Showing papers in "American Mineralogist in 1987"

Quartz-fayalite-iron and quartz-fayalite-magnetite equilibria and the free energy of formation of fayalite (Fe 2 SiO 4 ) and magnetite (Fe 3 O 4 )

Abstract: The quartz-fayalite-iron and quartz-fayalite-magnetite equilibria have been studied in the temperature range 1000-1400 and 1050-1300 K, respectively, using an electrochemical technique. The results are in excellent agreement with the calorimetric data on fayalite (Robie et al., 1982). For magnetite, the results demonstrate conclusively that there is no zero-point entropy and that the earlier heat-content measurements of Coughlin et al. (1951) should be preferred to the measurements by differential scanning calorimetry of Gronvold and Sveen (1974) above the Curie point. For quartz-fayalite-iron,

Muscovite dehydroxylation; high-temperature studies

Abstract: Ansl:n.Lcr Unit-cell dimensions of muscovite-2M, from the Diamond mine, South Dakota, were determined to 1000'C, single-crystal X-ray structural refinements were made at 20'C and 300'C, and additional structural refinements at 20, 525. and 650'C were made on similar material from Panasqueira, Portugal. Single-crystal data showed linear expansivity of the metric dimensions to about 850"C followed by a phase change to muscovite dehydroxylate with increases in the b and c parameters and a decrease in a near 850'C. The mean atomic distances ofthe K polyhedron increased more rapidly than those for the other polyhedra as temperature increased. The result shows that (l) the K-O(2) bond is longer than KO(l) and K-O(3) at each temperature and (2) the K4(2) bond distance [2.867(3) A at 20'Cl expands with temperature more rapidly [with a mean thermal-expansion coefficient (MTEC) of 5.41 than the other two interlayer bonds tK-O(l): 2.832(3) A at 20'C, MTEC : 4.7; K-O(3): 2.844(3) A at 20'C, MTEC : 4.51. It is concluded that the proton position weakens (lengthens) the K4(2) bond relative to K-O(l) and K-O(3) and that a preferred path for dehydroxylation results. Misfit between the tetrahedral and octahedral sheets is not a factor in thermal decomposition, since the tetrahedral rotation angle is about 8.5" at 950"c. Thermal analysis (rce, orc) indicated a very broad dehydration peak, interpreted as two ovedapping and poorly resolved dehydration peaks at apparently 550 and 750'C on the basis ofcomparison to pyrophyllite. This result suggests that dehydroxylation is not a homogeneous process. A model for dehydroxylation of muscovite was developed from Pauling bond-strength summation calculations. The strength of the AI-OH bond is greatly affected by the coordination number of neighboring polyhedra. When neighboring polyhedra are in octahedral coordination, the hydroxyl group is lost at lower temperatures than when neighboring polyhedra are in fivefold coordination (after partial dehydroxylation). This efect results from the distribution ofoversaturated apical oxygens that affect also the proton position and the AI-OH bond strength. The model is applicable to pyrophyllite and similar dioctahedral 2: I layer silicates and is supported by previously reported infrared, Nun, and rnna data.

Synchrotron radiation XANES spectroscopy of Ti in minerals; effects of Ti bonding distances, Ti valence, and site geometry on absorption edge structure

Abstract: AssrRAcr The near K-edge X-ray absorption spectra of Ti in a suite of silicate and oxide minerals have been examined in an effort to improve Ti characterization in solids. The spectra consist ofelectronic excitations, occurring in the "pre-edge" region, and multiple scattering resonances on the side and top ofthe edge. The pre-edge features are insensitive to Ti-O bond length, but are sensitive to valence, occurring about 2.0 eV lower in energy in Ti3+ samples. The second pre-edge feature is also sensitive to octahedral site distortion and to the presence of tetrahedral Tia* owing to intensification created by d-p orbital mixing. The multiple scattering features shift to lower energy as Ti-O bond length increases and also change in number, intensity, and energy in response to varying ligand symmetry' Analysis ofthese spectral features suggests that tetrahedral Ti4* probably occurs only at very small concentration levels in silicates, even when the total Sia* and Al3+ concentration is well below that necessary to fill available tetrahedral sites, as in the melanite-schorlomite garnets. The levels of Ti3* are more difficult to determine because of an apparent sensitivity of the pre-edge Tio* features to bulk chemistry. However, the overall results are consistent with only small amounts of Ti3+, even in samples from localities where other analyses (wet chemical and Mossbauer) have indicated significant Ti3*.

beta -Mg 2 SiO 4 ; a potential host for water in the mantle?

Abstract: Electrostatic potentials and Pauling bond strengths have been calculated for a group of oxygen and hydroxyl sites in highpressure silicate and oxide phases. The O1 position in β-Mg2SiO4 more closely resembles a hydroxyl than an oxygen in its electrostatic potential and coordination. This raises the possibility that β-Mg2SiO4 may be a host phase for water in the lower part of the upper mantle, if charge balance can be maintained by partial M-site vacancy as is typical of high-pressure pyroxenes. Partial hydroxyl occupancy of O1 is consistent with synthesis experiments, crystal-structure analysis, and Raman spectroscopic data, but further experimental work is required to confirm and evaluate the significance of this possibility. If it can be demonstrated experimentally that this does occur, there would be substantial consequences for geophysical and geochemical models of the upper mantle.

Sericite from the Silverton Caldera, Colorado; correlation among structure, composition, origin, and particle thickness

Abstract: "sericite" is a petrographic term used to indicate highly birefringent, fine-grained, micaceous material that is viewed under the optical microscope. Other analy"tical techniques, including X-ray diffraction (xno) analysis, transmission-electron microscopy (reu), and chemical analysis, reveal that sericite from the Silverton caldera is very fine grained muscovite or phengite, with a layer charge close to 1.0 equivalent per O,.(OH), for the micaceous layers. The structure and chemistry of this sericite differ from that of coarsergrained micas, however, because the sericite particles are so thin that exposed basal surfaces comprise a significant proportion of the sample. The presence of these surfaces leads to swelling (expandability), to larger cation exchange capacities, and to smaller fixed cation contents in the structural formulae. These surface properties correlate with the thickness of the clay particles parallel to c*, in agreement with the theory of interparticle difraction. Expandability also correlates with the Kubler index (thereby suggesting that this "crystallinity" index is a function of particle thickness), with an intensity ratio (Ir), and with the wave number of an 824-834 cm ' infrared absorption band. The amount of octahedral Fe + Mg can be estimated from the wave number of a 528-540 cm ' infrared absorption band. Based on xno analysis and computer modeling, the sericites are classified as R > 3, mixed-layer illite/smectites that range in expandability from >4o/o to

Solubility of carbon dioxide in albitic melt

Abstract: Infrared spectroscopy has been used to measure the concentrations of molecular CO_2, and carbonate in albitic (NaAlSi_3O_8) glasses quenched from melts equilibrated with CO_2, vapor at high pressures (15-30 kbar) and temperatures (1450-1625 °C). At constant temperature, the concentrations of carbonate and molecular CO_2, as well as the CO^2_3/CO_2, ratio increase with increasing pressure under vapor-saturated conditions. The second derivative of these species concentrations with respect to pressure under vapor-saturated conditions at constant temperature is positive over the range of conditions studied. At constant pressure under vapor-saturated conditions, the solubility of molecular CO_2, decreases with increasing temperature, but the concentration of carbonate increases. The net effect is that total CO_2, solubility is nearly independent of temperature. According to our results, CO_2 solubility and speciation change gradually over the range of conditions that we have studied and do not indicate major or abrupt structural changes in albitic melts in this P-Z range. Our results can be described thermodynamically in terms of two reactions. The first, CO_2(vapor) = CO_2,molecular(melt), describes the heterogeneous equilibrium between melt and vapor. The second, CO_2,molecular(melt) + O^2-(melt) = CO^2_3-(melt), models the homogeneous equilibrium between melt species. Volume and enthalpy changes of these two reactions have been constrained by our solubility and speciation data. We emphasize that the solubilities of volatile components that dissolve in melts in several different forms must be treated by such coupled heterogeneous and homogeneous equilibria and that spectroscopic methods provide direct insights into them.

Crystal structures of lizardite-1T and lizardite-2H1 from Coli, Italy

Abstract: The occurrence of lizardite-|T andlizardite-2HI from Coli, Italy, is reported. The two polytypes can be identified by their morphology. The 1T polytype occurs as truncated trigonal pyramids and the 2H1 polytype as truncated hexagonal pyramids or hexagonal plates. Lizardite-1Zhas the composition (Mgr.rrFe.oTAlooexsir e4Alo.o6)Os(OH)0, andlizardite-2H I has the composition (Mgr rrFeo orAlo,o)(Si, e3Alo r)O5(OH)4. Unit-cell parameters are a:5.325(5), c :7.259(7) A, space group P3lm, and a: 5.318(4), c : 14.541(7) A, space group P6rcm for the lT and 2Hl polytypes, respectively. The crystal structures of the two polytypes have been refined to R : 0.074 and0.024 (1T and 2Hl, rcspectively). Whereas the tetrahedral-octahedral layer exhibits negative ditrigonalization in the .lT structure (o : 1.7'), positive distortion (ot: +6.4) occurs in the 2HI structure. This pattern fits the existing predictions about the layer configurations within the diferent polytypes of lizardite.

Crystal structures and crystal chemistry of the uranyl oxide hydrates becquerelite, billietite, and protasite

Abstract: The related crystal structures of three uranyl oxide hydrate minerals, becquerelite, billietite, and protasite, have been determined by single-crystalX-ray diffraction. The chemical formulae have been determined by electron-microprobe analyses. Becquerelite, Ca[(U02)604(OH)6].8H20,is orthorhombic, Pn21a,a = 13.8378(8), b = 12.3781(12),c = 14.9238(9) A, Z = 4, R(Fobs)= 0.083 (2853 reflections). Billietite, Ba[(U02)604(OH)6]. 4H20, is orthorhombic, Pbn21,a = 12.0720(22), b = 30.167(4), c = 7.1455(5) A, Z = 4, R(Fobs) = 0.139 (4104 reflections). Protasite, Ba[(U02)30l0H)2]. 3H20, is monoclinic, Pn, a = 12.2949(16),b = 7.2206(10), c = 6.9558(8) A, (j = 90.401(15)°,Z = 2, R(Fobs) = 0.073 (2505 reflections). Each uranyl ion is coordinated to five other oxygen atoms in a plane nearly perpendicular to the uranyl axis forming infinite sheets that resemble those of aU308 in projection. The sheets are bonded together by large interlayer cations and water molecules.

Enthalpies of formation of dolomite and of magnesian calcites.

Abstract: (Ca,Mg) carbonates have been studied by solution calorimetry at 85"C in an acid solvent (lM IJ)I) that was saturated at room temperature with NaCl and contained 0.7 5M CaCl, and 0.25M MgClr. The enthalpy of formation of ordered dolomite from calcite and mag' nesite at 85"C is -5.74 + 0.25 kJ/mol, for a formula unit of dolomite containing I mol of cations. A synthetically disordered dolomite has an enthalpy of formation of + | .23 ! 0.32 kJlmol. At 85"C, four synthetic magnesian calcites with mole fraction, X, of MgCO, between 0.01 and 0.13 show exothermic heats of mixing relative to CaCO. and MgCOr. Combined with positive excess free energies of mixing, the enthalpy data suggest substantial negative excess entropies of mixing in the synthetic magnesian calcites. It is possible that considerable local order, perhaps clustering of Mg into randomly spaced Mg-rich cation layers, exists in synthetic magnesian calcites equilibrated near 700'C. The thermodynamic behavior of synthetic magrresian calcites may differ from that of biogenic samples.