Glaucophane

(mineralogy) Na₂Mg₃Al₂Si₈ A blue to black monoclinic sodium amphibole; blue to black coloration with marked pleochroism.

A monoclinic sodic amphibole with composition close to Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂. This mineral exhibits a characteristic blue color with distinct pleochroism from colorless to lavender blue when viewed in thin section by plane-polarized light. Outcrops of glaucophane-rich metamorphic rocks are commonly blue and tend to have good foliation; these rocks are called blueschists. See also Amphibole; Blueschist; Pleochroism.

Glaucophane is an index mineral of blueschist, which is generated under unusually high pressures at low temperatures in a tectonic environment exclusively associated with a subducted lithospheric slab or related tectonic loading. The glaucophane-bearing assemblages occur in recrystallized graywackes and pelitic rocks and in metabasites and metacherts of oceanic affinity; they are typically found in subduction zone complexes at plate boundaries, a setting first recognized in the Jurassic and Cretaceous Franciscan Complex of northern California. Blueschists are most common and best developed in Mesozoic and Cenozoic terranes: some Paleozoic and even latest Precambrian blueschists have been described in Russia and China. Blueschists formed earlier in geologic time may have been eroded or been recrystallized under normal geothermal conditions. The preservation of glaucophane in blueschists of continental or island arc margins indicates either rapid uplift or maintenance of low geothermal gradients by steady-state subduction for tens of million years. See also Graywacke; Subduction zones.

Glaucophane
General
Category	Silicate mineral
Chemical formula	Na2(Mg,Fe)3Al2Si8O22(OH)2
Identification
Color	Gray, lavender-blue
Crystal habit	Massive granular to columnar
Crystal system	Monoclinic - Prismatic
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
References	[1][2][3]

Glaucophane is a mineral belonging to the amphibole group, chemical formula Na₂(Mg,Fe)₃Al₂Si₈O₂₂(OH)₂. It is named from its typical blue color (in Greek, glaucophane means "blue appearing"). Glaucophane crystallizes in the monoclinic system.

The blue color is very diagnostic for this species. It, along with the closely related mineral riebeckite are the only common amphibole minerals that are typically blue. Glaucophane forms a solid solution series with ferroglaucophane; Na₂(Fe,Mg)₃Al₂Si₈O₂₂(OH)₂. Glaucophane is the magnesium-rich member and ferroglaucophane is the iron-rich member. Ferroglaucophane is similar to glaucophane but is denser, generally darker in color with a diminished pearly luster. Glaucophane's hardness is 5 - 6, and its specific gravity is approximately 3 - 3.2.

Glaucophane forms in metamorphic rocks that are either particularly rich in sodium or that 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. It is glaucophane's color that gives the blueschist facies its name. Glaucophane is also found in eclogites that have undergone retrograde metamorphism.^[1]

There is also a rare amphibole called holmquistite, chemical formula Li₂Mg₃Al₂Si₈O₂₂(OH)₂, which occurs only in lithium-rich continental rocks. For many years, holmquistite was mistaken for glaucophane, as the two look identical in thin section.

See also

• List of Minerals

References

1. ^ ^{a b} http://rruff.geo.arizona.edu/doclib/hom/glaucophane.pdf Handbook of Mineralogy
2. ^ http://webmineral.com/data/Glaucophane.shtml Webmineral
3. ^ http://www.mindat.org/min-1704.html

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