High-symmetry sulfate-rich imayoshiite from the Shijiangshan mine, Inner Mongolia, China, and its crystal structure

Abstract

Imayoshiite, Ca₃Al(CO₃)[B(OH)₄](OH)₆·12H₂O, is a rare mineral belonging to the ettringite group. The imayoshiite discussed in this paper was discovered in hydrothermally altered pockets at the Shijiangshan mine, Hexigten Banner, Chifeng City, Inner Mongolia, China. Before its discovery at the Shijiangshan mine, the only known locality for this mineral was Suisho-dani, Ise City, Mie Prefecture, Japan. The formation of Shijiangshan imayoshiite relates to the low-temperature hydrothermal processes during the late skarn stage. Its euhedral crystals exhibit a combination of hexagonal dipyramid {10-12} and hexagonal prism {10-10}. The empirical chemical formula of the sample is Ca₃(Al_0.78Si_0.11Fe_0.01)_∑0.90{[B(OH)₄]_0.91[CO₃]_0.60[SO₄]_0.45}_∑1.96(OH)_5.80·12.39H₂O, with Z = 2 (based on Ca_apfu = 3). Due to its disordered structure and specific crystal-chemical features, the crystal’s structure was solved and refined with a higher symmetry space group (P6₃/mcm) compared to the previously reported (P6₃). Using this space group and unit-cell parameters of a = 11.0663(6) Å, c = 10.6387(6) Å, and V = 1128.30(14) ų, the refinement converged to R = 0.0492. X-ray Photoelectron Spectroscopy (XPS) showed that all S atoms are present as SO₄²⁻. This study presents new data on the high-symmetry variety of imayoshiite, i.e., imayoshiite-P6₃/mcm, and summarizes the complex ligand morphology resulting from disorder in the ettringite group.