Glaucophane "eyes-chain" from Germany.

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.