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
Manganaxinite
General
Category	Cyclosilicates
Formula	(Ca,Fe,Mn)3Al2BO3Si4O12OH or Ca2(Fe,Mn)Al2BSi4O15(OH)
IMA symbol	Ax[1]
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	[2][3]

The axinite group includes:

• Axinite-(Fe) or ferroaxinite, Ca₂Fe²⁺Al₂BOSi₄O₁₅(OH) iron rich, clove-brown, brown, plum-blue, pearl-gray^[4]
• Axinite-(Mg) or magnesioaxinite, Ca₂MgAl₂BOSi₄O₁₅(OH) magnesium rich, pale blue to pale violet; light brown to light pink^[5]
• Axinite-(Mn) or manganaxinite, Ca₂Mn²⁺Al₂BOSi₄O₁₅(OH) manganese rich, honey-yellow, clove-brown, brown to blue^[6]
• Tinzenite (CaFe²⁺Mn²⁺)₃Al₂BOSi₄O₁₅(OH) iron – manganese intermediate, yellow, brownish yellow-green^[7]

Axinite is sometimes used as a gemstone.^[8]

Gallery

• 
  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