| Piemontite | |
|---|---|
 Piémontite from the type locality: Prabornaz Mine, Italy | |
| General | |
| Category | Sorosilicates Epidote |
| Formula (repeating unit) | 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] |
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]
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]
