Crystal structure of coalingite

Abstract

Summary: The crystal structure of coalingite (Mg₁₀Fe₂(OH)₂₄(CO₃)·2H₂O) has been determined using single-crystal X-ray methods. The mineral is trigonal, with space group Rm, a_H = 3·12, c_H = 37·4 Å, Z = ½, and (0001) cleavage. The structure is of a layer type, and is based on a structural element about 12·5 Å thick in the c-direction and consisting of two brucite-like layers and one disordered layer containing carbonate ions and water molecules and resembling those in sjögrenite and pyroaurite. The unit cell comprises three of these structural elements stacked together in the c-direction. The Mg²⁺ and Fe³⁺ ions are randomly distributed among all the octahedral sites of the brucite-like layers. The structure closely resembles those of sjögrenite and pyroaurite, but has two brucite-like layers between each CO₃²⁻−H₂O layer where these have one. There is a tendency to random interstratification, and the crystals appear to contain intergrown regions of brucite and of sjögrenite or pyroaurite. Coalingite-K probably has a similar structure, but with three brucite-like layers between each -H₂O layer; its idealized formula is probably Mg₁₆Fe₂(OH)₃₆(CO₃).2H₂O.