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Results for “American Mineralogist”

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At least 109 records · Page 6Linked to original sources
USGS1988

Optical (diffuse reflectance) and Mossbauer spectroscopic study of nontronite and related Fe-bearing smectites

Near-ultraviolet to near-infrared optical (diffuse reflectance) spectra of several nontronites and related Fe-bearing smectites [(Fe2+,Fe3+)-bearing saponite and (Fe2+,Fe3+)-bearing montmorillonite] are presented and interpreted. Mossbauer spectra at 298 K are also presented to help interpret the optical spectra. The optical spectra of nontronites are dominated by the ligand field transitions of Fe3+ in octahedral coordination sites. In addition to the ligand field transitions of single Fe3+ cations, a broad absorption band centered near 22000 cm-1 is observed that may be due to the simultaneous excitation of two Fe3+ cations to the 4T1g (4G) state. Alternatively, this band may represent excitations to the 2A2g and 2T1g ligand field states. For most samples, the amount of tetrahedrally coordinated Fe3+ was below that detectable by Mossbauer spectroscopy (1-3% of total Fe). However, the optical spectra of all of the nontronites show an absorption band near 23000 cm-1. This band is assigned to the 6A1 ??? 4E,4A1 transition of tetrahedrally coordinated Fe3+. The optical spectra of mixed-valence Fe-bearing smectites show a broad absorption band at 14000-15000 cm-1 owing to Fe2+ ??? Fe3+ charge transfer. -from Authors

American Mineralogist↗
USGS1987

Chalcopyrite disease in sphalerite: pathology and epidemiology.

This descriptive paper identifies three widely occurring textures designated as 'watermelon', 'dusting' and 'bimodal' that characterize the replacement of original Fe-bearing sphalerite by an aggregate of chalcopyrite and low-Fe sphalerite as an integral part of the mineralization process. Replacement probably predominates over alternative modes of origin for small chalcopyrite blebs in sphalerite from most vein and sea-floor massive sulphide deposits that formed in the 200-400oC temperature range and that have not been subsequently subjected to higher T. Sphalerite from the epithermal Ag-Pb-Zn deposit at Creede, Colorado, displays a rich variety of features ("bead chains') that are primary crystal dislocations decorated by exsolved chalcopyrite.-J.A.Z.

American Mineralogist↗
USGS1987

Crystal structure and crystal chemistry of melanovanadite, a natural vanadium bronze.

The crystal structure of melanovanadite from Minas Ragra, Peru, has been determined in space group P1. The triclinic unit cell (non-standard) has a 6.360(2), b 18.090(9), c 6.276(2) A, alpha 110.18(4)o, beta 101.62(3)o, gamma 82.86(4)o. A subcell with b' = b/2 was found by crystal-structure analysis to contain CaV4O10.5H2O. The subcell has a layer structure in which the vanadate sheet consists of corner-shared tetrahedral VO4 and double square-pyramidal V2O8 groups, similar to that previously found in synthetic CsV2O5. Refinement of the full structure (R = 0.056) showed that the Ca atom, which half-occupies a general position in the subcell, is 90% ordered at one of these sites in the whole unit cell. Bond length-bond strength estimates indicate that the tetrahedra contain V5+, and the square pyramids, V4+.-J.A.Z.

American Mineralogist↗
USGS1987

Quartz: heat capacities from 340 to 1000 K and revised values for the thermodynamic properties.

New heat-capacity data for quartz have been measured over the T interval 340-1000 K by differential scanning calorimetry. The data were combined with recent heat-content and heat-capacity data to provide a significantly revised set of thermodynamic properties for alpha -quartz and to resolve the problem of disparate heat-content and heat-capacity data for alpha - and beta -quartz.-J.A.Z.

American Mineralogist↗
USGS1988

Ostwald ripening and interparticle-diffraction effects for illite crystals

The Warren-Averbach method, an X-ray diffraction (XRD) method used to measure mean particle thickness and particle-thickness distribution, is used to restudy sericite from the Silverton caldera. Apparent particle-thickness distributions indicate that the clays may have undergone Ostwald ripening and that this process has modified the K-Ar ages of the samples. The mechanism of Ostwald ripening can account for many of the features found for the hydrothermal alteration of illite. Expandabilities measured by the XRD peak-position method for illite/smectites (I/S) from various locations are smaller than expandabilities measured by transmission electron microscopy (TEM) and by the Warren-Averbach (W-A) method. This disparity is interpreted as being related to the presence of nonswelling basal surfaces that form the ends of stacks of illite particles (short-stack effect), stacks that, according to the theory of interparticle diffraction, diffract as coherent X-ray scattering domains. -from Authors

American Mineralogist↗
USGS1988

Electrical conductivity of diopside: evidence for oxygen vacancies

Impedance spectra for two natural single crystals of diopside were obtained at 800 to 1300??C and 1-bar pressure over the frequency range 0.001 Hz to 100 kHz in a system closed to all components but oxygen. At both higher and lower fO2 values, no fO2 dependence of conductivity was observed, indicating the presence of different conduction mechanisms. At temperatures less than 1000??C, the activation energy is 1.3 eV, also suggesting a different conduction mechanism. Thus, at least four regimes are necessary to describe the conductivity of this diopside in T-fO2 space. The approximately -1/(7 ?? 1) value of d(log ??)/d(log fO2) in a high-temperature geologic region suggests a reaction by which oxygen vacancies control the conductivity. This relatively pure diopside is much less conducting than olivine or orthopyroxene. A second diopside with greater Fe content but otherwise similar in composition to the near-end-member diopside, is more conducting, has a smaller activation energy (1.0 eV) over the range 1050 to 1225??C, and shows only a weak negative fO2 dependence; suggesting that oxygen vacancies are present but are not the dominant defect in controlling the conductivity. -from Authors

American Mineralogist↗
USGS1986

Chloritoid-sillimanite assemblage from North Carolina.

The occurrence of sillimanite and Fe-rich chloritoid in apparent equilibrium in a quartzite near Charlotte, North Carolina, is reported. The implications for the kyanite-andalusite-sillimanite triple point are discussed.-J.A.Z.

American Mineralogist↗
USGS1986

Garnet megacrysts of the Williams diatremes, north-central Montana.

The physical characteristics of garnet megacrysts from the Williams diatremes are described, analysed and compared with other garnet megacryst suites. The only correlation found between the physical characteristics and the composition of the megacrysts related deep-red colour to high Cr content.-J.A.Z.

American Mineralogist↗
USGS1986

Mineralogical studies of the nitrate deposits of Chile. V. Iquiqueite, Na4K3Mg(CrO4)B24O39(OH).12H2O, a new saline mineral.

Iquiqueite (Na4K3Mg(CrO4)B24O39(OH).12H2O, a 11.6369(14), c 30.158(7) A, P31c, Z = 3) occurs as a widespread minor constituent in the nitrate fields of northern Chile. It is particularly abundant in the vicinity of Zapiga, Tarapaca province. Associated minerals include nitratite, halite, nitre, darapskite, blodite, glauberite, dietzeite, bruggenite, ulexite and gypsum. Iquiqueite forms thin, yellow, hexagonal platelets (5-50 mu m in diameter, <5 mu m in thickness) that are disseminated singly or in vermiform aggregates in nitrate ore. Observed forms are c(0001) and m(1010). Cleavage is perfect on (0001) and imperfect on (1010); H. = or <2. D(calc.) 2.05 g/cm3 and measured sp. gr. 2.05 + or - 0.09. The mineral is uniaxial negative, epsilon 1.447(2), omega 1.502(2). The XRD pattern has the six strongest lines 3.02(100), 2.856(100), 10.11(85), 6.04(85), 3.28(85), 3.22(85) A. The name is for the city of Iquique, Chile.-J.A.Z.

American Mineralogist↗
USGS1986

Mineralogy and geochemical evolution of the Little Three pegmatite-aplite layered intrusive, Ramona, California

Several layered pegmatite-aplite intrusives exposed at the Little Three mine, Ramona, display closely associated fine-grained to giant-textured mineral assemblages which are believed to have co-evolved from a hydrous aluminosilicate residual melt with an exsolved supercritical vapour phase. Calculations of phase relations between the major pegmatite-aplite mineral assemblages and supercritical aqueous fluid were made, assuming equilibrium and closed-system behaviour as a first-order model.-J.A.Z.

California↗
USGS1986

Heat capacities and thermodynamic functions for beryl, Be3Al2Si6O18, phenakite, Be2SiO4, euclase, BeAlSiO4(OH), bertrandite, Be4Si2O7(OH)2, and chrysoberyl, BeAl2O4.

The heat capacities of beryl, phenakite, euclase and bertrandite have been measured between approx 5 and 800 K by combined quasi-adiabatic cryogenic calorimetry and differential scanning calorimetry. The heat capacities of chrysoberyl have been measured from 340 to 800 K. The resulting data have been combined with solution and phase-equilibrium experimental data and simultaneously adjusted using the programme PHAS20 to provide an internally consistent set of thermodynamic properties for several important beryllium phases. The experimental heat capacities and tables of derived thermodynamic properties are presented.-J.A.Z.

American Mineralogist↗
USGS1986

Minasgeraisite, a new member of the gadolinite group from Minas Gerais, Brazil.

Minasgeraisite, Y2CaBe2Si2O10, a 9.833(2), b 7.562(1), c 4.702(1) A, beta 90.46(6)o, P21/a, Z = 2, is a new member of the gadolinite group. Found in quantities of only several hundred mg in the Jaguaracu pegmatite, Minas Gerais, Brazil, it occurs as 0.2 to 1.0 mm-diameter rosettes, coating and intergrown with milarite, albite, quartz and muscovite. The mineral is lavender to lilac purple and has a sheaf-like habit, with a grain size of <3-5 mu m. It is not magnetic and not fluorescent under either short-wave or long-wave ultraviolet radiations; H. approx 6-7, D(meas.) >4.25, D(calc.) 4.90 g/cm3, lustre is earthy to subvitreous and the streak is pale purple. Minasgeraisite is slowly soluble in common acids. The mineral has one excellent cleavage, (100) by analogy with gadolinite, and another good cleavage (001). Minasgeraisite is biaxial positive with alpha (colourless) 1.740(4), beta (pale greyish yellow) 1.754(4), gamma (lavender purple) 1.786(4), 2Vgamma 68o. Strongest powder XRD lines include 3.11(100), 2.830(100), 2.540(90), 1.768(35) A.-J.A.Z.

American Mineralogist↗
USGS1986

Evolution of the Coast batholith along the Skagway Traverse, Alaska and British Columbia.

Reconnaissance geological mapping of a strip 10-15 km wide across the Coast batholith from Haines to Skagway to the vicinity of Log Cabin was performed. The petrography, major- and minor-element chemistry, geochronology (using the U/Pb method on zircons), and initial ratios of 87Sr/86Sr of a suite of representative samples from each major unit are reported.-J.A.Z.

American Mineralogist↗
USGS1986

Kassite from the Diamond Jo quarry, Magnet Cove, Hot Spring County, Arkansas: The problem of cafetite and kassite

Small (<0.5 mm), brownish-pink platy rosettes and yellow spherules, in cavities in nepheline syenite at the Diamond Jo quarry, Magnet Cove, Hot Spring County, Arkansas, have been identified as kassite, CaTi 2 O 4 (OH) 2 , a mineral previously known only from the Kola Peninsula, USSR. The Magnet Cove kassite is orthorhombic, in space group Ammm, A2mm, A222 , or A2,22 , with unit-cell parameters a = 12.10, b = 31.65, c = 4.95 Å, and Z = 16; d calc = 3.28 g/cm 3 . Principal X-ray powder lines [ d in Å, ( I )] are 7.85(10), 3.26(7), 2.565(7), 1.910(8), 1.898(6). The X-ray powder and single-crystal data and density correspond to those reported by earlier workers for cafetite, (Ca,Mg)(Fe,Al) 2 Ti 4 O 12 ·4H 2 O, from Kola, but the chemical and physical properties correspond to those given in their description of kassite. These earlier workers may have intermixed samples during their X-ray study of kassite and cafetite.

Arkansas↗