Piemontite (Piedmontite)
Current inventory: 0 gems
 

  
Piemontite is named after the Piemonte (Piedmont) region in northwestern Italy where it was discovered.

Discovered in 1853; IMA status: Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

Ca₂(Al,Mn³⁺,Fe³⁺)₃(SiO₄)(Si₂O₇)O(OH)

Calcium Aluminum Manganese Iron Silcate Oxide Hydroxide

Molecular Weight:

488.18 gm

Composition:

Calcium

16.42 %

Ca

22.97 %

CaO

Manganese

10.13 %

Mn

13.08 %

MnO

Aluminum

9.95 %

Al

18.80 %

Al₂O₃

Iron

3.43 %

Fe

4.42 %

FeO

Silicon

17.26 %

Si

36.92 %

SiO₂

Hydrogen

0.21 %

H

1.85 %

H₂O

Oxygen

42.61 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/C.23-30

Nickel-Strunz 10th Ed. ID:

9.BG.05

9 : SILICATES (Germanates)
B : Sorosilicates
G : Sorosilicates with mixed SiO₄ and Si₂O₇ groups; cations in octahedral [6] and greater coordination

Related to:

Epidote Supergroup. Clinozoisite Group (subgroup).

Members of Group:

Clinozoisite Group: Clinozoisite, Clinozoisite-(Sr), Epidote, Epidote-(Pb), Epidote-(Sr), Hancockite, Manganipiemontite-(Sr), Mukhinite, Piemontite, Piemontite-(Pb), Piemontite-(Sr)

Varieties:

Plumbian Piemontite, Strontian Piemontite

Synonyms:

Piedmontite

Crystal Data

Crystallography:

Monoclinic - Prismatic

Crystal Habit:

In prismatic, bladed or acicular crystals, to 8 cm; in clusters of radiating crystals; as anhedral grains and grain aggregates.

Twinning:

{100}, lamellar, uncommon

Physical Properties

Cleavage:

Perfect on {001}, Poor on {100}

Fracture:

Irregular/Uneven

Tenacity:

Brittle

Moh's Hardness:

6.0 - 6.5

Density:

3.46 - 3.54 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Optical Properties

Color:

Reddish Brown, deep Red, Purplish Red to almost Black; in thin section, Yellow, Pink, Violet to deep Red

Transparency:

Translucent to Opaque

Luster:

Vitreous

Refractive Index:

1.725 - 1.832  Biaxial ( + )

Birefringence:

0.0250 - 0.0760

Dispersion:

Strong; r > v; less commonly r < v

Pleochroism:

Visible; X = light yellow, orange to pink; Y = pale violet to deep lavender; Z = pink to deep red

Occurances

Geological Setting:

In regionally metamorphosed rocks of the greenschist to amphibolite facies; in metasomatized manganese deposits; in low-temperature hydrothermal veins in altered rhyolites, andesites, and diorites.

Common Associations:

Calcite, Epidote, Glaucophane, Orthoclase, Quartz, Tremolite

Common Impurities:

Fe, Ti, Na, K, H₂O

Type Locality:

Prabornaz Mine (Praborna Mine), Saint-Marcel, Aosta Valley, Italy

Year Discovered:

1853

View mineral photos:

Piemontite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Piemontite, also spelled Piedmontite, is a rare member of the Epidote Group of minerals that includes Allanite, Clinozoisite, Epidote, Piemontite, Tanzanite, Thulite and Zoisite. Piemontite crystals are rarely large enough for faceting so it is usually only available as cabochons made from clusters of radiating crystals or aggregates or as Quartz gems with Piemontite inclusions.

The name "Piemontite" has two different uses: Piemontite is the mineral species described here, but more commonly "Piemontite" refers incorrectly to deep red colored Mn3+ bearing Epidotes that are actually just varieties of the species Epidote, and not the species Piemontite. Many "Piemontite" mineral specimens in collections are actually Mn-rich red varieties of Epidote.

There are many localities for Piemontite: in the Praborna mine, south of St. Marcel, Val d'Aosta, Piedmont, Italy. In Sweden, on Sörhårås and Rakten, Ultevis, Norrbotten, and at Jakobsberg and Långban, Väarmland. At Tachgagalt, Anti-Atlas Mountains, Morocco. In the USA, at Garnet Lake and in the Agnew Meadow mine, Madera County, and in the Braito mine, Plumas County, California; in the Tucson Mountains, Pima County, Arizona; at the Idarado mine, Ouray County, Colorado; in Yuba Canyon, Peavine Mountain, Washoe County, Nevada; and on Pine Mountain, Adams County, Pennsylvania. In the Wessels mine, near Kuruman, Cape Province, South Africa. At the Kajlidongri mine, Jhabua district, Madhya Pradesh, India. From Otakiyama, Tokushima Prefecture, and other places in Japan. Large crystals from eight km northeast of Old Boolcoomata, South Australia.
 

  
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