metallic iron in the most reduced samples (type I) results from the shift of the iron saturation surface toward more Fe-rich compositions with decreasing temperature. The rims and lamellae of spinel around and within ilmenite in the type-III samples result from the combination of Exchange 1 and the well-known substitution Fe 2+ + Ti 4+ = 2Fe 3+ . These experimental results show that natural spinel + ilmenite microintergrowths do not always result from either an oxidation or a reduction process. In particular, textures similar to those of type I, which were observed in lunar basalts, may simply record cooling under O 2 -conserving conditions. In most terrestrial rocks, however, spinel + ilmenite intergrowths are certainly best interpreted as resulting from oxidation of at least a subsystem of the rock.

Phlogopite-quartz stability in the system KAlO 2 -MgO-SiO 2 -H 2 O represents a simple model for the stability of biotite in many common, crustal rock types during granulitefacies metamorphism and in the crystallization of intermediate to felsic magmas. Despite many previous experimental and theoretical topological studies, disagreement remains on the stoichiometries and P- T locations of many of the six equilibria emanating from the invariant point at which phlogopite, β-quartz, high sanidine, orthoenstatite, aqueous fluid, and hydrous alumino-silicate melt coexist. In their experimental and modeling study, Vielzeuf and Clemens (1992) resolved most of the disagreement among data sets for the fluid-absent melting reaction Phl + Qtz = En + Sa + M (Fl). The five remaining equilibria form the main subject of this paper: Phl + 3Qtz = 3En + Sa + H 2 O (M) Phl + Qtz + Sa + Fl= M (En) En + Qtz + Sa + Fl= M (Phl) Phl + Qtz + Fl= En + M (Sa) En + Sa + Fl= Phl + M (Qtz). Previous data on the equilibria are reevaluated, and I present further experimental constraints showing that the data sets are mostly reconcilable and that some seeming contradictions result from misinterpretations of the experimental products in relation to the compositions of the starting materials. I present the most accurate possible depiction of the P- T topological relations, given our current knowledge of the system. The very tight experimental constraints on the locus of reaction (M) make it possible to refine the value for the free energy of formation of Al-Si-disordered phlogopite, reducing the uncertainty on the calorimetrically determined value (Clemens et aI., 1987) by 46%. The new value for ΔG 0 f Phl is -5838.16 ± 3.37 kl/mol. Contrary to the conclusions of some previous studies, reactions (Sa) and (En) are nowhere near coincident. The (Sa) reaction exhibits graceful slope changes that vary in sympathy with those for (F1). The ultimate controls on this are probably subtle variations, with pressure, of the partial molar volume and entropy of H 2 O in the hydrous aluminosilicate melt. Reaction (Qtz) probably has a slightly negative (almost flat) dP/dT slope, with melt on the high-T side, which seems more logical than previous depictions of the reaction with a positive slope and melt on the low- T side. Melt MgO contents are always very low. Thus, although reactions (En) and (Phl) lie at lower T than Sa + Qtz + Fl = M, the amount of solidus depression due to the presence of MgO is very small. If (Fl) is taken as a model for fluid-absent biotite breakdown in quartzofeldspathic rocks and metagraywackes at granulite grade, its constant positive or infinite dPldT slope would allow adiabatic ascent of the melt to very high crustal levels. This agrees with the inferred origin of many granitoids and silicic volcanic complexes. On the other hand, many migmatites have formed through fluid-saturated partial melting reactions in the amphibolitefacies middle crust. If this occurred by reactions similar to (Sa), the negative dPldT slopes of such equilibria would seem to prohibit magma ascent. However, if the natural reactions exhibit slope changes similar to that of (Sa), it may be possible for some H 2 O-saturated melts formed by biotite breakdown to ascend significant distances without crossing their solidi. Finally, experimental and thermodynamic constraints suggest that the anthophyllite dehydration reaction (Ant = 7En + Qtz + H 2 O) intersects reaction (Sa) at between 600 and 700 MPa. This would render (Sa) metastable and introduce new equilibria. The talc dehydration reaction (7Tc = 3Ant + 4Qtz + 4H 2 O) also appears likely to intersect one of these, at some pressure above 800 MPa, introducing further complications. Though they are likely to be stable, none of these new talc and anthophyllite melting equilibria have been encountered experimentally. This is almost certainly a consequence of the huge kinetic barrier to the crystallization of anthophyllite and may mean that it is impossible to determine experimentally the complete stable phase diagram. However, this is a problem peculiar to experimental studies in KAlO 2 -MgO-SiO 2 -H 2 O and does not detract from the usefulness of the metastable equilibria for modeling petrogenetic processes because the talc and anthophyllite equilibria are unlikely to interfere in natural Fe-bearing systems.

The pressure-temperature conditions of the reactions governing the high-pressure stability of talc were investigated in experiments on the bulk composition Mg 3 Si 4 O 10 (OH) 2 + H 2 O 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 THERMOCALC v2.4 (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 <730°C. At ~5 GPa, 710 °C, there is an invariant point involving talc, 10 Å phase, enstatite, coesite, and vapor. This point marks the highest pressure at which talc is stable. Above it, the thermal stability of 10 Å phase expands with increasing pressure. Its maximum stability is unknown. Talc is a common hydrothermal alteration product of peridotite and may transport H 2 O in subducting slabs from shallow depths to depths of ~150 km. It may also crystallize in overlying mantle-wedge peridotite after infiltration of fluid from the slab, and its dehydration in the mantle wedge may lead to partial melting. The 10 Å phase may transport H 2 O in silica-enriched hydrated peridotite to depths of at least 200 km.

Localized depletion of 18 O and 13 C in a thin subhorizontal marble layer in the Adamello contact aureole, Southern Alps, Italy, resulted from fluid infiltration focused along a crosscutting dike. Values of δ 18 O and δ 13 C in calcite from the 1 m long profile decrease systematically from sedimentary values of δ 18 O = 22‰(SMOW) and δ 13 C= 0‰(PDB) to δ 18 O = 12.5‰ and δ 13 C ≈ -7‰ near the dike. The presence of clinozoisite and garnet in the 5-15 cm thick marble layers near the granodiorite dike indicates H 2 O-rich fluid conditions (X CO₂ ≈ 0.01 ). The O and C isotope profiles were compared with one- and two-dimensional models of advective-dispersive isotope transport. Individually the isotope profiles fit one-dimensional transport models well. However one-dimensional models, using equilibrium fluidrock exchange or a kinetic formulation, do not explain the relative locations or shapes of the two isotope-exchange profiles given the petrologic constraint of X CO₂ ≈ 0.01 for the infiltrating fluid. Excellent agreement with the δ 18 O and δ 13 C data is obtained using a twodimensional model that specifies (1) a high-permeability zone in marble near the dike that focuses fluid flow parallel to the dike and (2) a lower permeability zone in marble away from the dike where isotope exchange is dominated by molecular diffusion. The combined constraints imposed by phase equilibria and two isotope tracers allow two-dimensional fluid flow to be inferred from one-dimensional data. The results emphasize that isotope distributions resulting from multidimensional flow may fortuitously fit one-dimensional transport models if isotope tracers are considered independently. The use of multiple tracers coupled to fluid-composition constraints is therefore essential to discriminate between various transport models.

Computer simulation techniques were used to investigate defect formation and the diffusion of O and OH groups in grossular garnet. Diffusion of both species along an octa-hedral edge was simulated, and activation energies for the jump were calculated. This Jump represents the greatest distance along the shortest diffusion route through the grossular lattice, and hence the calculated activation energy is comparable to experimentally determined values. The activation energy for 0 diffusion in the extrinsic regime is predicted to be 106 kJ/mol, whereas for the high-temperature intrinsic regime, we calculate an activation energy of 383 kJ/mol. Extrinsic diffusion of the OH group along the same pathway also has an activation energy of 106 kJ/mol. However, when structurally bound H 2 O m the form of hydrogarnet defects is introduced into the system the OH diffusion activation energy is reduced to 67 kJ/mol. The high activation energy combined with experimentally available diffusion coefficients for O in garnet suggest that O mobility is very low at metamorphic temperatures.

Calcic garnet from a calc-silicate hornfels from the contact-metamorphic aureole of the Duluth complex, northeastern Minnesota, has TiO 2 compositions ranging from 0 to 13 wt%. The concentrations of Fe, Mg, Al, Ti, and Si are highly correlated, and 93% of the total variance can be ascribed to three principal components formed by the additive component grossular and several exchange components. The first is a combination of the exchangesTiSi -1 , MgTiAl -2 , and MgCa -1 . The second represents the substitutions of Al for Fe and of Fe for Ca. The third represents the substitution of Cr for Al. Most of the variance occurs along a linear trend from Ca 3 Al 1.72 Fe 0.28 Si 3 O 12 to Ca 3 Mg 0.33 Al 0.67 Fe 0.67 Ti 1.0 Si 2.33 O 12 . Within this trend, a gap in composition occurs between 6 and 12 wt% TiO 2 . Separate grains of Ti-rich and Ti-poor garnet occur in contact with sharp boundaries. No exsolution lamellae were observed, however. Immiscibility appears to be blocked by the presence of Cr.

Petrographic observations of subsolvus granites at Strange Lake indicate that the sodium zirconosilicate elpidite crystallized under magmatic conditions, but that the calcium zirconosilicates armstrongite and gittinsite are secondary. This interpretation is consistent with the extensive solid solution displayed by elpidite and the restricted compositions of armstrongite and gittinsite. Both calcium zirconosilicate minerals show textural evidence of having replaced elpidite, and in the case of gittinsite, with major volume loss. In the near-surface environment, gittinsite plus quartz fill the volume formerly occupied by elpidite. At greater depth, gittinsite and armstrongite partially replaced elpidite but are not accompanied by quartz, and abundant pore space is observed where gittinsite is the principal secondary phase. Below 70 m elpidite is generally unaltered. Replacement of elpidite by armstrongite is interpreted to have been a result of the cation-exchange reaction in which volume is nearly conserved, and replacement by gittinsite is thought to have resulted from the reaction which is accompanied by a 65% volume reduction. An alteration model is proposed in which external Ca-rich, quartz-undersaturated fluids dissolved elpidite and replaced it with gittinsite, where a H₄SiO⁰₄ was buffered mainly by the fluid (high water-rock ratio), and with armstrongite plus gittinsite, or armstrongite alone, where a H₄SiO⁰₄ was buffered to higher values by the rock (low water-rock ratio). The formation of gittinsite created extensive pore space that was subsequently filled, in the upper part of the pluton, when the fluid became saturated with quartz as its temperature decreased during the final stages of alteration.

The Kawashimo alkali basalt of the southwest Japan arc has peridotite xenoliths with a wide lithological range, from lherzolite {Fo of olivine, 89; Cr' [=Cr/(Cr + Al) atomic ratio] of spinel O.IO}to harzburgite (Fo of olivine, 91; Cr' of spinel, 0.54). Reaction zones between orthopyroxene and alkali-basalt melt are low-pressure analogues of mantle-melt interaction products and consist of two subzones: a fine-grained inner subzone (adjacent to orthopyroxene) and a relatively coarse-grained outer subzone. In both xenolith types the reaction products are olivine + diopsidic clinopyroxene ± spinel ± glass, but the spinel concentrations are remarkably different around lherzolite from those around harzburgite. Cr-bearing spinel is concentrated only in the outer subzone on harzburgite orthopyroxene (Cr' > 0.14); the inner subzone on harzburgite orthopyroxene and both the inner and outer subzones on lherzolite orthopyroxene (Cr' = 0.05) are almost free of spinel. The remarkable enrichment of spinel in the outer subzone on the harzburgite orthopyroxene suggests a mechanism of spinel concentration, Le., the origin of podiform chromitite is related to interaction between Cr-rich orthopyroxene and basaltic melt. This observation for the Kawashimo xenoliths is concordant with the near absence of podiform chromitite in Iherzolitic mantle. Chromian spinel could be concentrated if a relatively silica-rich secondary melt, produced by interaction between pyroxene-undersaturated magma and harzburgite orthopyroxene, is mixed with a primitive magma in the upper mantle.

Phase equilibria in the system NASH calculated using Berman’s (1988) data base are inconsistent with field and petrological data on the stability of paragonite and the assemblage albite + kaolinite. This inconsistency appears to result from erroneous values for entropy and enthalpy for kaolinite. Although the entropy value of 203.7 J/(mol· K) in Berman’s (1988) data base is based on some calorimetric measurements, it is -15 J/(mol·K) greater than values derived by various summation methods. Using a set of experimental and empirical constraints, it is possible to derive by mathematical programming an entropy of 187.46 J/(mol·K) and a corresponding enthalpy of 4129.92 kJ/mol, values that are very close to estimates obtained by oxide summation methods. These values are therefore consistent with most available experimental data involving kaolinite and produce phase diagrams that are also consistent with geological observations. The discrepancy between most calorimetrically based entropies and values derived by oxide summation methods is attributed to impurities of amorphous Al(OH) 3 in the samples used for calorimetry.

The crystal structure of synthetic pääkkönenite, Sb 2.07 As 0.93 S 2.00 , as determined. This study revealed that, in spite of the strong orthorhombic pseudosymmetry shown by Weissenberg photographs, the structure is monoclinic (space group C2/m), with a = 10.75(2), b = 3.959(3), c = 12.49(2) Å, β= 115.25(8)°, V = 481(1) Å 3 , and Z = 4. The crystal structure was solved by direct methods using intensity data collected from a twinned crystal. The refinement led to an R index of 4.58%. The observed pseudosymmetry was shown to arise from {001} twinning. The structure can be described as a sequence of sheets stacked perpendicular to the a axis and mutually displaced by ½b. Each sheet consists of Sb 4 S 4 As 2 ribbon-like units similar to the Sb 4 S 6 chains of stibnite but linked together by As-As bonds. The structural relationships with stibnite are discussed to explain the formation mechanism of pääkkönenite and the presence of twinning. X-ray powder data for synthetic pääkkönenite are also reported.

Reederite-(Y), ideally, (Na,Mn,Fe) 15 (Y,REE) 2 (CO 3 )(SO 3 F)Cl, is a new mineral from Mont-Saint-Hilaire, Quebec. It occurs as blocky, yellow to orange-brown grains up to 2 mm. Associated minerals include trona, shortite, petersenite-(Ce), catapleiite, minor analcime, and manganotychite. The mineral has a vitreous luster and white streak. It is soft (Mohs hardness 3-3.5) and brittle, with a conchoidal fracture and perfect {001} cleavage. Reederite-(Y) is uniaxial negative, ω = 1.548(1) and ε = 1.537(1). It is hexagonal, space group P6̅, a = 8.773(1), c = 10.746(2) Å, and Z = 1. The strongest X-ray powder diffraction lines are [d (Å), I, (hkl)] 2.532, 100, (212); 4.39, 80 (102); 2.774, 80, (113); 2.240, 80, (213); 6.20,40, (101); 1.657,40 (116,314,322,410); and 2.067,30, (105,303). The infrared spectrum is given. An electron microprobe analysis gave Na 2 O 34.04, CaO 0.70, MnO 1.23, Al 2 O 3 1.31, Y 2 O 3 10.24, La 2 O 3 1.39, Ce 2 O 3 3.54, Nd 2 O 3 1.99, Er 2 O 3 1.19, DY 2 O 3 1.39 (plus minor amounts of Fe, Pr, Sm, Gd, and Yb oxides), SO 3 5.07, CO 2 (calc) 31.91, F 1.86, and Cl 2.05, -O ≡ F + Cl 1.24, total 99.14 wt%. D meas = 2.91 g/cm 3 , D calc = 2.85 g/cm 3 . The structure has been refined to R = 3.2%. The carbonate layers are thick slabs accommodating (CO 3 ) 2- groups oriented perpendicular to the {001} layering, the large cation polyhedra, and the Cl - and (SO 3 F) - anions. The Na atoms adopt a variety of distinct coordinations. This is the first reported natural occurrence of a fluorosulfate anion.

Plumboferrite from the type locality at Jakobsberg, Filipstad, Sweden, has been reexamined. It occurs in assemblages confined to bands in metamorphic carbonate rock and is associated with hematite, magnesioferrite-jacobsite-magnetite solid solutions, lindqvistite, calcite, andradite, phlogopite, hedyphane, svabite, hematophanite, native copper, and cuprite. Six samples investigated fall in a narrow composition range: PbO 33.9-34.6, Sb 2 Os 0.0-0.3, Fe 2 O 3 62.4-63.9, MnO 0.6-1.8, TiO 2 0.1-0.5, SiO 2 0.0-0.2, Al 2 O 3 0.0-0.1, MgO 0.1-0.4 (weight percent). The mineral is moderately anisotropic and optically uniaxial (-). Reflectance values obtained in air are 25.6-24.2% (470 nm), 24.5-23.5% (546 nm), 23.7- 22.8 (589 nm), and 22.6-22.1% (650 nm). VHN 100 = 882 and D calc = 6.12(1) g/cm3. Single-crystal X-ray studies show that plumboferrite is hexagonal, essentially P6 3 /mmc, with the cell dimensions a = 5.931(1), c = 23.551(2) Å, and V = 717.4(2) Å 3 (refined from powder data). Overexposed precession X-ray photographs indicate the presence of a supercell with a' = √3a = 10.27 Å and cʹ = 3c = 70.7 Å (hexagonal setting). The structure of piumbo ferrite has been refined from the 394 most significant (I> 5σ l ) X-ray reflections with (sin θ)/λ ≤ 0.81 Å -1 to R = 3.9%. It has a defect magnetoplumbite-type structure that can be described in terms of two basic structural units, the R and the S (spinel) blocks. The stacking sequence of such blocks along c is RSRʹSʹ, where ideally R = (Pb 2 Fe 5 O 11-6 ) and S = (Fe 6 O 8 ) for plumboferrite. The formula (with Z = 2 for the subcell) for the investigated material approaches Pb 2 Me 0.33 Fe 10.67 O 18.33 with, Me = Mn 2+ , Mg, rather than PbFe 4 O 7 , which is still prevalent in the literature. The departure from stoichiometry (19 O atoms per formula unit) reflects partially occupied O3 positions in the R blocks. The weak superlattice reflections observed (not included in the present refinements) probably result from the ordering of split Pb positions, O vacancies, or both.

Partial molar densities of oxides in natural silicate melts can be estimated to useful precision by exploiting their strong correlation with densities of the corresponding solid oxides, reflecting the close structural similarity between silicate solids and silicate melts. Using this extrapolation method, partial molar density (in grams per mole-cubic centimeter at 1400 °C) is inferred to be 2.0 for SO 3 and 2.3 for P 2 OS; i.e., the partial molar volumes, V i, m(in cubic centimeters per mole), are 40 and 61, respectively. The inferred d V i,m /dT (in cubic centimeters per mole-kelvin) are 0.0056 for SO 3 and 0.0085 for P 2 OS 5. Analogous estimates are also given for V 2 O 5 , Cr 2 O 3 , V 2 O 3 , ZrO 2 , La 2 O 3 , Ce 2 O 3 , and Nd 2 O 3 . The inferred uncertainty for the V i,m estimates is 12% (1σ, relative), although the V i,m for Cr 2 O 3 is somewhat more uncertain because on melting, this oxide probably changes from octahedral to largely tetrahedral coordination.

The location of minor amounts of Ni (0.44 wt% NiO) in the structure of San Carlos olivine has been determined using Ni K-edge fluorescence EXAFS spectroscopy. The mean Ni-O distance, d(Ni-O) = 2.08 Å, suggests a preferential location of Ni in the M1 site. The observed distance is smaller than the Mg-O distance in the M1 site of α-Mg 2 SiO 4 [mean d(M1-O) = 2.10 Å] and is similar to the average M1-O distance in olivines close to the San Carlos composition [mean d(M1-0) = 2.09 A]. The most important relaxation effect resulting from the Ni for Mg substitution is a smaller mean M1-O distance. The medium-range distribution of Ni (up to 4.5 Å from the central Ni atom) in the San Carlos olivine structure was estimated by modeling EXAFS spectra, considering various M1 and M2 cation distributions around M1 (Ni) sites. These models suggest medium-range ordering of Ni and Fe in adjacent M1 and M2 sites of the olivine structure and nonideal behavior of Ni in San Carlos olivine.