Piemontite is a sorosilicate mineral in the monoclinic crystal system with the chemical formula Ca2(Al,Mn3+,Fe3+)3(SiO4)(Si2O7)O(OH). It is a member of the epidote group.
Red to reddish-brown or red-black in colour, Piemontite has a red streak and a vitreous lustre.
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.
Piemontite on Quartz, from No. 5 shaft, Messina mine, Limpopo Province
, South Africa. Red piemontite microcrystals cover three sides of a doubly terminated quartz crystal. Size: 7.1 x 3.0 x 2.6 cm.