Piemontite

Summary

Piemontite
Piémontite.jpg
Piémontite from the type locality: Prabornaz Mine, Italy
General
CategorySorosilicates
Epidote
Formula
(repeating unit)
Ca2(Al,Mn3+,Fe3+)3(SiO4)(Si2O7)O(OH)
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/m
Identification
ColourReddish-brown, reddish-black
Crystal habitSlender prismatic, blocky to massive
TwinningOn [100] uncommon
Cleavage[001] good, [100] distinct
FractureUneven to splintery
TenacityBrittle
Mohs scale hardness6 - 6.5
LustreVitreous
StreakRed
DiaphaneityTranslucent to nearly opaque
Density3.46 - 3.54
Optical propertiesBiaxial (+) 2V = 64 - 106
Refractive indexnα = 1.725 - 1.756 nβ = 1.730 - 1.789 nγ = 1.750 - 1.832
Birefringenceδ = 0.025 - 0.076
PleochroismVisible
Dispersionr>v very strong
References[1][2][3]

Piemontite is a sorosilicate mineral in the monoclinic crystal system with the chemical formula Ca2(Al,Mn3+,Fe3+)3(SiO4)(Si2O7)O(OH).[1] It is a member of the epidote group.[3]

Red to reddish-brown or red-black in colour, Piemontite has a red streak and a vitreous lustre.[3]

The type locality is the Prabornaz Mine, in Saint-Marcel, Aosta Valley, Italy.[3]

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.[1]

References

  1. ^ a b c Handbook of Mineralogy
  2. ^ Webmineral.com website
  3. ^ a b c d Mindat reference page for Piemontite


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 x 3.0 x 2.6 cm.