| Glaucophane | |
|---|---|
 Bleu Gemm glaucophane with fuchsite | |
| General | |
| Category | Inosilicates Sodic amphibole group |
| Formula | ☐Na2(Mg3Al2)Si8O22(OH)2 |
| IMA symbol | Gln[1] |
| Strunz classification | 9.DE.25 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH-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 | [2][3][4][5] |
Glaucophane is amineral and a mineral group belonging to the sodicamphibole supergroup of the double chaininosilicates, with the chemical formula Na2(Mg3Al2)Si8O22(OH)2.
Glaucophane crystallizes in themonoclinic system.
Glaucophane is named for its typical blue color. InGreek,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 mineralriebeckite, to which it forms a series with, and their intermediatecrossite, are the only well knownamphiboles that are commonly blue. Glaucophane forms asolid solution series withferroglaucophane (Na2(Fe,Mg)3Al2Si8O22(OH)2). Glaucophane is themagnesium-richendmember and ferroglaucophane is theiron-rich endmember.
Ferroglaucophane is similar to glaucophane but is slightly denser and hence increasedspecific gravity. The two endmembers are indistinguishable in hand specimens and are stronglypleochroic. Glaucophane'shardness is 5–6 and its specific gravity is approximately 3–3.2.
The blueschistmetamorphic facies gets its name from abundant blue minerals glaucophane andlawsonite. Glaucophane generally forms in blueschist metamorphic rocks of gabbroic or basaltic composition that are rich insodium and have experienced low temperature-high pressuremetamorphism such as would occur along asubduction zone.
This material has undergone intense pressure and moderate heat as it was subducted downward toward themantle. Glaucophane is also found ineclogites that have undergone retrograde metamorphism.[2]
There is also a rare amphibole calledholmquistite, 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.
