Glaucophane

Glaucophane
Bleu Gemm glaucophane with fuchsite
General
Category	Inosilicates Sodic amphibole group
Formula (repeating unit)	☐Na2(Mg3Al2)Si8O22(OH)2
Strunz classification	9.DE.25
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Color	Gray, navy blue, lavender-blue
Crystal habit	slender long prisms, Massive granular to columnar
Cleavage	Good on [110] and on [001]
Fracture	Brittle - conchoidal
Mohs scale hardness	6.0 - 6.5
Luster	Vitreous - pearly
Streak	Grayish blue
Diaphaneity	Translucent
Specific gravity	3 - 3.15
Optical properties	Biaxial (-)
Refractive index	nα = 1.606 - 1.637 nβ = 1.615 - 1.650 nγ = 1.627 - 1.655
Birefringence	δ = 0.021
Pleochroism	Strong
Dispersion	Strong
References

Glaucophane is the name of a mineral and a mineral group belonging to the sodic amphibole supergroup of the double chain inosilicates, with the chemical formula ☐Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂.

Glaucophane crystallizes in the monoclinic system.

Glaucophane is named for its typical blue color. In Greek, glaucophane means "blue appearing". As the major mineral component, it is glaucophane's color that gives the "blueschist" metamorphic rock type its name.

The blue color is very diagnostic for this species. Glaucophane, along with the closely related mineral riebeckite, to which it forms a series with, and their intermediate crossite, are the only well known amphiboles that are commonly blue. Glaucophane forms a solid solution series with ferroglaucophane; Na₂(Fe,Mg)₃Al₂Si₈O₂₂(OH)₂. Glaucophane is the magnesium-rich endmember and ferroglaucophane is the iron-rich endmember. Ferroglaucophane is similar to glaucophane but is slightly denser and hence increased specific gravity. The two endmembers are indistinguishable in hand specimens and are strongly pleochroic. Glaucophane's hardness is 5 - 6 and its specific gravity is approximately 3 - 3.2.

The blueschist metamorphic facies gets its name from abundant blue minerals glaucophane and lawsonite. Glaucophane generally forms in blueschist metamorphic rocks of gabbroic or basaltic composition that are rich in sodium and have experienced low temperature-high pressure metamorphism such as would occur along a subduction zone. This material has undergone intense pressure and moderate heat as it was subducted downward toward the mantle. Glaucophane is also found in eclogites that have undergone retrograde metamorphism.