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

Aluminum hydroxysulfate, AlSO₄(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 AlSO₄(OH) crystal at 700°C and ∼1.0 GPa in a hydrothermal diamond-anvil cell from a mixture of 95% H₂SO₄ and Al₂O₃ powder and its structure determination from single-crystal X-ray diffraction data. AlSO₄(OH) is monoclinic with space group C2/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 AlO₆ octahedra that run parallel to the c-axis. These chains are linked together by SO₄ tetrahedra and hydrogen bonds, forming an octahedral-tetrahedral framework. Except for the numbers and positions of H atoms, AlSO₄(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 AlSO₄(OH) is the same as that obtained by Shanks et al. (1981) through hydrolysis of Al₂(SO₄)₃ solutions at temperatures above 310°C. To date, AlSO₄(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)SO₄ 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.