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THE CRYSTAL STRUCTURE OF HUBEITE, A NOVEL SOROSILICATE MINERAL

Mark A. Cooper and Frank C. Hawthorne

Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

E-mail address: frank_hawthorne{at}umanitoba.ca

The crystal structure of hubeite, Ca₂ Mn²⁺ Fe³⁺ [Si₄ O₁₂ (OH)] (H₂O)₂, triclinic, space group P, a 9.9653(3), b 13.9171(3), c 6.5703(2) Å, 133.264(1), ß 101.414(1), 66.302(1)°, V 603.47(6) ų, Z = 2, has been solved by direct methods and has been refined to an R index of 2.2% based on 2875 observed (5) reflections measured with MoK X-radiation. There are two Ca sites, one [8]- and one [7]-coordinated, with <Ca–> (: O, OH, H₂O) distances of 2.527 and 2.485 Å, respectively. There is one Mn site octahedrally coordinated by five O-atoms and one (OH) group, with a <Mn–O> distance of 2.224 Å, indicating occupancy of this site by Mn²⁺. There is one Fe site octahedrally coordinated by O-atoms, with an <Fe–O> distance of 2.025 Å, indicating occupancy of this site by Fe³⁺. There are four Si sites, each occupied solely by Si, with a <Si–O> distance of 1.627 Å. One of the silicate tetrahedra is an acid silicate group: SiO₃(OH). The four Si tetrahedra link by sharing corners to form a linear [Si₄₁₃] group, and hence hubeite is a sorosilicate. The overall structure consists of layers of [Si₄₁₃] groups alternating with layers of edge-sharing [6]-, [7]- and [8]-coordinated Ca, Mn²⁺ and Fe³⁺ polyhedra parallel to (001). These layers stack in the [001] direction, forming a framework with tunnels that extend along [001] and contain the isolated (H₂O) groups. The structure of hubeite is related to the structures of inesite, rhodonite and babingtonite.

Keywords: hubeite, crystal structure, sorosilicate, Hubei, China.

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