PIEMONTITE

    Class : Silicates
    Subclass : Sorosilicates
    Crystal system : Monoclinic
    Chemistry : Ca2(Mn,Fe)Al2(SiO4)3(OH)
    Rarity : Very common

Piemontite is a metamorphic silicate common in greenschist and amphibolite facies rocks, sometimes blueschists, as well as in metamorphic manganese deposits where it appears during late hydrothermal phases. It is also a mineral of hydrothermal alteration of veins, pegmatites and certain acidic to intermediate lavas. Piedmontite is a manganiferous silicate from the epidote group with which it forms a continuous series by replacement of iron by manganese. Chemically quite complex, it can admit significant quantities of strontium and rare earths, supporting the hypothesis of a complete solid solution with manganiferous allanite. It owes its name to its region of discovery : Italian Piedmont. Piemontite is found in lamellar or acicular prismatic crystals reaching 8 cm, striated according to elongation. Its color varies from carmine red to wine red brown and reddish black. The piemontite schists of New Zealand constitute an aesthetic rock used in ornamentation. Quartz with piemontite inclusions are also cut for jewelry.

Main photo : Piemontite from Castagnola, Tuscany, Italy © Gianandrea Ghirri

Piemontite from Pilar Cliffs, New Mexico, USA © Michael C. Michayluk
Piemontite from Pizzo Alzasca, Ticino, Switzerland © Enrico Bonacina
Piemontite from Praborna Mine, Saint-Marcel, Italy © Luigi Chiappino
6.80 ct piemontite quartz cabochon from Tanzania

Piemontite in the World

One of the classic deposits of piedmontite is Saint Marcel in the Italian Piedmont, but very beautiful crystals also come from Glen Coe in Scotland, and large specimens from Old Boolcoomata (South Australia). Piedmontite is common in the United States, notably in Arizona (Tucson Mountains), New Mexico (Pilar), and in many locations in California. It is also present in good specimens in metamorphic manganese deposits: Tachgagalt (Morocco), Langban (Sweden), Wessels, near Kuruman (South Africa), and the Kajlidongri mine in India. Piedmontite is the source of the purple-red color of "antique red porphyry" and sometimes used as an ornamental stone (New Zealand Lake Wakatipu and Haast River deposits, for example).

Piemontite in France

In France, piemontite is reported at Cap Garonne (Var), at St-Jean-de-Maurienne (Savoie), on the island of Groix (Morbihan), at Costabonne (Pyrénées-Orientales) and at the Anglade mine (Ariège).

Twinning

A twin is known on {100} and sometimes lamellar.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 6 to 6.5
Density : 3.46 to 3.54
Fracture : Irregular
Streak : Red

TP : Translucent to opaque
RI : 1.725 to 1.832
Birefringence : 0.025 to 0.076
Optical character : Biaxial +
Pleochroism : Visible
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : ParamagneticRadioactivity : None

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