ULTRAHIGH POTASSIUM CONTENT IN THE CLINOPYROXENE STRUCTURE: AN X-RAY SINGLE-CRYSTAL STUDY

Abstract

The crystal structures and chemical compositions of two crystals of clinopyroxene (labelled K1 and K2) synthesized in the model system CaMgSi 2 O 6 -KAlSi 2 O 6 at 7 GPa, have been investigated. Quantitative analysis leads to the following formulae: (Ca 0.75 K 0.23 Mg 0.02)(Mg 0.73 Al 0.27)(Si 1.97 Al 0.03)O 6.00 and (Ca 0.90 K 0.07 Mg 0.03)(Mg 0.86 Al 0.14)(Si 1.94 Al 0.06)O 6.00 , for K1 and K2 respectively. Lattice parameters are: a = 9.803(2), b = 8.985(2), c = 5.263(1) Å, = 105.69(1)° for K1 and a = 9.744 (1), b = 8.904(2), c = 5.273(1) Å, b = 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.