Glaucophane
Image: Didier Descouens (CC BY-SA 4.0)
Mineral NameGlaucophane
Chemical FormulaNa₂Mg₃Al₂Si₈O₂₂(OH)₂
Mohs Hardness6
Crystal Systemmonoclinic
Lustervitreous
Streak Colorwhite to pale blue-gray
Cleavageperfect on {110} at 56° and 124°
Fractureuneven
Specific Gravity3.1
Colorsblue, lavender-blue, bluish-black, green
Transparencytranslucent
Type LocalityGroix Island, Groix, Morbihan, Brittany, France
Mineral GroupSilicates (Amphibole Group)
Usescollector, scientific
Associated Mineralsfuchsite, muscovite, epidote, garnet, lawsonite
Rarityuncommon
Description
This specimen features a dense aggregate of deep blue to bluish-black, acicular glaucophane crystals, intergrown with bright green, platy fuchsite (a variety of muscovite). The overall appearance is a striking contrast of blue and green minerals.
Geological Context
Glaucophane is a characteristic mineral of blueschist facies metamorphic rocks, which form under conditions of high pressure and relatively low temperature. These conditions are typically found in subduction zones, making glaucophane an important indicator mineral for such tectonic settings.
