GLAUCOPHANE

    Class : Silicates
    Subclass : Inosilicates
    Crystal system : Monoclinic
    Chemistry : Na2Mg3Al2Si8O22(OH)2
    Rarity : Quite common

Glaucophane belongs to the group of sodium amphiboles and forms a series with ferroglaucophane. It is essentially a mineral from rocks of regional metamorphism of low temperature and high pressure (blue schist facies, the blue tint being precisely linked to the presence of this mineral). It is associated with epidote, lawsonite, almandine and muscovite. It is also present as relics in retromorphosed eclogites. Its name comes from the Greek glaukos (green-blue) and phainein (to appear) because of its color. Glaucophane occurs in often indistinct crystals, sometimes in elongated prisms with rounded terminal faces, but more often in lamellar, fibrous, granular or bacillary masses of a lavender gray-blue to dark blue and blue-black color.

Main photo : Glaucophane from Rio Oremo, Piedmont, Italy © Tony Peterson

Glaucophanite from Campana, Bastia, Upper Corsica, France © Erwin Löffler
Glaucophane from Glaucophane Ridge, San Benito County, California, USA © Kelly Nash
Glaucophane from Cazadero, California, USA © Eugene & Sharon Cisneros
Glaucophane from Rio Oremo, Piedmont, Italy © Franco Luca Bonino

Glaucophane in the World

Large prismatic glaucophane crystals come from Laytonville (California) and several other sites in this state (Tiburon Peninsula, Valley Ford, San Benito, etc...). Still in the United States, we know glaucophane in the carbonatite of Powderhorn (Colorado). Glaucophane is also common in Japan (Horokanai, Chichibu, etc...). Very well-known samples come from St-Marcel and Rio Oremo (Italy).

Glaucophane in France

In France, glaucophane is well represented in the copper serpentinites of Saint-Véran (Hautes-Alpes), and in Corsica, near Bastia. The superb and justly famous glaucophanites from the island of Groix (Morbihan) are in fact made up of ferroglaucophane with epidote and almandine.

Twinning

Twins are single or multiple and parallel to {100}.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 5 to 6
Density : 3.08 to 3.22
Fracture : Irregular to conchoidal
Streak : Gray to gray-blue

TP : Translucent
RI : 1.606 to 1.655
Birefringence : 0.021
Optical character : Biaxial -
Pleochroism : Strong
Fluorescence : None

Solubility : Insoluble


Magnetism : Paramagnetic
Radioactivity : None

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