Piemontite

Piemontite is a sorosilicate mineral in the monoclinic crystal system with the chemical formula Ca₂(Al,Mn³⁺,Fe³⁺)₃(SiO₄)(Si₂O₇)O(OH).^[2] It is a member of the epidote group.^[4]

Piemontite
Piémontite from the type locality: Prabornaz Mine, Italy
General
Category	Sorosilicates Epidote
Formula	Ca2(Al,Mn3+,Fe3+)3(SiO4)(Si2O7)O(OH)
IMA symbol	Pmt[1]
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/m
Identification
Color	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
Luster	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	[2][3][4]

Red to reddish-brown or red-black in color, piemontite has a red streak and a vitreous lustre.^[4] Manganese (Mn³⁺) causes the red color.^[5]

The type locality is the Prabornaz Mine, in Saint-Marcel, Aosta Valley, Italy.^[4]

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.^[2]

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 × 3.0 × 2.6 cm.