Calcinaksite, KNaCa(Si₄O₁₀) H₂O, a new mineral from the Eifel volcanic area, Germany

Abstract

The new mineral calcinaksite, ideally KNaCa(Si₄O₁₀) · H₂O, the first hydrous and Ca-dominant member of the litidionite group, is found in a xenolith of metamorphosed carbonate-rich rock from the southern lava flow of the Bellerberg volcano, Eastern Eifel region, Rheinland-Pfalz, Germany. It is associated with wollastonite, gehlenite, brownmillerite, Ca₂SiO₄ (larnite or calcio-olivine), quartz, aragonite, calcite, jennite, tobermorite and ettringite. Calcinaksite occurs as clusters of colourless to light-grey subhedral prismatic crystals. The mineral is brittle, with Mohs’ hardness of 5; D_meas is 2.62(2) g/cm³ and D_calc is 2.623 g/cm³. The IR spectrum shows the presence of H₂O molecules forming three different H-bonds. Calcinaksite is optically biaxial (+), α = 1.542(2), β = 1.550(2), γ = 1.565(3), 2V _meas = 75(10). The chemical composition (electron-microprobe data, H₂O determined by the Alimarin method, wt%) is: Na₂O 6.69, K₂O 12.01, CaO 15.04, FeO 0.59, SiO₂ 61.46, H₂O 4.9, total 100.69. The empirical formula is H_2.11 K_0.99Na_0.84Ca_1.04Fe_0.03Si_3.98O₁₁. The crystal structure was solved and refined to R ₁ = 0.053, wR ₂ = 0.075 based upon 3057 reflections having I > 3σ(I). Calcinaksite is triclinic, space group P $\overline{1}$ _, a = 7.021(2), b = 8.250(3), c = 10.145(2) Å. α = 102.23(2)°, β = 100.34(2)°, γ = 115.09(3)°, V = 495.4(3) ų, Z = 2. The strongest reflections of the X-ray powder pattern [d, Å (I,%) (hkl)] are: 3.431 (70) (−121, −211, −210, 012, 0–22), 3.300 (67) (−031), 3.173 (95) (−103, −201, −220, 003, 111), 3.060 (100) (−212, 2–11, −221, 200, −1-13, 021, −202), 2.851 (83) (0–23, −122, 1–13, 1–31), 2.664 (62) (1–23, −222, 201).