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

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) ų, 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) ų, 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.