Piemontite
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 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. 
(same H-M symbol)
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]
References
- ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- ^ a b c Handbook of Mineralogy
- ^ Webmineral.com website
- ^ a b c d Mindat reference page for Piemontite
- ^ "Minerals Colored by Metal Ions". minerals.gps.caltech.edu. Retrieved 2023-03-01.
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| Simple | |
|---|---|
| Mixed |
- Axinite (borosilicate)
- Geigerite (arsenate)
- Manganese nodule (various)
- Samsonite (sulfosalt)
- Zincobotryogen (sulfate)
- Wolframite (tungstate)
- Hübnerite (tungstate)
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