Does anyone not love stibnite? Those spectacular elongated crystals are favorites of collectors and museum-goers alike. Stibnite is Sb2S3. The Sb atoms are in 7-fold coordination with sulfur, but unlike many minerals with odd coordinations, these coordination polyhedra are quite regular. The atomic structure is fairly simple and easy to see.
The coordination polyhedra are augmented triangular prisms, where a square pyramid is built out on one face of a triangular prism. Unlike many minerals where you have to squint fairly hard to see a resemblance to a named polyhedron, these polyhedra are pretty regular. There are two slightly different kinds. One has regular planar square prism faces, the other has slightly kinked squares (the kink edge is shown with a dashed line. These polyhedra link up in pairs perpendicular to each other as shown above. Pyramid faces are yellow, prism faces are blue and the ends of the prisms are pink.
At left above is an oblique view of the stibnite structure. The two types of polyhedra are green and gold. At right is a view perpendicular to the c-axis, the axis of elongation of the crystals. Although the c-dimension of the unit cell is only a third as large as the a- and b- dimensions, unit cells stack neatly in the c direction, probably accounting for the crystal habit.The kink edges are in gray. At right is a view of the structure with the c-axis horizontal and the a-axis vertical
The view above shows two views with the b-axis vertical. At left, the c-axis is horizontal and at right the a-axis is horizontal. The two types of polyhedra are in green and gold. The kink edges are in gray. The edges of the unit cell in the a and b directions are very nearly equal, 1.1234 and 1.1314 nm, respectively. The c edge is much smaller: 0.3837 nm.
Created 22 April 2013, Last Update 02 May 2013
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