Hydrothermal synthesis and crystal structure of AlSO4(OH): A titanite-group member

Abstract

Aluminum hydroxysulfate, AlSO 4 (OH), is postulated to play a vital role in controlling the solubility of aluminum in sulfate-rich acidic soils and ground waters, but it has not yet been confirmed in nature. This study reports the synthesis of an AlSO 4 (OH) crystal at 700 °C and ~1.0 GPa in a hydrothermal diamond-anvil cell from a mixture of 95% H 2 SO 4 and Al 2 O 3 powder and its structure determination from single-crystal X-ray diffraction data. AlSO 4 (OH) is monoclinic with space group C 2/ c and unit-cell parameters a = 7.1110(4), b = 7.0311(5), c = 7.0088(4) A, β = 119.281(2)°, and V = 305.65(3) A 3 . Its crystal structure is characterized by kinked chains of corner-sharing AlO 6 octahedra that run parallel to the c -axis. These chains are linked together by SO 4 tetrahedra and hydrogen bonds, forming an octahedral-tetrahedral framework. Except for the numbers and positions of H atoms, AlSO 4 (OH) is isostructural with the kieserite-type minerals, a subgroup of the titanite group of minerals. A comparison of powder X-ray diffraction patterns indicates that our AlSO 4 (OH) is the same as that obtained by Shanks et al. (1981) through hydrolysis of Al 2 (SO 4 ) 3 solutions at temperatures above 310 °C. To date, AlSO 4 (OH) has been synthesized only at temperatures above 290 °C, implying that it may not stable in low-temperature environments, such as acidic soils and mine waters. The possible environments to find Al(OH)SO 4 may include places where sulfur-rich magma-derived fluids react with aluminous rocks under elevated temperature and pressure, and on Venus where a sulfur-rich atmosphere interacts with surface rocks at temperatures above 400 °C.