Istituto di Geoscienze e Georisorse     
Bindi L., Safonov O., Litvin Y., Perchuk L., Menchetti S. Ultrahigh potassium content in the clinopyroxene structure: an X-ray single-crystal study. In: European Journal of Mineralogy, vol. 14 (5) pp. 929 - 934. Schweizerbart, 2002.
The crystal structures and chemical compositions of two crystals of clinopyroxene (labelled K1 and K2) synthesized in the model system CaMgSi₂O₆ - KAlSi₂O₆ at 7 GPa, have been investigated. Quantitative analysis leads to the following formulae: (Ca₀.₇₅K₀.₂₃Mg₀.₀₂)(Mg₀.₇₃Al₀.₂₇)(Si₁.₉₇Al₀.₀₃)O₆.₀₀ and (Ca₀.₉₀K₀.₀₇Mg₀.₀₃)(Mg₀.₈₆Al₀.₁₄)(Si1.₉₄Al₀.₀₆)O₆.₀₀, for K1 and K2 respectively. Lattice parameters are: a = 9.803(2), b = 8.985(2), c = 5.263(1) , β = 105.69(1) for K1 and a = 9.744 (1), b = 8.904(2), c = 5.273(1) β = 106.14(1) for K2. The structures were refined to R_(all) = 1.16 and 2.02 %, respectively, using 982 independent reflections. Substitution of K for Ca causes strong modifications to the average structure, mainly a lengthening of the M2-O3C1 bond distance and a shortening of the T-O3A1 bond distance. The crystal K1 shows the largest volume for a M2 polyhedron ever reported for the clinopyroxene structure with divalent cations in this site. No evidence was found for a small cation such as Mg coexisting with Ca in the M2 site that might act as a structural stabilizer for the accommodation of K in the clinopyroxene structure.
URL: http://www.schweizerbart.de/j/ejm/
DOI: 10.1127/0935-1221/2002/0014-0929
Subject potassium-bearing clinopyroxene
structure refinement
single-crystal X-ray diffraction
synthetic crystals
chemical analysis

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