Piemontite

Piemontite is a sorosilicate mineral in the monoclinic crystal system with the chemical formula Ca2(Al,Mn(3+),Fe(3+))3(SiO4)(Si2O7)O(OH). It is a member of the epidote group.

Red to reddish-brown or red-black in color, piemontite has a red streak and a vitreous lustre. Manganese (Mn3+) causes the red color.

The type locality is the Prabornaz Mine, in Saint-Marcel, Aosta Valley, Italy.

It occurs metamorphic rocks of the greenschist to amphibolite metamorphic facies and in low-temperature hydrothermal veins in altered volcanic rocks. It also occurs in metasomatized deposits of manganese ore. Associated minerals include: epidote, tremolite, glaucophane, orthoclase, quartz and calcite.