Piemontite

piemontite

epidote

tremolite

glaucophane

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Formulae:
There are three piemontite minerals:
piemontite: Ca₂(Al₂Mn³⁺)[Si₂O₇]{SiO₄]O(OH)
Piemontite-(Pb): CaPb(Al₂Mn³⁺)[Si₂O₇]{SiO₄]O(OH)
Piemontite-(Sr): CaSr(Al₂Mn³⁺)[Si₂O₇]{SiO₄]O(OH)

This page refers to piemontite.
Sorosilicate (Si₂O₇ groups), manganese-bearing mineral, epidote group
Crystal System: Monoclinic
Specific gravity: 3.46 to 3.54 measured, 3.45 calculated
Hardness: 6 to 6½
Streak: Reddish
Colour: Red, red-violet, red-brown to reddish black
Common impurities: Fe,Ti,Na,K,H₂O
Environments:

Metamorphic environments
Hydrothermal environments

Piemontite is the manganese analogue of clinozoisite, with Mn³⁺ substituting for Al. Piemontite is not found in many manganese-rich deposits where it might be expected. This is because piemontite can form only where there is sufficient manganese and the conditions are sufficiently oxidising to create Mn³⁺ (R&M 86.5.449-455). Piemontite occurs in regionally metamorphosed rocks of the greenschist to amphibolite facies, in metasomatized manganese deposits and in low-temperature hydrothermal veins in altered rhyolite, andesite and diorite. Associated minerals include epidote, tremolite, glaucophane, orthoclase, quartz and calcite (HOM).

Localities

At the type locality, the Prabornaz Mine, Saint-Marcel, Aosta Valley, Italy, piemontite forms dark red, bladed crystals and masses in the upper zone of a metamorphic manganese ore deposit (R&M 86.5.449-455).

At the Cassagna Mine, Ne, Genoa, Liguria, Italy, piemontite is found as well formed crystals to 3 mm in size with pyrolusite and quartz (R&M 86.5.449-455).

In Japan, piemontite schist is found in the Sonoki and Sangun metamorphic zones (R&M 86.5.449-455).

At the Tone mine, Kinkai-Tone-machi, Nagasaki City, Nagasaki Prefecture, Japan, piemontite occurs as crystals associated with braunite (R&M 86.5.449-455).

At the Otakiyama manganese deposit, Bizan, Tokushima City, Tokushima prefecture, Shikoku Island, Japan, piemontite is found as 3-cm acicular crystals (R&M 86.5.449-455).

At the Messina Mine, Musina, Musina Local Municipality, Vhembe District Municipality, Limpopo, South Africa, translucent to transparent crystals of piemontite to 6 mm in size are found with colourless quartz (R&M 86.5.449-455).

At Shadow Lake, Minarets District, Ritter Range, Madera county, California, USA, specimens containing well crystallised piemontite were taken from a shear zone in schistose volcanic rocks about three hundred yards east of the outlet of Shadow Lake. The usual mineral association is piemontite, tremolite and vein quartz, which were deposited in that order. In addition some cavities in the rock are lined with small, orange-coloured crystals of a manganese-bearing garnet. In thin section tremolite, itself enclosed in later quartz, can be seen replacing piemontite (AM 18.493-500).

At the Pilar Cliffs, Taos county, New Mexico, USA, rough, dark red crystals of piemontite have been found (R&M 86.5.449-455).

At Culp Ridge, Glatfelter Farm, Hamiltonban Township, Adams county, Pennsylvania, USA, piemontite has been found as dense veins of radiating acicular crystals (R&M 86.5.449-455).

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