Mineral Information and Data:

Piemontite
Ca2Mn3+Al2(Si2O7)(SiO4)O(OH) [CNMNC approved formula]

Named after the Piemonte (Piedmont) region in northwestern Italy. First published in 1853. Approved and defined (IMA, 1962).

IMA status : Approved Species 1962 ()
CLASSIFICATION 
Dana
(8th edition) :
58.02.01a.05  (This Dana# Changed)
  (58) Sorosilicate Insular, Mixed, Single, and Larger Tetrahedral Groups
  (58.02) with cations in [6] and higher coordination; single and double groups (n=1,2)
  (58.02.01a) Epidote group (Clinozoisite subgroup)
  
Nickel-Strunz
(10th edition) :
09.BG.05a
  (09) Silicates
  (09.B) Sorosilicates (dimer) structures
  (09.BG) Sorosilicates with mixed SiO4 and Si2O7 groups; cations in octahedral [6] and greater coordination
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Space group: P21/m
Unit cell: a = 8.878(10) Å, b = 5.692(5) Å, c = 10.201(10) Å, β = 115.25° , Z=2
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.
  
Color: Reddish brown, deep red, purplish red to almost black; in thin section, yellow, pink, violet to deep red
Diaphaneity: Translucent to nearly opaque
Luster: Vitreous
Hardness (Mohs): 6 - 6.5
Measured Density: 3.46 - 3.54 g/cm3
Cleavage: {001}, perfect; {100}, poor.
Tenacity: Brittle
Fracture: Uneven
Streak: Reddish
  
Geologic 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.
Mineral Association: Epidote, tremolite, glaucophane, orthoclase, quartz, calcite
  
Comments: Dana number changed from 58.2.1a.11

 

Piemontite
Origin: N'Chwaning mine, Kuruman, No. Cape, South Africa
Sample size: Microscopic image
Photo and Owner: Lou Perloff

 

Piemontite
Origin: Peavine Mountain, Washoe Co., Nevada, United States
Sample size: 3.5 x 2.5 x 2 cm
Photo: Tom Loomis

 

Piemontite
(zoom-in of above)

<<< BACK TO MINERAL SEARCH