Steudelite, (Na3☐)[(K,Na)17Ca7]Ca4(Al24Si24O96)(SO3)6F6·4H2O, a new cancrinite-group mineral with afghanite-type framework topology

The new cancrinite-group mineral steudelite, ideally (Na3☐)[(K,Na)17Ca7]Ca4(Al24Si24O96)(SO3)6F6·4H2O, was discovered in syenite ejectum from the Sacrofano paleovolcano, Latium region, Italy. The associated minerals are sanidine, diopside, andradite, biotite, leucite, haüyne, sacrofanite, biachellaite, liottite, and secondary dioctahedral smectite. Steudelite forms colourless, thick-tabular, isometric and short-prismatic crystals up to 7 mm across. Cleavage is distinct on {10 1¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{1}$$\end{document} 0}; perfect parting on {0001} is observed. Steudelite is brittle, with uneven fracture. The Mohs’ hardness is 5. Measured and calculated density are equal to 2.51 (1) and 2.511 g cm–3, respectively. The IR spectrum shows the presence of H2O molecules, SO32– and minor SO42– anionic groups. The chemical composition of steudelite is (wt%; electron microprobe, H2O determined by the modified Penfield method; total sulfur apportioned between SO2 and SO3 based on structural data): Na2O 7.40, K2O, 8.42, CaO 3.54, Al2O3 26.46, Fe2O3 0.18, SiO2 30.96, SO2 4.74, SO3 5.18, F 1.66, Cl 0.84, H2O 1.6, –O≡Cl,F − 0.89, total 100.09. The empirical formula is H8.22Na11.06K8.28Ca11.18(Al24.04Fe3+0.10Si23.86O96)(SO3)3.43(SO4)3.00F4.04Cl1.10O3.89. The crystal structure of steudelite is based on the afghanite-type aluminosilicate framework containing a column of cancrinite cages and isolated cancrinite and liottite cages whose populations are {Na3.14Ca0.14☐0.72[(H2O,OH)3.72Cl0.28]}, {Ca4F4.02Cl0.60}, and {K8.40Na8.13Ca7.10(SO3)3.24(SO4)2.76}, respectively. The SO32– and SO42– groups occur in the liottite cage, at two sites with the occupancies S1[(SO4)0.69(SO3)0.31] and S2[SO3]2. Steudelite is hexagonal, space group P-62c, with a = 12.89529 (15), c = 21.2778 (3) Å, V = 3064.21 (8) Å3, and Z = 1. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 11.15 (28) (100), 4.799 (25) (104), 3.973 (16) (105), 3.721 (47) (300), 3.305 (100) (214, 303, 400), 2.661 (17) (008), 2.149 (21) (330). The mineral is named in honour of the outstanding German chemist, a specialist in chemistry of sulfur Prof. Ralf Steudel (1937–2021).