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Piémontite from the type locality: Prabornaz Mine, Italy
Category Sorosilicates
(repeating unit)
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group P21/m
Colour Reddish-brown, reddish-black
Crystal habit Slender prismatic, blocky to massive
Twinning On [100] uncommon
Cleavage [001] good, [100] distinct
Fracture Uneven to splintery
Tenacity Brittle
Mohs scale hardness 6 - 6.5
Lustre Vitreous
Streak Red
Diaphaneity Translucent to nearly opaque
Density 3.46 - 3.54
Optical properties Biaxial (+) 2V = 64 - 106
Refractive index nα = 1.725 - 1.756 nβ = 1.730 - 1.789 nγ = 1.750 - 1.832
Birefringence δ = 0.025 - 0.076
Pleochroism Visible
Dispersion r>v very strong
References [1][2][3]

Piemontite is a sorosilicate mineral in the monoclinic crystal system with the chemical formula Ca2(Al,Mn3+,Fe3+)3(SiO4)(Si2O7)O(OH).[1] It is a member of the epidote group.[3]

Red to reddish-brown or red-black in colour, Piemontite has a red streak and a vitreous lustre.[3]

The type locality is the Prabornaz Mine, in Saint-Marcel, Aosta Valley, Italy.[3]

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.[1]


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.