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
Category	Cyclosilicates
Formula (repeating unit)	(Ca,Fe,Mn)3Al2BO3Si4O12OH or Ca2(Fe,Mn)Al2BSi4O15(OH)
Strunz classification	9.BD.20
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Identification
Color	Reddish brown to yellow to colorless. Blue, violet, grey.
Crystal habit	Tabular, wedge shaped crystals
Cleavage	Good on {100}
Fracture	Conchoidal
Mohs scale hardness	6.0–7.5
Luster	Vitreous
Streak	White
Specific gravity	3.18–3.37
Optical properties	Biaxial (-)
Refractive index	nα = 1.672–1.693 nβ = 1.677–1.701 nγ = 1.681–1.704
Birefringence	δ = 0.011
Pleochroism	Strong
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]