Piemontite - Locality: St. Marcel, Aosta, Italy — Photo: Aangelo · CC BY-SA 3.0
Piemontite
The red-to-black manganese member of the epidote group — the 'piedmontite' of metamorphic rocks.
Intro adapted from Wikipedia (CC BY-SA).
Piemontite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₂(Al₂Mn³⁺)[Si₂O₇][SiO₄]O(OH).
It crystallizes in the monoclinic crystal system (space group P2_1/m). It is a member of the Epidote structural group.
Key properties: Mohs hardness 6; specific gravity 3.46–3.54; typically red or brown or black or yellow; Reddish streak; Vitreous luster; refractive index 1.764; optically biaxial.
IMA symbol Pmt, IMA status: approved, named in 1853.
Found in regionally metamorphosed rocks of the greenschist to amphibolite facies; in metasomatized manganese deposits; in low-temperature hydrothermal veins in altered rhyolites, andesites, and diorites.
It is found associated with Epidote, tremolite, glaucophane, orthoclase, quartz, calcite.
The name commemorates the Piemonte (Piedmont) region in northwestern Italy.
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