Aragonite, (v. Tarnowitzite)

Mineral: Aragonite
Chemistry: CaCO3
Size: 70 mm
Owner: Carl Acosta
Photographer: Jeff Scovil

Tarnowitzite is the varietal name given to lead-bearing aragonite.   This specimen shows the habit typical for this variety, with pseudo-hexagonal crystals, tapering slightly towards the base, and terminated by pseudo-hexagonal pyramids.

In fact, aragonite crystallises in the orthorhombic system, and this pseudo-hexagonal habit is the result of cyclical twinning, with each twin involving three individuals.