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We report data on the composition and crystal structure of the most K-rich (3.61 wt% K 2 O) natural clinopyroxene yet discovered. The studied crystal was found as a tiny inclusion in garnet from a garnet-clinopyroxene rock of the Kumdy-Kol microdiamond mine, Kokchetav complex, Northern Kazakhstan. Microprobe analysis yields the formula (Ca 0.61 Fe 0.13 Mg 0.04 Mn 0.01 K 0.17 Na 0.05 ) (Al 0.61 Mg 0.39 )(Si 1.61 Al 0.39 )O 6.00 . Lattice parameters are: a = 9.773(1), b = 8.926(1), c = 5.269(1) Å, β = 105.75(1)°. The structure was refined up to R all  = 2.42% using 982 independent reflections. Substitution of K for Ca causes significant modification of the average structure. No evidence for an additional M2’ position was found. Crystal-chemical characteristics are compared with published data on both natural and synthetic K-bearing clinopyroxenes.

american mineralogist

Letter. Ultrapotassic clinopyroxene from the Kumdy-Kol microdiamond mine, Kokchetav Complex, Kazakhstan: Occurrence, composition and crystal-chemical characterization