High temperature transformations and thermal expansion of halotrichite FeAl₂ (SO₄)₄⋅22H₂O

Abstract

Halotrichite is a widespread mineral in post-volcanic environments and oxidation zones of ore deposits. Halotrichite is stable at temperature up to 70 °C; further heating leads to the formation of an X-ray amorphous phase I. There are reflections of millosevichite (prevailing) and mikasaite appearing in the range of temperatures 340–660 °C. Millosevichite and mikasaite are decomposing at temperatures 660 °C with the formation of an X-ray amorphous phase II. According to data of the synchronous thermal analysis, the transition from halotrichite into anhydrous sulfates is accompanied by the loss of H2O molecules, which makes about 42.9 wt %, the transition to the X-ray amorphous phase II is caused by the loss of SO3, which is ca. 37.4 wt %, associated with two endothermal effects. The thermal expansion of halotrichite is sharply anisotropic, the maximum expansion is determined by the shear deformations of the lattice in its monoclinic plane along the bisectrix of the obtuse angle β, and the minimum one – in the direction of strong S–O–Fe bonds inside [Fe(SO4)(H2O)5]0 complexes. The significant volumetric expansion of halotrichite (9(3)∙10-5 ºC-1) occurs due to the determing role of hydrogen bonds in composition of the crystal structure.