摘要:

AbstractAn in-situ study of cubanite, CuFe2S3, was performed in a diamond-anvil cell using Mossbauer spectroscopy and energy-dispersive X-ray diffraction at room temperature and pressures up to 5 GPa. Mossbauer spectra of orthorhombic cubanite show a single Fe site with rapid electron transfer between Fe2+and Fe3+, a hyperfine magnetic field that is relatively insensitive to pressure, and a center shift that decreases with pressure because of increasing covalency. A phase transition occurs above 3.3 GPa that involves a change from the orthorhombic cubanite structure to a derivative of the hexagonal NiAs (B8) structure, with a zero-pressure volume decrease of 29%. The large difference in volume is caused by a change from tetrahedral to octahedral coordination and a significant shortening of metal-metal bonds. Volume-compression data were fitted to a second-order Birch- Murnaghan equation of state (K′0= 4) with the results K0= 64 ± 3 GPa (orthorhombic phase) and K0= 157 ± 16 GPa (high-pressure phase). Mossbauer data of the high-pressure phase indicate a single Fe site with no magnetic ordering and a valence intermediate between Fe2+nd Fe3+. Consideration of likely ordering patterns in the high-pressure phase indicates that localized electron transfer could occur along face-shared pairs of Fe octahedra, and extended electron delocalization could occur along paths formed by faceand edge-shared octahedra.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-201

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractFrom in-situ structure and site occupancy refinements of a synthetic orthopyroxene, (Mg0.75Fe0.25)2Si2O6, by single-crystal X-ray diffraction at 296,1000, 1100,1200, and 1300 K (with reversals), two significant phenomena were observed: (I) with increasing temper­ature, the silicate B chain straightens much faster, especially above 1200 K, and becomes straighter than the A chain at 1300 K, with the 03-03-03 angles of the A and B chains being 170.8 and 173.1°, respectively; and (2) the In KDvalues (where KD= FeM1MgM2/ FeM2MgM1) vary linearly with MT (K) between 1000 and 1200 K, but nonlinearly between 1200 and 1300 K. The drastic straightening of the silicate chains at 1300 K is accompanied by the attainment of very similar configurations of the SiO4tetrahedra, SiA and SiB, in shapes, sizes, and out-of-plane tiltings. The rigid-body thermal vibration analysis suggests that the librational motion of the SiA tetrahedron (8. I°2) is slightly larger than that of SiB (5.2°2) at 296 K but becomes considerably smaller (48.0°2) than that of SiB (77.3°2) at 1300 K. The Ml octahedron remains nearly regular from 296 to 1300 K, whereas the M2 coordination changes from sixfold (296 K) to sevenfold (1200 K) and to sixfold (1300 K) because of the bridging 03B moving out and 03B′ moving into the coordination sphere (r ≤ 3.0 Å) at elevated temperatures. At 1300 K, the highly distorted M2 octahedron shares two edges with SiA and SiB, rather than one with SiA at 296 K, and the structure has all features of the high-temperature orthopyroxene phase predicted by Pannhorst (1979), which, in fact, is a transitional structural state between orthopyroxene and the so-called protoenstatite. On the basis of the configurations of the silicate chains in Mg- and Fe-rich orthopyroxenes at high temperature, an explanation is given as to why Mg-rich orthopy­roxenes tend to transform to the protoenstatite structure with increasing temperature, whereas Fe-rich ones tend to transform to a clinopyroxene with space group C2/c. The anomalous behavior of the Fe-Mg order-disorder above 1200 K is attributed to the exis­tence of the transitional state, particularly the changing charge distribution around the M2 site.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-202

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractMonazite and xenotime, the RE(PO4) dimorphs, are the most ubiquitous rare earth (RE) minerals, yet accurate structure studies of the natural phases have not been reported. Here we report the results of high-precision structure studies of both the natural phases and the synthetic RE(PO4) phases for all individual stable rare earth elements.Monazite is monoclinic, P21/n, and xenotime is isostructural with zircon (space group I41/amd). Both atomic arrangements are based on [001] chains of intervening phosphate tetrahedra and RE polyhedra, with a REO8polyhedron in xenotime that accommodates the heavy lanthanides (Tb-Lu in the synthetic phases) and a REO9, polyhedron in monazite that preferentially incorporates the larger light rare earth elements (La-Gd). As the struc­ture “transforms” from xenotime to monazite, the crystallographic properties are com­parable along the [001] chains, with structural adjustments to the different sizes of RE atoms occurring principally in (001).There are distinct similarities between the structures that are evident when their atomic arrangements are projected down [001]. In that projection, the chains exist in (100) planes, with two planes per unit cell. In monazite the planes are offset by 2.2 Å along [010], relative to those in xenotime, in order to accommodate the larger light RE atoms. The shift of the planes in monazite allows the RE atom in that phase to bond to an additional O2′ atom to complete the REO9polyhedron.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-203

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractA homogeneous megacryst of schorlomite was investigated to determine the valence states of Fe and Ti and the crystallographic sites occupied by these elements. The chemical composition of the specimen was analyzed by electron microprobe, wet-chemical analysis, FTlR, and INAA. The results from X-ray absorption near-edge structure spectroscopy (XANES) are consistent with exclusively Ti4+occupying the octahedral site only. The tetrahedral site is deficient in Si and the results of low-temperature57Fe Mossbauer spec­troscopy indicate that the remainder of the site is occupied by Fe3+and substantial Fe2+. A spin-allowed intensified crystal-field transition of[4]Fe2+is present in the near-infrared spectrum. The optical absorption spectrum is dominated by an intense band centered at 500 nm with a full width of 8000 cm-1at half maximum peak height; this band is assigned to an Fe2+-Ti4+intervalence charge transfer transition between,[4]Fe2+and[6]Ti. The cation site occupancies in this specimen of schorlomite can be expressed by the following formula: {Ca2.866Mg0.800Na0.038Mn0.019}Σ3.003[Ti4+1.058Fe3+0.631Al0.137Fe2+0.057Mg0.055Zr0.039V3+0.014Mn0.013] Σ2.004- (Si2.348Fe3+0.339Fe2+0.311[4H]0.005)Σ3.003O12.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-204

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractThe minerals zunyite, Al13Si5O20(OH9F)18Q5and harkerite, Ca24Mgg[AlSi4(O5OH)16]2- (CO3)8(BO3)8(H2O9Q)5have been studied by means of solid-state27Al MAS NMR. Zunyite contains Si5O16pentamers and harkerite contains AlSi4(O5OH)16pentamers. These pen- tameric groups are unique because their T-O-Si angles are almost 180°. Chemical analysis of the zunyite sample shows that it has excess Al: the Al-Si ratio is 2.9, compared with the ideal of 2.6.High-speed spinning27Al MAS NMR spectra (11-13 kHz) showed two more signals than the spectra obtained by Kunwar et al. (1984). The signal with δiso= 46.8 ± 0.5 ppm represents the excess Al, which enters the central Si1 site of the Si5O16pentamer. This assumption is confirmed by the fact that δisoof Al in the AlSi4(O,OH)16pentamer in harkerite is 44 ± 1 ppm. Additional proof comes from comparing the electrostatic energy and the quadrupole interaction of Al in either a Si1 or Si2 configuration.The Al site in the pentamers of zunyite and harkerite can be considered as a q4(4Si) site. In this case, Al-O-Si angles are correlated with the27Al chemical shift (Lippmaa et al., 1986). This correlation holds well for the27Al data for harkerite. The value for zunyite indicates that the structure adapts to the incorporation of Al in the Sil site by a narrowing of the Al-O-Si angle to 171 ± 2.5°. The lower limit of the chemical shift range for Al in framework aluminosilicates is decreased by 12 ppm, from 55.8 ppm for mordenite to 44 ppm for harkerite.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-205

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractImpedance spectroscopy helps distinguish the contributions that grain interiors and grain boundaries make to electrical resistance of silicate minerals and rocks. The technique also distinguishes the low-frequency response due to the presence of instrument electrodes. We measured olivine, orthopyroxene, clinopyroxenes, and both natural and synthetic clinopyroxenite. Measurements were made at 1 bar, from 750 to 1150 °C, and over a fre­quency range from106Hz; some measurements were also made at 300-850 °C and 10-20 kbar. The grain-interior response lies at highest frequency, the sample- electrode response at low frequencies, and the grain boundary response at mid-frequencies. Grain interiors show as semicircular impedance arcs when plotted on the complex plane, and sample-electrode responses of hot single crystals and of hot dry rocks are exhibited as depressed arcs. In comparison, monofrequency measurements contain no information to identify the source of the response; at 1 kHz they detect only the resistance sum of grain interiors and grain boundaries and at low frequency (≤ 1 Hz) are likely to sense all three components. The major experimental problem is to find electrodes that make good contact with the sample and that are stable with time. The effect of pressure (10 kbar, 300-800 °C) is to diminish the resistance associated with grain boundaries and the sample-electrode interface, in the laboratory and presumably in nature. Monofrequency measurements at 1 bar may underestimate the conductivity of rocks at similar temperature but higher pressure.A network of electrical elements is presented for use in interpreting impedance spectra and conductive paths in hot or cold, wet or dry, minerals and rocks at any pressure. In dry rocks, a series network path predominates; in wet rocks, aqueous pore fluid and crystals both conduct. Finite resistance across the sample-electrode interface is evidence that elec­tronic charge carriers are present at the surface, and presumably within, the silicate min­erals and rocks measured.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-206

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractDozyite is a new mineral species involving regular interstratification of trioctahedral serpentine and trioctahedral chlorite units in a 1:1 ratio. It occurs as colorless crystals in an altered skarn adjacent to the Ertsberg East Cu, Au, and Ag mine in central Irian Jaya, Indonesia. The name is after Jean Jacques Dozy, the Dutch geologist who discovered and named the Ertsberg ore province in 1936. Unit-cell parameters are a = 5.323(3), b = 9.214(9), c = 21.45(2) A, β= 94.43(6)°, and V = 1049(2) Å3.It has space group Cm. There is a 21-Å periodicity in the 00l and in most other reflections where k = 3n. The reflections where k ≠3n are continuously streaked. Excellent regularity of alternation of the component serpentine and chlorite units is indicated by the coefficient of variation CV = 0.26 for the 001 reflections. A simplified ideal bulk composition is (Mg7Al2)(Si4Al2)O15- (OH)12with Z = 2, halfway between the compositions of the closely associated discrete chlorite (clinochlore) and the discrete serpentine (amesite). We believe the components in this occurrence of dozyite are clinochlore and amesite and that the interstratification was formed during the conversion of early clinochlore to amesite by Al metasomatism. The structure of dozyite contains a la chlorite unit followed by a serpentine 1:1 layer that is in the same position that the lower tetrahedral sheet of a repeating chlorite unit would occupy in the one-layer monoclinic Iaa-2 chlorite polytype, but rotated 180° so that the octahedral cations alternate I,I,II per 21 Å. The next chlorite unit follows with zero shift of its lower sixfold rings relative to those of the serpentine 1:1 layer.A second occurrence of dozyite has been recognized in a Cr-rich serpentinite from the Wood Chrome mine in Lancaster County, Pennsylvania. It represents a different polytype, with β= 90°, and a different composition relative to the Ertsberg material.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-207

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractPelitic schists in the contact aureole of the Iate-Alpine Vedrette di Ries granodiorite show an epitaxial replacement of kyanite by Fe-rich staurolite. The reaction is observed within nodules of fine-grained staurolite crystals (100-200 μm) containing oriented la­mellae of kyanite.Microprobe, backscattered electron microscopy (BSEM), and high-resolution transmis­sion electron microscopy (HRTEM) investigations demonstrate the presence of submicro- scopic intergrowths of staurolite and kyanite. The orientation of both phases, ast‖bKyand cst‖cKy,is in agreement with previous observations. Individual staurolite and kyanite la­mellae are between one unit cell and some tens of unit cells wide. Detailed observation of the termination of staurolite lamellae within kyanite suggests that the replacement occurs by a solid-state reaction, in which the primary cubic close-packed O framework of kyanite is preserved. The epitaxial replacement implies a reorganization of the site occupancy accompanied by addition of Fe + Mg + H and removal of Si + Al from the reaction site.The epitaxial replacement of metastable kyanite occurred during contact metamorphism within the andalusite stability field, under temperature and pressure conditions of 500-550 °C and>2.9 kbar, respectively.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-208

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractOptically anisotropic and isotropic haiiyne has been observed in phonolite from Niang- niang Hill, Jiangsu Province, southeastern China. Selected-area electron diffraction (SAED) and transmission electron microscopy (TEM) studies show the anisotropic haiiyne has a one-dimensional superstructure with a period of 6dI10subalong one of the<110) directions of the ideal cubic haiiyne subcell. The superstructure may be considered as a periodic arrangement of (110) layer domains with P23 symmetry along [110]. Neighboring domains are related by a twin relationship. The unit-cell parameters for this haiiyne superstructure are thus metrically orthorhombic, with asup= 2d1̄10sub,bsup= 6d110sub, Csup= csub, but the most likely space group is monoclinic, Pn. SAED patterns and TEM images of two opti­cally isotropic haiiyne samples show they have either a three-dimensional superstructure or no superstructure. The average structure of the isotropic haiiyne with the superstructure is cubic, even though it is composed of twin domains with one-, two-, and three-dimen­sional superstructures. The superstructure may have formed by ordering Of[Na3Ca(SO4)]3+and [(Na,K)4(OH)]3+clusters in the cubo-octahedral cages of the haüyne framework during an arrested phase transition from P4̄3n to P23 symmetry.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-209

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter

摘要:

AbstractH2O solubility has been determined in haplogranitic melts (system SiO2-NaAlSi3O8- KAlSi3O8, Qz-Ab-Or) in the range 0.5-8 kbar and 800-1350 °C Three types of starting materials were used: dry glass cylinders, prehydrated glass pieces, or dry glass blocks surrounded by glass powder. All the starting materials gave consistent results. The H2O contents of the glasses were determined by Karl-Fischer titration. Dissolved H2O was demonstrated to be distributed homogeneously throughout the isobarically quenched melts (glasses) using infrared spectroscopy.The compositional dependence of H2O solubility was mainly determined at 0.5 kbar, 900 and 1000 °C; I kbar, 8500C; and 4.8 kbar, 8000C. Seventeen compositions containing 25, 35, or 45 wt% normative Qz and with various Or/(Or + Ab) ratios (0.86-0.09, Ab and Or expressed as normative weight percent) were investigated. At 0.5 kbar, H2O sol­ubility was little affected by the anhydrous composition. By contrast, molar H2O solubility in aluminosilicate melts was significantly dependent upon anhydrous composition between I and 5 kbar. The highest solubility values were obtained for the most Ab-rich melts. This alkali effect has important implications for the physical and chemical properties of granitic melts.The effect of pressure (P) on H2O solubility at P ≥ 3 kbar is greater than that reported in previous studies. Between 3 and 8 kbar at 800 °C, there is a (nearly linear) positive correlation between P and H2O solubility. The effect of temperature (T) on H2O solubility was investigated for a composition Qz28Ab38Or34(normative weight percent) in the P-T range 0.5-8 kbar and 800-1350 °C. Water solubility ranged from retrograde (with increas­ing T) at P ≤ 4 kbar through temperature independence at approximately 4.5 kbar to prograde at P = 5 kbar.Calculated H2O solubilities using the model of Burnham and Nekvasil (1986) are slightly high at 0.5 kbar and significantly low at 5 kbar, compared with the experimental data. This implies that calculated H2O activities for haplogranitic systems using the H2O content of the melt may be overestimated at high pressure (P ≥ 5 kbar). Using the ther­modynamic model of Silver and Stolper (1985) and assuming a proportion of molecular H2O and OH groups close to that defined for albite melts by Silver and Stolper (1989), we found that the partial molar volume of H2O in a melt with a composition Qz28Ab38Or34has to be close to 10-12 cm3/mol to obtain a good agreement between the calculated and the experimentally determined H2O solubility curves in the pressure range 1-8 kbar at 900 °C.

ISSN:0003-004X    

DOI:10.2138/am-1995-1-210

出版商:Mineralogical Society of America    

年代:1995

数据来源: Degruyter