Betzite, Na₆Ca₂(Al₆Si₆O₂₄)Cl₄, a New Cancrinite-Group Mineral from the Eifel Paleovolcanic Region, Germany

Betzite, ideally Na₆Ca₂(Al₆Si₆O₂₄)Cl₄, a new cancrinite-group mineral, was discovered in a metasomatically altered (pyrometamorphosed) calcic xenolith, hosted by alkaline basalt at the Bellerberg paleovolcano in the Eastern Eifel region, Rhineland-Palatinate, Germany. The associated minerals are anorthite, phlogopite, diopside, grossular, fluorite, calcite, a tobermorite-like mineral, and vanadoallanite-(Ce). Betzite occurs as colorless hexagonal prismatic crystals up to 2 mm long and up to 0.5 mm thick. The new mineral is brittle, with a Mohs' hardness of 5½. Distinct cleavage on {100} and parting on {0001} are observed. The D_meas = 2.38(2) g/cm³ and D_calc = 2.363 g/cm³. Betzite is optically uniaxial (+) with ω = 1.528(2) and ε = 1.545(3). The IR spectrum is given. The chemical composition of betzite is (wt.%; electron microprobe, H₂O determined by the modified Penfield method): Na₂O 11.88, K₂O 4.82, CaO 10.74, MgO 0.21, Al₂O₃ 27.32, Fe₂O₃ 0.68, SiO₂ 32.84, SO₃ 1.89, Cl 10.48, H₂O 1.10, −O≡Cl −2.37, total 99.59. The empirical formula is Na_4.22K_1.13Ca_2.11Mg_0.06(Si_6.01Al_5.90Fe³⁺_0.09O₂₄)Cl_3.25(SO₄)_0.26(H_1.34O_0.64). The crystal structure was determined using single-crystal X-ray diffraction data. It is hexagonal, space group P6₃, a = 12.8166(9) Å, c = 5.3562(3) Å, V = 761.95(12) ų (at a temperature of 100 K) and Z = 3. Betzite is a dimorph of quadridavyne, with a disordered distribution of extra-framework components occupying channels. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 11.14 (31) (100), 4.833 (93) (101), 3.715 (95) (300), 3.313 (100) (211), 2.787 (37) (400), 2.681 (56) (002, 131), 2.474 (35) (112, 401), 2.146 (24) (330). The mineral is named in honor of the German amateur mineralogist Volker Betz (b. 1947).