Abstract
Thecrystalstructuresandchemicalcompositionsoftwocrystalsofclinopyroxene(labelledK1andK2)synthesizedinthe 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)O6.00and (Ca0.90K0.07Mg0.03)(Mg0.86Al0.14)(Si1.94Al0.06)O6.00, for K1 and K2 respectively. Latticeparametersare:a =9.803(2),b=8.985(2),c =5.263(1)A, =105.69(1)° forK1anda =9.744(1),b=8.904(2),c =5.273(1) A, b = 106.14(1)° for K2. The structures were refined to Rall = 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 withdivalentcations inthissite. No evidence was found fora smallcation such as Mg coexistingwithCa in the M2 site that might act as a structural stabilizer for the accommodation of K in the clinopyroxene structure.
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