Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “American Mineralogist”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 127 records · Page 7Linked to original sources
USGS1986

Renierite, Cu10ZnGe2Fe4S16-Cu11GeAsFe4S16: a coupled solid solution series.

The composition of renierite is found to be Cu10(Zn1-xCux)Ge2-xAsxFe4S16 (0 = or < x = or < 1), with continuous solid solution between the zincian and arsenian end-members, Cu10ZnGe2Fe4S16 and Cu11GeAsFe4S16, through the coupled substitution Zn(II) + Ge(IV) = Cu(I) + As(V). This is the first reported example of extensive coupled solid solution in a sulphide mineral. Arsenian renierite, not previously characterized, is similar to zincian renierite in polished section, with a slightly redder colour and lower anisotropy. It is reddish orange with relief very similar to that of bornite, though it is harder (VHN25 = 286) and does not tarnish in air. It is slightly bireflective, with colours varying from orange-yellow to reddish orange in nearly crossed polarizers. The strongest powder XRD lines are: 3.042(100), 1.861(29), 1.869(16), 1.594(11) and 1.017(10) A; D(calc.) 4.50 g/cm3. Specimens have been found at the Ruby Creek copper deposit, Alaska, where zincian renierite also occurs, and at the Inexco no. 1 mine, Jamestown, Colorado.-J.A.Z.

American Mineralogist↗
USGS1986

Nature of phases synthesized along the join (Mg,Mn)2Si2O6.

Eight compositions were synthesized along this join. Orthopyroxene crystallized at X = Mn/(Mn + Mg) = 0.0-0.3, clinopyroxene at X 0.4-0.5, and pyroxmangite at X 0.65-1.0, consistent with the composition ranges that have been inferred by examination of metamorphic rock assemblages. Unit-cell dimensions for these phases and for rhodonite (X = 1.0) are presented, along with microprobe chemical analyses and phase diagrams.-J.A.Z.

American Mineralogist↗
USGS1991

Low-K granophyres of the Stillwater Complex, Montana

Small bodies of granophyre occur as a volumetrically insignificant but ubiquitous component of the Banded series of the Stillwater Complex. White to pink granophyre typically occurs as veins, 1-12cm thick and as much as 100m long. A geochemically similar body of coalescing alaskite dikes, associated with an occurrence of Pt-group elements in the Banded series of the complex, crops out approximately 2km south-southeast of Picket Pin Mountain over an area 130 by 210m. Considering host rocks and chemistry, these rocks are comparable to the most siliceous examples of oceanic plagiogranite. The Stillwater granophyres, however, are enriched in Si, Th, U, and LREEs, and depleted in K, Fe, and Eu, relative to oceanic granophyres. -from Authors

American Mineralogist↗
USGS1991

Commentary on the sphalerite geobarometer

The FeS content of sphalerite in assemblages with pyrite and pyrrhotite has been widely used and widely criticized as a geobarometer. The commonly accepted form of the geobarometer is based on the composition of sphalerite being independent of temperature below about 550??C at all pressures up to at least 10 kbar, but strong thermodynamic arguments require a significant temperature dependence in this region. Most minor components have a negligible effect on the relevant equilibria, but the effect of CuS is somewhat more significant. Molar and partial molar volumes for binary (ZnS-FeS) and ternary (ZnS-FeS-CuS) sphalerite solutions are presented; the ternary data are consistent with charge transfer between Cu and Fe. -from Authors

American Mineralogist↗
USGS1986

Thalenite from Arizona.

Thalenite occurs as a minor constituent of a single small pegmatite within an extensive area of granite a few miles S of Kingman, Arizona. Partly crystalline and partly metamict, this thalenite has composition Y3(Si3O10)(OH), with extensive substitution of Y by REE, especially Dy, Er and Yb. Upon heating, even at moderate T, both the crystalline and the metamict thalenite are converted to a phase with a structure corresponding with that of thortveitite, Sc2Si2O7.-J.A.Z.

American Mineralogist↗
USGS1986

Aluminum in hornblende: an empirical igneous geobarometer.

Electron-microprobe analyses of hornblendes from five calc-alkaline plutonic complexes representing low- and high-pressure regimes define a tightly clustered linear trend in terms of total Al (AlT) and tetrahedral Al (Aliv) contents. Data collated from the literature on calcic amphiboles from other plutonic complexes and from phase equilibrium experiments using natural rocks or synthetic analogue compositions show a similar AlT-Aliv trend and systematic pressure effects.-J.A.Z.

American Mineralogist↗
USGS1989

Mineralogical studies of the nitrate deposits of Chile: VI. Hectorfloresite, Na9(IO3)(SO4)4, a new saline mineral

The new mineral hectorfloresite, known to occur in only one locality in the nitrate fields of northern Chile, consists of tiny prismatic crystals, generally less than 1 mm long and 0.2 mm in diameter, in cavities in dense nitrate ore consisting of saline-cemented silt, sand, and small rock fragments. The physical, optical and crystallographic properties of the new mineral are described. -from Authors

American Mineralogist↗
USGS1989

Availability of a library of infrared (2.1-25.0 μm) mineral spectra

All previously published libraries of infrared mineral spectra are in the form of transmittance. Reflectance spectra are, however, more useful for remote sensing and some potential laboratory applications, such as the use of an infrared microscope for mineral identification on polished sections. This note points out that construction of a new library of infrared (2.1-25.0 &mu; m) mineral spectra is in progress. Both transmittance and reflectance measurements of a selection of 63 different, well-characteized minerals have been published to date. These data are available in both hard copy and digital form.

American Mineralogist↗
USGS1990

Crystal chemistry of the natural vanadium bronzes

The crystal chemistry of the natural vanadium bronze minerals is reviewed on the basis of published information and new studies (mainly by X-ray powder-diffraction methods) using type material wherever possible. The known V bronze minerals are divided into three categories: 1) the hewettite group, 2) the straczekite group, 3) other structure types including navajoite, schubnelite, fervanite, shcherbinaite, bannermanite, and melanovanadite. All known structures associated with the fibrous V bronzes (fiber spacing 3.6 A??) can be considered as various lateral linkages (into sheets or networks) of only two types of polyvanadate chains: 1) a divanadate chain (V2O6)n consisting of alternating square pyramids, and 2) a tetravanadate chain (V4O12)n consisting of four highly condensed single octahedral chains. -from Authors

American Mineralogist↗
USGS1990

Thermodynamic properties for bunsenite, NiO, magnetite, Fe3O4, and hematite, Fe2O3, with comments on selected oxygen buffer reactions

Smoothed values of the heat capacities and derived thermodynamic functions are given for bunsenite, magnetite, and hematite for the temperature interval 298.15 to 1800 K. The Gibbs free energy for the reaction Ni + 0.5O2 = NiO is given by the equation ??rG0T = -238.39 + 0.1146T - 3.72 ?? 10-3T ln T and is valid from 298.15 K to 1700 K. The Gibbs free energy (in kJ) of the reaction 2 magnetite + 3 quartz = 3 fayalite + O2 may be calculated from the equation ??rG0T = 474.155 - 0.16120 T in kJ and between 800 and 1400 K. The Gibbs free energy (in kJ) of the reaction 6 hematite = 4 magnetite + O2 may be calculated from the following equations: ??rG0T = 496.215 - 0.27114T, ??rG0T = 514.690 - 0.29753T, ??rG0T = 501.348 - 0.2854T. -from Author

American Mineralogist↗
USGS1990

The assemblage WO2 + H2O as a steady-state hydrogen source in moderately reduced hydrothermal experiments

The values of fH2 for the assemblage WO2 + WO2.72 + H2O (designated as WO) have been measured in sealed Au capsules under an external pressure of 2 kbar CH4 and between 650 and 800??C using Ag-AgBr-HBr sensors of fH2. The fH2 values obtained can be represented by the equation log(fWOH2)2kbar,T(??0.06) = (-1924.9 ??(T,K) + 4.06 and are found to be slightly greater than those associated with the previously calibrated C-CH4 buffer. -from Authors

American Mineralogist↗
USGS1990

The thermodynamic properties of dumortierite Si 3 B[Al 6.75 [] 0.25 O 17.25 (OH) 0.75 ]

The heat capacities of dumortierite have been measured from 7 to 1000 K by quasiadiabatic low-temperature and differential scanning calorimetry. The corrected calorimetric entropy at 298.15 K calculated from the experimental data is 330.2 + 5.0 J/(mol.K) based upon the chemistry of the sample, average compositions proposed by Alexander et al. (1986), and correction to the structural formula given by Moore and Araki (1978). Moore and Araki (1978) have shown the Al(l) site to be disordered, and chemical analyses suggest that there is some Al substitution for Si. A minimum configurational entropy of 4.74 J/(mol'K) should be added to the corrected calorimetric entropy for third law calculations. The enthalpy and Gibbs free energy of formation of dumortierite from the elements have been estimated from synthesis and decomposition data and are -9109 + 20 and -8568 + 20 kJ/mol, respectively, at 298.15 K and I bar

American Mineralogist↗
USGS1990

Relations of zoned pegmatites to other pegmatites, granite, and metamorphic rocks in the southern Black Hills, South Dakota

The pegmatite field and the Harney Peak Granite of the southern Black Hills, South Dakota, form an igneous system that progresses from slightly biotitic muscovite granite through layered pegmatitic granite, with alternating sodic and potassic rocks, to simple plagioclase-quartz-perthite pegmatites, and on to zoned pegmatites. Most of the country rocks are Lower Proterozoic mica schists. At 1700 Ga, intrusion of the Harney Peak Granite created a large dome in these rocks, a thermal aureole with a staurolite, a first sillimanite isograd, and a small area of metamorphism above the second sillimanite isograd. The zoned pegmatites have a strong tendency to occur in clusters, and the types of pegmatites are different in different clusters. A less obvious tendency is a regional zonation in which rare-mineral pegmatites become more abundant and muscovite pegmatites less abundant toward the outskirts of the region. The composition of the granite indicates that its magma originated by partial melting of metasedimentary mica schists similar to those at the present surface. The pegmatitic nature of most of the granite probably reflects exsolution of an aqueous phase. -from Authors

American Mineralogist↗
USGS1990

Temperature, size, and depth of the magma reservoir for the Taylor Creek Rhyolite, New Mexico

The 55 km3 mid-Tertiary Taylor Creek Rhyolite in southwestern New Mexico consists of 20 lava domes and flows. This rhyolite is metaluminous to weakly peraluminous. Compositional zonation in feldspar phenocrysts is very minor and nonsystematic. The compositions of each feldspar species vary little throughout the suite of analyzed samples. This chemical homogeneity of phenocrysts reflects similar whole-rock homogeneity and suggests that the lavas were tapped from a single large reservoir of magma. Ages of sanidine phenocrysts determined using 40Ar/39Ar indicate that the Taylor Creek Rhyolite lavas were emplaced during a period of less than 0.42 my. and possibly less than 0.13 m.y., which is consistent with the single-reservoir scenario. Two-feldspar geothermometry suggests that Taylor Creek Rhyolite phenocrysts crystallized at about 775??C, at an assumed pressure of 2 kbar. Fe-Ti-oxide geothermometry suggests phenocryst growth at about 800??C. Experimental studies suggest that quartz and potassium-feldspar crystals that grow from H2O-undersaturated granitic magmas should exhibit resorption texture, a texture ubiquitous to Taylor Creek Rhyolite quartz and sanidine phenocrysts. We tentatively conclude that the Taylor Creek Rhyolite magma was H2O undersaturated and subliquidus at an unspecified pressure greater than 0.5 kbar during phenocryst growth and that Taylor Creek Rhyolite pyroclastic deposits formed because volatile saturation developed during the ascent of magma to sites of eruption. -from Authors

American Mineralogist↗
USGS1990

Fluorian garnets from the host rocks of the Skaergaard intrusion: implications for metamorphic fluid composition

Zoned, silica-deficient, calcic garnets containing up to 5 mol% F substitution for O formed during contact metamorphism of basalts by the Skaergaard intrusion in East Greenland. Fluorian calcic garnets occur as a retrograde alteration of prograde wollastonite and clinopyroxene that fills vesicles and vugs in lavas 30-70 m from the intrusion. The F content of garnet is extremely sensitive to minor changes in fluid composition. The calculations show that a decrease in pH or an increase in log aF- of 0.3 at constant pressure and temperature will decrease the F concentration in garnet from 5 to 0 mol%. The results of this study show that fluorian hydrous grandites provide a mineralogical record of the activities of F species in coexisting metamorphic and hydrothermal fluids. -from Authors

American Mineralogist↗
USGS1990

Molecular orbital (SCF-X-α-SW) theory of Fe 2+- Mn 3+ , Fe 3+- Mn 2+ , and Fe 3+- Mn 3+ charge transfer and magnetic exchange in oxides and silicates

Metal-metal charge-transfer and magnetic exchange interactions have important effects on the optical spectra, crystal chemistry, and physics of minerals. Previous molecular orbital calculations have provided insight on the nature of Fe2+-Fe3+ and Fe2+-Ti4+ charge-transfer transitions in oxides and silicates. In this work, spin-unrestricted molecular orbital calculations on (FeMnO10) clusters are used to study the nature of magnetic exchange and electron delocalization (charge transfer) associated with Fe3+-Mn2+, Fe3+-Mn3+, and Fe2+-Mn3+ interactions in oxides and silicates.

American Mineralogist↗
USGS1990

The Stillwater Complex and its anorthosites: an accident of magmatic underplating?

The Stillwater Complex, emplaced 2700??40 Ma, is exposed at the edge of a 4000-km2 block of Late Archean rocks that formed 40 to 110 m.y. yearlier. Voluminous plagioclase cumulates (anorthosites) within the Middle Banded series of the complex are difficult to explain either by in situ fractionation of mafic magma or by popular models for mixing of two magma types. Current models for the evolution of the lowermost continental crust by magmatic underplating suggest that a major crust-forming event of about 100 m.y. duration would satisfy geologic and geochemical constraints for the formation of the Stillwater Complex and the related granitoids. -from Authors

American Mineralogist↗