Axinite

Axinite is a brown to violet-brown, or reddish-brown bladed group of minerals composed of calcium aluminium boro-silicate, (Ca,Fe,Mn)3Al2BO3Si4O12OH. Axinite is pyroelectric and piezoelectric.

Axinite
Axinite – Deposit Topotype, France
General
CategoryCyclosilicates
Formula
(repeating unit)		(Ca,Fe,Mn)3Al2BO3Si4O12OH or Ca2(Fe,Mn)Al2BSi4O15(OH)
Strunz classification9.BD.20
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Identification
ColorReddish brown to yellow to colorless. Blue, violet, grey.
Crystal habitTabular, wedge shaped crystals
CleavageGood on {100}
FractureConchoidal
Mohs scale hardness6.0–7.5
LusterVitreous
StreakWhite
Specific gravity3.18–3.37
Optical propertiesBiaxial (-)
Refractive indexnα = 1.672–1.693
nβ = 1.677–1.701
nγ = 1.681–1.704
Birefringenceδ = 0.011
PleochroismStrong
References[1][2]

The axinite group includes:

  • Axinite-(Fe) or ferroaxinite, Ca2Fe2+Al2BOSi4O15(OH) iron rich, clove-brown, brown, plum-blue, pearl-gray[3]
  • Axinite-(Mg) or magnesioaxinite, Ca2MgAl2BOSi4O15(OH) magnesium rich, pale blue to pale violet; light brown to light pink[4]
  • Axinite-(Mn) or manganaxinite, Ca2Mn2+Al2BOSi4O15(OH) manganese rich, honey-yellow, clove-brown, brown to blue[5]
  • Tinzenite, (CaFe2+Mn2+)3Al2BOSi4O15(OH) iron – manganese intermediate, yellow, brownish yellow-green[6]

Axinite is sometimes used as a gemstone.[7]

  • Clove-brown axinite crystals to 2.3 cm set atop matrix from the West Bor Pit at Dalnegorsk, Russia
  • Chloritized bladed crystals of Axinite forming on Adularia from the Swiss Alps
  • Tinzenite on Calcite, 4.5 × 3.5 × 3 cm. Wessels Mine, Kalahari manganese fields, Northern Cape Province, South Africa
  • Manganaxinite (Axinite-(Mn)), with sharp curving crystals to 4 cm. West Bor Pit at Dalnegorsk, Russia
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References
  1. Axinite Archived November 19, 2008, at the Wayback Machine. Mineral Galleries
  2. Axinite. Mindat
  3. Handbook of Mineralogy: Ferroaxinite
  4. Handbook of Mineralogy: Magnesioaxinite
  5. Handbook of Mineralogy: Manganaxinite
  6. Handbook of Mineralogy: Tinzenite
  7. Tables of Gemstone Identification By Roger Dedeyne, Ivo Quintens p.147
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