Crystal Structure and Raman Spectroscopy of Synthetic Potassium Richterite

The structures of two K-richterite crystals with the crystal chemical formulas (K 0.44 Na 0.32 0.24 ) Σ=1 (Ca 1.18 Na 0.82 ) Σ=2 Mg 5 Si 8 O 22 OH 2 and (K 0.83 Na 0.02 0.15 ) Σ=1 (Ca 1.11 Na 0.89 ) Σ=2 Mg 5 Si 8 O 22 OH 2 synthesized at a pressure of 3 GPa and a temperature of 1000°С in the MgSiO 3 + CaMgSi 2 O 6 + K 2 CO 3 + Na 2 CO 3 + CO 2 + H 2 O system were studied by single-crystal X-ray analysis and Raman spectroscopy. The monoclinic cell parameters were obtained: a = 10.0256(5) and 9.9748(11) Å, b = 17.9874(7) and 17.9879(16) Å, c = 5.2687(3) and 5.2746(6) Å, V e.c. = 916.17(18) and 918.52(8) Å 3 , β = 104.520(12)° and 104.821(5)°, sp. gr. С 2/ m (12), and Z = 2. The sites M (1), M (2), and M (3) are inhabited by Mg 2+ , while site M (4) is occupied simultaneously by Ca 2+ and Na + . The Na + cations not included in M (4) are located in position A , which also accommodates K + cations. Raman spectroscopy made it possible to reveal vacancies in position A in both samples. The structure corresponds to the “ideal” structure of richterite group minerals. The unit cell volume of the measured crystals is directly proportional to the K content in position A . Based on the generalization of new and published data, an equation of the dependence of V u.c. for amphiboles of the richterite Na(NaCa)Mg 5 Si 8 O 22 (OH) 2 –K-richterite K(NaCa)Mg 5 Si 8 O 22 (OH) 2 series with a low tremolite component on the K content in position A is proposed.