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Piemontite

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Piemontite
Piémontite from the type locality: Prabornaz Mine, Italy
General
CategorySorosilicates
Epidote
Formula
(repeating unit)
Ca2(Al,Mn3+,Fe3+)3(SiO4)(Si2O7)O(OH)
IMA symbolPmt[1]
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/m
Identification
ColorReddish-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
LusterVitreous
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[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]

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.

References

[edit]
  1. ^ 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.
  2. ^ a b c Handbook of Mineralogy
  3. ^ Webmineral.com website
  4. ^ a b c d Mindat reference page for Piemontite
  5. ^ "Minerals Colored by Metal Ions". minerals.gps.caltech.edu. Retrieved 2023-03-01.