Crystal structures of K-cymrite and kokchetavite from single-crystal X-ray diffraction

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Marco E. Ciriotti
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Crystal structures of K-cymrite and kokchetavite from single-crystal X-ray diffraction

Messaggio da Marco E. Ciriotti » mar 30 mar, 2021 10:31

Referenza:
▪ Romanenko, A.V., Rashchenko, S.V., Sokol, A.G., Korsakov, A.V., Seryotkin, Y.V., Glazyrin, K.V., Musiyachenko, K. (2021): Crystal structures of K-cymrite and kokchetavite from single-crystal X-ray diffraction. American Mineralogist, 106, 404-409.

Abstract:
We determined for the first time the crystal structures of high-pressure K-cymrite (KAlSi3O8⋅H2O) and its dehydrated form kokchetavite (KAlSi3O8) using single-crystal X‑ray diffraction. The K-cymrite structure has been successfully refined in the hexagonal space group P6/mmm [a = 5.3361(3) Å, c = 7.7081(7) Å, V = 190.08(3) Å3, R1 = 0.036 for 127 unique observed reflections], which is in agreement with previous models from powder X‑ray diffraction. In contrast, kokchetavite shows superstructural reflections, suggesting a different unit cell and a space group of P6/mcc [a = 10.5757(3) Å, c = 15.6404(6) Å, V = 1514.94(10) Å3, R1 = 0.068 for 1455 unique observed reflections]. Upon dehydration, single-crystal grains of K-cymrite transform into single-crystal grains of kokchetavite. The latter questions a previous interpretation of kokchetavite crystals in mineral inclusions as a product of direct crystallization from fluid/melt. The Raman spectrum of K-cymrite shows a strong polarization dependence, which is important in identification of the mineral inclusions.
Marco E. Ciriotti

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