Showing papers in "American Mineralogist in 1990"

Two new geobarometers for garnet amphibolites, with applications to southeastern Vermont

Abstract: Two new geobarometers for the assemblage garnet + hornblende + plagioclase + quartz have been calibrated on the basis of the equilibrium 6 Anorthite + 3 Tremolite = 2 Grossular + 1 Pyrope + 3 Tschermakite + 6 Quartz and its Fe end-member equivalent. Data representing 37 natural samples that equilibrated at conditions of 2.5 to 13 kbar and 500 to 800°C were fitted to the general equation -RT In Keq= A - BT + (P - I)C by using a weighted least-squares method. Multiple correlation coefficients are high (0.98 and 0.97). Lines of constant Keqhave extremely shallow slopes in P- T space (- 2 to + 8 bars per degree), suggesting that pressures may be deduced precisely, even where temperatures are only poorly constrained. Typical analytical errors and temperature imprecisions propagate to very small errors in pressure (about ::!:500 bars). Caution is advised in applying the barometers outside the range of calibrant-mineral compositions. Application of the calibrations to samples from southeastern Vermont near the Strafford, Chester, and Athens domes documents relatively high pressure metamorphism (7 to 10 kbar) for these structures.

Crystal structure determinations of synthetic sodium, magnesium, and potassium birnessite using TEM and the Rietveld method

Abstract: Determination de la structure cristalline des sous-mailles des phases synthetiques analogues a la birnessite riches en Na, Mg et K. Cristallisation dans C2/m avec pour Na: a=5,175, b=2,850, c=7,337 A, β=103,18°; pour Mg: a=5,049, b=2,845, c=7,051 A, β=96,65°; pour K: a=5,149, b=2,843, c=7,176 A, β=100,76°. La structure generale de la birnessite est analogue a celle du chalcopanite. La surstructure proviendrait de l'ordonnation de l'eau et des cations dans les couches intermediaires, positions differentes dans les trois structures

Hydroxide in pyroxene; variations in the natural environment

Abstract: A suite of 51 pyroxenes from a wide range of geological environments was examined with infrared spectroscopy. Small amounts of OH occur in nearly all the samples, suggesting that OH groups are a minor component of most pyroxenes. The OH concentrations vary (0.002-0.12 wt% OH) as a function of geological setting with the greatest amounts occurring in mantle-derived samples. The intensities of certain OH absorption bands are somewhat correlated with pyroxene composition, in particular with the presence of trivalent cations. This suggests that crystal chemical or compositional factors also control the OH incorporation. The OH incorporated in the pyroxene structure is easily distinguishable from OH owing to amphibole lamellae, which usually occur in diopsides and often in orthopyroxenes. OH in pyroxene is probably a function of the activity of hydrous components during crystallization but may also depend on postcrystallization changes in the geological environment.

High-pressure crystal chemistry of stishovite

Abstract: A high-pressure single-crystal X-ray difraction study of stishovite, the highest pressure polymorph of SiO, known, has been completed to 16 GPa. The compressibility of stishovite is anisotropic with a approximately twice as compressible as c. Consequently, the axial ratio c/a increases with pressure. The observed unit-cell compression gives an isothermal bulk modulus of 313 GPa using a Birch-Murnaghan equation of state. With increasing pressure, the primary structural response is the compression of the Si-O bonds and O' ' ' O separations. Neither the O-Si-O angle nor Si-O-Si angles show a sigrificant change with pressure. The longer axial Si-O bonds are more compressible than the shorter equatorial Si-O bonds. Among the O. . .O separations, O(l). . .O(1) (i.e., the c axis) is least compressible, followed by the O( I ) . . . O(2) separation, the shared octahedral edge, and lastly by the o(l)...o(3) separation. The incompressibility of the o(l).'.o(l) separation can be explained by the nature of the strong Si'..Si repulsive forces across the shared octahedral edge. Moreover, the decrease in the O(l). . .O(2) separation, which is the shared octahedral edge, increases the shielding between the Si atoms. The SiOu octahedra in stishovite show no change in distortion with increasing pressure. The polyhedral bulk modulus of SiO. is 342 GPa, the largest known value among octahedral units of rutile-type oxides.

Clay mineral authigenesis in coal and shale from the anthracite region, Pennsylvania

Abstract: Textural, chemical, and mineralogical analyses of authigenic clays in anthracite-rank coal and associated shale from eastern Pennsylvania have allowed a better understanding of the parameters controlling diagenesis and perhaps coalification in this region. Minerals in anthracite occur in distinct assemblages associated with the following microenvironments: coal matrix, two orthogonal joint sets (termed systematic and nonsystematic cleat), and a third joint set. Mineralogical differences among microenvironments allow inferences about clay-mineral origins and the parameters that controlled mineral authigenesis. Kaolinite occurs in the shale, and all microenvironments within the coal seam. Authigenic minerals that replaced preexisting kaolinite during the latest stage ofcoalification (anthracitization) at T > 200 'C include NHu-rich illite, pyrophyllite, and the following minerals, which are primarily restricted to the systematic cleat set: tosudite (Rl-ordered, dioctahedral, mixed-layer chlorite/smectite), sudoite (di, trioctahedral chlorite) and rectorite (Rlordered, mixedJayer paragonite/smectite). Alteration of smectite to illite may be responsible for formation of authigenic illite and Na-bearing illite, which are present only in the shale and coal matrix, and Fe-, Al-rich chlorite and quartz in the third joint set. The chemical components for authigenesis appear to have multiple sources: AI and Si from preexisting kaolinite and quartz, N from local organic matter, Mg and Na (for tosudite, sudoite, and rectorite) largely from metasomatic hydrothermal fluids, Fe and Mg for Fe-, Al-rich chlorite from smectite illitization. Minerals in the shale, the coal matrix, and the nonsystematic cleat set are interpreted to represent authigenesis in a low-permeability environment (closed-system alteration); however, the assemblage sudoite * tosudite * rectorite in the systematic cleat set is interpreted to be the result ofone or two stages ofhydrothermal alteration (open-system alteration). We suggest hat differences in minerals between the two nearly perpendicular cleat sets are the result of permeability differences which were maintained by an anisotropic lateral stress field created by plate convergence during the Alleghanian orogeny. Hydrothermal alteration may be related to large-scale basinal flow induced by Alleghanianage uplift; such migrating fluids could also have transported heat from depth and thereby significantly increased the rate and rank ofcoalification in this region.

Hydroxyl contents of accessory minerals in mantle eclogites and related rocks

Abstract: Infrared spectra in the region 3000 to 3800 cm^(-1) have been measured on coesite, kyanite, sanidine, and rutile occurring in inclusions of eclogite, and on garnet and spinel in an inclusion of alkremite from South African kimberites. Coesite and sanidine contain virtually no H_2O or OH, although one sanidine crystal was observed to contain abundant H2O, apparently as fluid inclusions of molecular H2O. Kyanite contains significant OH, consistent with previous results for similar samples. Corundum gives no indication of the presence of H_2O or OH. Rutile containing significant amounts of Fe, Cr, Al, and Nb also contains significant amounts of structural OH. Garnet in alkremite contains small amounts of molecular H_2O, whereas the spinel coexisting with this garnet gives no indication of H_2O or OH, indicating that the spinel structure is not able to accommodate significant amounts of OH. Ammonium contents of all samples were less than the detection limit (about 10 ppm).

The system tonalite-H_2O at 15 kbar and the genesis of calc-alkaline magmas

Abstract: Experimental phase relations and well-characterized mineral and melt compositions have been determined for a Sierra Nevada tonalite (andesite) with 2.5 to 10 wt% H_2O added at 15 kbar and 850 to 1100 °C. All results are from experiments conducted in Au capsules (minimal Fe loss to capsule) at oxygen fugacities measured to be near Ni-NiO. With increasing water content, the liquidus mineral changes from clinopyroxene at <3 wt% H20 to garnet, followed within 50 °C by clinopyroxene at 3 to 9 wt% H_2O, to hornblende at water contents greater than ~9 wt%. Microprobe analyses of glasses quenched from within the tonalite melting interval show an enrichment in Ca relative to (Mg + Fe), where Fe represents total Fe as Fe^(2+), with increasing SiO_2 instead of the typical calcalkaline trend of approximately constant Ca/(Mg + Fe) from basalt through rhyolite. The results document the importance of garnet and clinopyroxene in andesitic to dacitic compositions at deep-crustal to uppermost-mantle pressures, and they indicate that it is not possible to directly produce magmas of calc-alkaline composition at 15 kbar and the investigated range of water contents by either (1) partial melting of tonalitic to gabbroic (eclogitic) lower crust or (2) crystal fractionation from melts of andesitic to basaltic composition. Melting or fractionation at lower pressures or with lower water contents may produce calc-alkaline liquids as plagioclase replaces garnet as the major Al-bearing phase. Cale-alkaline rocks showing evidence of garnet-liquid equilibration in their REE patterns must also have undergone lower-pressure fractionation in order to explain their major-element compositions.