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At least 145 records · Page 8Linked to original sources
USGS1990

Heat capacities and thermodynamic properties of annite (aluminous iron biotite)

The heat capacities have been measured between 7 and 650 K by quasi-adiabatic calorimetry and differential scanning calorimetry. At 298.15 K and 1 bar, the calorimetric entropy for our sample is 354.9??0.7 J/(mol.K). A minimum configurational entropy of 18.7 J/(mol.K) for full disorder of Al/Si in the tetrahedral sites should be added to the calorimetric entropy for third-law calculations. The heat capacity equation [Cp in units of J/mol.K)] Cp0 = 583.586 + 0.075246T - 3420.60T-0.5 - (4.4551 ?? 106)T-2 fits the experimental and estimated heat capacities for our sample (valid range 250 to 1000 K) with an average deviation of 0.37%. -from Authors

American Mineralogist↗
USGS1991

Boromuscovite, a new member of the mica group, from the Little Three mine pegmatite, Ramona district, San Diego County, California

Boromuscovite, ideally KAl2(Si3B)O10(OH,F)2, in which [4]Al is replaced by B relative to muscovite, occurs as a late-stage, postpocket rupture mineral within the New Spaulding Pocket, main Little Three pegmatite dike. The mineral is white to cream colored and occurs as a porcelaneous veneer and coating on primary minerals. The average grain size is less than 3-4 ??m, but the coatings may be as much as 1 cm or more thick. Fragments of topaz, albite, elbaite, and other pocket minerals are included in the coating. The boromuscovite precipitated from a late-stage hydothermal fluid; it occurs only as a snowlike coating. Chemical composition, unit-cell parameters, Mohs hardness, cleavage, fracture, and optical properties are reported. -from Authors

American Mineralogist↗
USGS1991

Heat capacities and entropies of sillimanite, fibrolite, andalusite, kyanite, and quartz and the Al2SiO5 phase diagram

Low-temperature heat capacities for sillimanite, fibrolite, and both fine-grained and coarse-grained quartz have been measured. Superambient heat capacities have been measured for four sillimanite, two andalusite, one kyanite, and two fibrolite samples. Equations are recommended for the temperature dependence of the heat capacities of kyanite, andalusite, sillimanite, and fibrolite. The heat capacity functions have been combined with thermal expansion (fibrolite and sillimanite reported here), enthalpy of solution, and phase equilibrium data in order to construct a phase diagram for the Al2SiO5 polymorphs. -from Authors

American Mineralogist↗
USGS1991

Precise determinations of the equilibria kyanite <--> sillimanite and kyanite <--> andalusite and a revised triple point for Al 2 SiO 5 polymorphs

Investigates the equilibria kyanite ??? sillimanite and kyanite ??? andalusite using the piston-cylinder apparatus to refine the phase relationships among Al2SiO3 polymorphs. Experiments at the US Geological Survey constrain the kyanite ??? sillimanite equilibrium to 14.3-14.6, 11.4-11.7, 9.5-10.0, and 6.0-6.5 kbar at 1000, 900, 800, and 600??C, respectively. At UCLA, experiments limite the equilibrium to 14.4-14.6, >12.9, 11.8-12.2 and 7.0-8.0 kbar at 1000, 950, 900, and 700??C, respectively. The equilibrium kyanite ??? andalusite was reversed between 7.4 and 8.0 kbar, 800??C at UCLA. -from Authors

American Mineralogist↗
USGS1991

Revised values for the thermodynamic properties of boehmite, AlO(OH) , and related species and phases in the system Al-H-O

Heat capacity measurements are reported for a well-characterized boehmite that differ significantly from results reported earlier by Shomate and Cook (1946) for a monohydrate of alumina. It is suggested that the earlier measurements were made on a sample that was a mixture of phases and that use of that heat-capacity and derived thermodynamic data be discontinued. The entropy of boehmite derived in this study is 37.19 ?? 0.10 J/(mol.K) at 298.15 K. Based on our value for the entropy and accepting the recommended Gibbs free energy for Al(OH)-4, the Gibbs free energy and enthalpy of formation of boehmite are calculated to be -918.4 ?? 2.1 and -996.4 ?? 2.2 kJ/mol, respectively, from solubility data for boehmite. The Gibbs energy for boehmite is unchanged from that given by Hemingway et al. (1978). -from Authors

American Mineralogist↗
USGS1992

Crystalline solution series and order-disorder within the natrolite mineral group

Electron microprobe and X-ray analyses were made of natrolite, tetranatrolite, gonnardite, and thomsonite from the Magnet Cove alkaline igneous complex, Arkansas, and of selected specimens from the U.S. National Museum. This information and data from the literature indicate that natrolite, mesolite, scolecite, edingtonite, and tetraedingtonite show only small deviations from the ideal stoichiometry. In contrast, gonnardite, tetranatrolite, and thomsonite show large deviations from the ideal end-member compositions and compose three crystalline series. The structures of the natrolite minerals are defined by combining each of the three types of framework structures with various combinations of channel-occupying polyhedra. Various polysomatic series can be constructed by combining slices of two basic structures to form new hybrid structures. -from Authors

American Mineralogist↗
USGS1992

Solution properties of almandine-pyrope garnet as determined by phase equilibrium experiments

The thermodynamic mixing properties of almandine-pyrope garnet were derived from phase equilibrium experiments at temperatures of 900 and 1000??C and pressures from 8 to 14 kbar. Almandine has essentially ideal behavior in almandine-pyrope garnet over the composition range Alm89-Alm61 at the above experimental conditions. In all experimental products a systematic partitioning of Fe and Mg between garnet and ilmenite was seen with ln Kd ??? 1.59 which was not temperature sensitive. The results support the use of garnet mixing models that incorporate ideal or nearly ideal Fe-Mg parameters. -from Authors

American Mineralogist↗
USGS1991

Boron analysis by electron microprobe using MoB4C layered synthetic crystals

Preliminary electron microprobe studies of B distribution in minerals have been carried out using MoB4C-layered synthetic crystals to improve analytical sensitivity for B. Any microprobe measurements of the B contents of minerals using this crystal must include analyses for Cl to assess and correct for the interference of Cl X-rays on the BK?? peak. Microprobe analyses for B can be made routinely in tourmaline and other B-rich minerals, and minor B contents also can be determined in common rock-forming minerals. Incorporation of unusually high B contents in minerals other than borosilicates has been discovered in prograde and retrograde minerals in tourmalinites from the Broken Hill district, Australia, and may reflect high B activities produced during the metamorphism of tourmaline-rich rocks. -from Authors

American Mineralogist↗
USGS1992

Water, CO2, Cl, and F in melt inclusions in phenocrysts from three Holocene explosive eruptions, Crater Lake, Oregon

Rare melt inclusions ~ 100 μ m in diameter trapped near the boundaries of corroded patchy zones in plagioclase phenocrysts from Plinian pumice of three Holocene eruptions were analyzed by IR spectroscopy for molecular H 2 O, OH groups, and CO 2 and by electron microprobe for Cl and F. The three rhyodacitic eruptions, each of which began with a Plinian phase, occurred over ~200 yr. The Llao Rock and Cleetwood eruptions ended with degassed lava flows and the subsequent climactic eruption with voluminous ignimbrite. Groundmass glass and melt inclusion compositions (anhydrous) are similar. Inclusions with total H 2 O concentrations of 1.2–3.4 wt% are adjacent to fractures or are hourglass inclusions, suggesting partial degassing caused by depressurization. Melt inclusions in phenocrysts in climactic ignimbrite may have vesiculated for the same reason. Intact inclusions in Plinian pumices have total H 2 O concentrations believed to represent magmatic H 2 O contents (wt%): Llao Rock, 5.3, 5.3; Cleetwood,3.8,4.7; climactic 3.9 ± 0.2 (1 σ, n = 6). Ratios of OH/molecular H 2 O indicate closure temperatures of 200–500 °C that reflect syn- or posteruptive cooling. CO 2 concentrations are ≤25 ppm. H 2 O and CO 2 concentrations indicate saturation pressures of 1.0–1.8 kbar or depths ≥5 km. Six inclusions from climactic pumice average 400 ± 40 (1 σ ) ppm F, 1880 ± 70 ppm Cl. F and Cl concentrations of the other samples are similar and not well correlated with each other or with total H 2 O. Location of melt inclusions near boundaries of patchy zones, which are mantled by oscillatory-zoned overgrowths, suggests that their H 2 O concentrations represent magmatic values significantly before eruption. Although mean H 2 O concentrations of analyzed melt inclusions decrease for the three successive eruptions, it is not certain that this indicates a corresponding change in magmatic H 2 O content during the interval between eruptions.

Oregon↗
USGS1993

Magma transport and metasomatism in the mantle: a critical review of current geochemical models

Conflicting geochemical models of metasomatic interactions between mantle peridotite and melt all assume that mantle reactions reflect chromatographic processes. Examination of field, petrological, and compositional data suggests that the hypothesis of chromatographic fractionation based on the supposition of large-scale percolative processes needs review and revision. Well-constrained rock and mineral data from xenoliths indicate that many elements that behave incompatibly in equilibrium crystallization processes are absorbed immediately when melts emerge from conduits into depleted peridotite. After reacting to equilibrium with the peridotite, melt that percolates away from the conduit is largely depleted of incompatible elements. Continued addition of melts extends the zone of equilibrium farther from the conduit. Such a process resembles ion-exchange chromatography for H2O purification, rather than the model of chromatographic species separation. -from Authors

American Mineralogist↗
USGS1993

A flexible Au-Ir cell with quick assembly for hydrothermal experiments

The paper describes a new flexible reaction cell for high-temperature and high-pressure experiments in hydrothermal apparatus. The interior of the cell is all Au, except for two inert Ir gaskets. The design features an all Au cap that can be easily and rapidly assembled and disassembled. The capacity of the cell is approximately 240 mL, with a height of 20 cm and an o.d. of 6 cm. -Authors

American Mineralogist↗
USGS1993

Micro-PIXE analysis of silicate reference standards

The accuracy and precision of the University of Guelph proton microprobe have been evaluated through trace-element analysis of well-characterized silicate glasses and minerals, including BHVO-1 glass, Kakanui augite and hornblende, and ten other natural samples of volcanic glass, amphibole, pyroxene, and garnet. Using the 2.39 wt% Mo in a NIST steel as the standard, excellent precision and agreement between reported and analyzed abundances were obtained for Fe, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, and Nb. -from Authors

American Mineralogist↗
USGS1993

Kosnarite, KZr2(PO4)3, a new mineral from Mount Mica and Black Mountain, Oxford County, Maine

Kosnarite, ideally KZr2(PO4)3, has been identified as part of a late-stage, secondary phosphate mineral assemblage from the Mount Mica pegmatite at Paris, and from the Black Mountain pegmatite, Rumford, Oxford County, Maine. Kosnarite from Mount Mica occurs as pseudocubic rhombohedral crystals, as much as 0.9 mm in maximum dimension, that display the dominant {102} form. Color ranges from pale blue to blue-green to nearly colorless. The mineral has a white streak, is transparent, has a vitreous luster, and is nonfluorescent in ultraviolet light. It has a hardness of 4.5, is brittle with a conchoidal fracture, and has perfect {102} cleavage. Kosnarite from Black Mountain is almost pure KZr2(PO4)3 with only trace amounts of Hf, Mn, Na, and Rb. The mineral is one of three known alkali zirconium phosphates; the others are gainesite and the Cs analogue of gainesite. -from Author

American Mineralogist↗
USGS1996

Reinvestigation of the annite = sanidine + magnetite + H2 reaction using the fH2-sensor technique

The decomposition of the iron mica, annite, to sanidine plus magnetite and vapor, KFe3AlSi3O10(OH)2 = KAlSi3O8 + Fe3O4 + H2, has been reexamined experimentally with the use of a variety of buffers coupled with fH2 sensors at 2 kbar and between 400 and 840 ??C. Various capsule configurations were used in this study to delineate the equilibrium constant for this reaction in conjunction with 57Fe Mo??ssbauer spectroscopy measurements to monitor the oxy-annite component in mica in selected experiments. Results at the most reducing and highest temperature conditions of this study extend the annite stability field to higher temperature and fo2 values than those defined in previous work. Lower temperature results indicate that the annite-sanidine-magnetite stability boundary does not intersect the hematite + magnetite + H2O buffer at 400??C as previously reported but rather is subparallel to the buffer curve at lower fo2 values. The equilibrium fH2 (in bars) for the assemblage annite + sanidine + magnetite + vapor at 2 kbar and between 400 and 840 ??C can be described by the relation log fH2 (??0.08) = 13.644 - (17368/T) + (5.168 ?? 106)/T2, where T is temperature in kelvins.

American Mineralogist↗
USGS1996

Roof-rock contamination of Taylor Creek Rhyolite, New Mexico, as recorded in hornblende phenocrysts and biotite xenocrysts

The Taylor Creek Rhyolite, a group of coeval, mid-Tertiary, silica-rich rhyolite lava domes in southwestern New Mexico, is notable for recording bulk-rock evidence of minor, yet easily measurable, contamination of its source magma reservoir resulting from assimilation of Proterozoic roof rock. Most of the evidence is recorded in trace element concentrations and 87Sr/86Sri ratios, which are far different in unconlaminated magma and roof rocks. Hornblende phenocrysts and biotite xenocrysts also record the effects of contamination. Electron microprobe analyses show that all hornblende grains are zoned to Mg-rich and Fe- and Mn-poor rims. Rim MgO content is typically about 10 wt% greater than core MgO content. Other hornblende constituents are not measurably variable. Biotite xenocrysts, trace mineral constituents, are present only in the domes that are most contaminated, as judged by bulk-rock variations in trace element concentrations and 87Sr/ 86Sri. Biotite grains are invariably partly to almost completely altered. Microprobe analyses of the cores of the least-altered grains show that large variations in Fe and Mg and that biotite contains 2-20 times as much Mg as fresh biotite phenocrysts in other silica-rich rhyolite lavas. Fe and Mg are negatively correlated in hornblende and biotite, consistent with mixing two end-member compositions. The mass ratio of contaminant to magma was probably less than 1:100, and major constituents, including Al, were not measurably affected in hornblende. Al-in-hornblende barometry yields essentially a constant calculated pressure of about 1.5 kbar, which is consistent with the interpretation that all contamination occurred in a boundary zone about 300 m thick at the top of the magma reservoir.

American Mineralogist↗