Crystal Structure and Raman Spectroscopy of Synthetic Potassium Richterite

The structures of two K-richterite crystals with the crystal chemical formulas (K0.44Na0.32 $$\square $$ 0.24)Σ=1(Ca1.18Na0.82)Σ=2Mg5Si8O22OH2 and (K0.83Na0.02 $$\square $$ 0.15)Σ=1(Ca1.11Na0.89)Σ=2Mg5Si8O22OH2 synthesized at a pressure of 3 GPa and a temperature of 1000°С in the MgSiO3 + CaMgSi2O6 + K2CO3 + Na2CO3 + CO2 + H2O 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, Ve.c. = 916.17(18) and 918.52(8) Å3, β = 104.520(12)° and 104.821(5)°, sp. gr. С2/m (12), and Z = 2. The sites M(1), M(2), and M(3) are inhabited by Mg2+, while site M(4) is occupied simultaneously by Ca2+ 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 Vu.c. for amphiboles of the richterite Na(NaCa)Mg5Si8O22(OH)2–K-richterite K(NaCa)Mg5Si8O22(OH)2 series with a low tremolite component on the K content in position A is proposed.