Austinite

Structure

The structure is composed of chains of edge-sharing polyhedra ZnO₆, and very distorted Ca(O,OH)₈ polyhedra linked through AsO₄ into a three-dimensional network.

Any crystal which has a mirror plane as one of its symmetry elements has the property that its mirror image (with any plane as the mirror plane) can always be superimposed on the original crystal by translation or rotation or both.
If there are no mirror planes as symmetry elements then the mirror image of a crystal cannot be brought into superposition with the original crystal by rotation or translation. This is enantiomorphism, and the mirror images are said to be enantiomorphs of each other. The possibility of enantiomorphic crystals is determined by the crystal symmetry, i.e., by the point group of the crystal species. There are 32 possible point groups, and 22 of these are capable of forming enantiomorphs. The enantiomorphs are designated right or left handed, according to whether they rotate the plane of polarised light to the right or to the left. Sometimes it is clear from the outward form of the crystal whether it is right or left handed, and sometimes optical methods are needed to determine this.

The commonest enantiomorphic mineral is quartz, with point group 32; all quartz crystals will be either right or left handed, but it may not be possible to distinguish this from the external form unless some critical crystal faces are present.

Austinite has point group 222, with no mirror planes, so austinite is also an enantiomorphic mineral, occurring as both right handed and left handed crystals, with right handed ones more common.