| Piemontite | |
| Category: | Sorosilicates Epidote |
| Imasymbol: | Pmt[1] |
| System: | Monoclinic |
| Class: | Prismatic (2/m) (same H-M symbol) |
| Symmetry: | P21/m |
| Color: | Reddish-brown, reddish-black |
| Habit: | Slender prismatic, blocky to massive |
| Twinning: | On [100] uncommon |
| Cleavage: | [001] good, [100] distinct |
| Fracture: | Uneven to splintery |
| Tenacity: | Brittle |
| Mohs: | 6 – 6.5 |
| Luster: | Vitreous |
| Refractive: | nα = 1.725 – 1.756 nβ = 1.730 – 1.789 nγ = 1.750 – 1.832 |
| Opticalprop: | Biaxial (+) 2V = 64–106 |
| Birefringence: | δ = 0.025 – 0.076 |
| Dispersion: | r>v very strong |
| Pleochroism: | Visible |
| Streak: | Red |
| Density: | 3.46 – 3.54 |
| Diaphaneity: | Translucent to nearly opaque |
| References: | [2] [3] |
Piemontite is a sorosilicate mineral in the monoclinic crystal system with the chemical formula .[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. Manganese (Mn3+) causes the red color.[5]
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.[2]