Crystal Structure and Raman Spectroscopy of Synthetic Potassium Richterite

Abstract

The structures of two K-richterite crystals with the crystal chemical formulas (K_0.44Na_0.32\(\square \)_0.24)_Σ=1(Ca_1.18Na_0.82)_Σ=2Mg₅Si₈O₂₂OH₂ and (K_0.83Na_0.02\(\square \)_0.15)_Σ=1(Ca_1.11Na_0.89)_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, V_e.c. = 916.17(18) and 918.52(8) ų, 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 V_u.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.