Chemical composition, statistical analysis of the unit cell, and electrostatic modeling of the structure of Al-saturated chlorite from metamorphosed rocks
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Sang Soo Lee
,
M. Darby Dyar
Abstract
Natural Al-saturated chlorite having a wide range of Mg:Fe ratios [Mg/(Mg + Fe) = 0.357 to 0.943] was studied to determine the effect of Mg-Fe substitution on the unit-cell parameters. With a nearly constant Al content [AlT/(AlT + Mg + Fe) = 0.362 to 0.416, where AlT represents total Al contents in both tetrahedral and octahedral sites], the only major variable affecting the unit-cell dimensions of natural chlorite is the Mg:Fe ratio. The value of about 0.4 for the Al content represents apparent Al saturation for chlorites in metamorphosed rocks. Unit-cell parameters were determined by least-squares refinement from 39 chlorite samples obtained over a large range in metamorphic grades, from X-ray data derived from Gandolfi pattern simulations (via single crystals and a single-crystal diffractometer) from this study and from traditional powder diffractometer data from the literature. Nine of the samples were analyzed for their Fe3+ contents (= 0.147 to 0.304 atoms) by Mössbauer spectroscopy; chemical compositions were generally obtained by electron-microprobe analysis.
Statistical tests show that the change of the crystallographic a and b axes are linear over the range of Mg:Fe ratios studied, whereas the crystallographic c axis and the β angle have no significant relationship to these Mg:Fe ratios. Equations relating the effect of Mg:Fe ratio on a and b are: a (Å) = −0.092 x + 5.408; x = Mg/(Mg + Fe) and b (Å) = − 0.162 x + 9.370, respectively.
Electrostatic models involving Pauling.s electrostatic valency principle for Al-saturated chlorite were determined assuming: (1) Al-Al avoidance for Al-substituted tetrahedra, (2) no vacancies, and (3) no Al3+ substitution in the M1 sites. Electrostatic restrictions occur for R3+ → R2+ substitutions in the M2 sites of the 2:1 layer. These restrictions produce an Al saturation in chlorite at AlT/(AlT + Mg + Fe) ratios of near 0.4. This saturation limit requires that 37.5% of the tetrahedral sites (1.5 out of 4 sites) are occupied by Al, and the electrostatic charge (electrostatic valency units, e.v.u.) close to +1 e.v.u. is favored for the interlayer to offset the net negative charge on the 2:1 layer.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Single-crystal FTIR and X-ray study of vishnevite, ideally [Na6(SO4)][Na2(H2O)2](Si6Al6O24)
- Crystallographic texture and microstructure of terebratulide brachiopod shell calcite: An optimized materials design with hierarchical architecture
- High-temperature phase relations and topological constraints in the quaternary system MgO-Al2O3-SiO2-Cr2O3: An experimental study
- Tilt and buckling modes, and acoustic anisotropy in layers with post-perovskite connectivity
- Optical absorption study of natural garnets of almandine-skiagite composition showing intervalence Fe2+ + Fe3+ → Fe3+ + Fe2+ charge-transfer transition
- Deriving formation constants for aqueous metal complexes from XANES spectra: Zn2+ and Fe2+ chloride complexes in hypersaline solutions
- Birnessite polytype systematics and identiÞ cation by powder X-ray diffraction
- Improved measurement of fission-track annealing in apatite using c-axis projection
- Improved modeling of fission-track annealing in apatite
- Intercalibration of FTIR and SIMS for hydrogen measurements in glasses and nominally anhydrous minerals
- Order and miscibility in the otavite–magnesite solid solution
- An infrared investigation of the otavite-magnesite solid solution
- Partitioning of calcium, magnesium, and transition metals between olivine and melt governed by the structure of the silicate melt at ambient pressure
- The geometric effects of VFe2+ for VMg substitution on the crystal structures of the grandidierite-ominelite series
- The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis
- Gram-Charlier development of the atomic displacement factors into mineral structures: The case of samsonite, Ag4MnSb2S6
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- Biopyribole evolution during tremolite synthesis from dolomite and quartz in CO2-H2O fluid
- The evolution of diamond morphology in the process of dissolution: Experimental data
- The pearceite-polybasite group of minerals: Crystal chemistry and new nomenclature rules
- Determination of layer stacking microstructures and intralayer transition of illite polytypes by high-resolution transmission electron microscopy (HRTEM)
- Structural behavior of Al3+ in peralkaline, metaluminous, and peraluminous silicate melts and glasses at ambient pressure
- The crystal structure of ingersonite, Ca3Mn2+Sb45+O14, and its relationships with pyrochlore
- Chemical composition, statistical analysis of the unit cell, and electrostatic modeling of the structure of Al-saturated chlorite from metamorphosed rocks
- XANES study of the oxidation state of Cr in lower mantle phases: Periclase and magnesium silicate perovskite
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