Mineral Information and Data:

Barroisite
[]NaCa[Mg3(Al,Fe3+)2](Si7Al)O22(OH)2 [CNMNC approved formula]

Named by G. Murgoci with no mention of etymology. First published in 1922.

IMA status : Redefined IMA Approved; 1997 ()
CLASSIFICATION 
Dana
(8th edition) :
66.01.03b.05  
  (66) Inosilicate Double-Width Unbranched Chains, W=2
  (66.01) with P=2 amphibole configuration
  (66.01.03b) Group 3, the sodic-calcic amphiboles
  
Nickel-Strunz
(10th edition) :
09.DE.20
  (09) Silicates
  (09.D) Inosilicates* Structural terminology per Liebau (1985)
  (09.DE) with 2-periodic double chains, Si4O11; Clinoamphiboles
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Space group: C2/m
Unit cell: a = 9.759(1)Å, b = 17.921(2)Å, c = 5.290(1)Å, β = 104:49(1)°, Z=2
Crystal Habit: As rims on, or cores of, other mixed-species amphibole crystals.
  
Color: Green; dark green in thin section
Diaphaneity: Semitransparent
Luster: Vitreous
Hardness (Mohs): 5 - 6
Measured Density: 3.21 (calculated) g/cm3
Cleavage: Perfect on {110}, intersecting at ~56° and ~124°; partings on {100}, {001}
Tenacity: Brittle
  
Polymorphism and Series: Forms a series with ferro-barroisite
Geologic Setting: In blueschist facies metamorphic rocks.
Mineral Association: Omphacite, glaucophane, crossite, actinolite, calcite
  

 

Barroisite
Dark-green crystals in matrix
Origin: Monte Beigua, Varezzo, Savano, Liguria, Italy
Sample size: 1.5 x 2 cm
Photo: Diederik Visser

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