Quartz

Top view of a transparent smoky quartz crystal, 2.2 cm wide, from W Erongo Mountains, Namibia. Very likely from a miarole pocket and naturally etched by aggressive hot fluids. The strange looking pattern on its surface developed because r- and z- faces ({ 1 0 -1 1 } and { 0 1 -1 1 }) were effected by the fluids to a different degree. Because the crystal is a Dauphiné law twin, and Dauphiné law twins are penetration twins, each of the faces represents a mosaic of irregular twin domains that correspond to the r- and z-rhombohedral crystallographic forms, so it does not really make sense to talk of r- or z-faces on Dauphiné twins. There is no simple method to determine if the bright patches correspond to r- or z-forms.

You will note that each irregular patch has a straight border at the edge of a crystal face. The reason is not that the twin domain ends there - just the opposite - it continues on the neighboring crystal face: if the surface of a patch belonging to a certain twin domain corresponds to a z-form, the continuation of that patch must correspond to an r-form on an adjacent crystal face, and as a consequence one surface is etched, the other one not.
Bought from a street dealer at the Erongo in 2006.

This Photo was Mindat.org Photo of the Day - 28th Aug 2010

This photo has been shown 1623 times

Photo added: 3rd Jul 2010