Ultrahigh potassium content in the clinopyroxene structure; an X-ray single-crystal study

The crystal structure and chemical composition of two crystals of clinopyroxene (K1 and K2) synthesized in the model system CaMgSi (sub 2) O (sub 6) -KAlSi (sub 2) O (sub 6) have been determined. The formula for K1 is (Ca (sub 0.75) K (sub 0.23) Mg (sub 0.02) )(Mg (sub 0.73) Al (sub 0.27) )(Si (sub 1.97) Al (sub 0.03) )O (sub 6.00) , with a 9.803, b 8.985, c 5.263 Aa, beta 105.69 degrees ; K2 has (Ca (sub 0.90) K (sub 0.07) Mg (sub 0.03) )(Mg (sub 0.86) Al (sub 0.14) ) (Si (sub 1.94) Al (sub 0.06) )O (sub 6.00) , with a 9.744, b 8.904, c 5.273 Aa, beta 106.14 degrees . The structures were refined to R 1.16 and 2.02, respectively. The substitution of K for Ca causes strong modifications to the average structure, mainly a lengthening of the M2-O3Cl bond distance and a shortening of T-O3Al. The crystal K1 shows the largest volume for a M2 polyhedron ever reported for a clinopyroxene structure with divalent cations in this site.