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Grunerite

 

Grunerite
Hand Specimen identification: Habit, color, two prominent cleavages at 56° to each other, and association help identify grunerite, but it cannot be distinguished from other members of the cummingtonite series without chemical or X-ray analysis.

Chemical Composition :Fe7Si8O22(OH)2

Hardness : 6

Specific Gravity3.1 to 3.6

Cleavage/Fracture : two perfect prismatic{110}/uneven

Luster/Transparency : silky, vitreous/transparent to translucent

Color : dark green or brown

Streak : white

Optical Properties : Grunerite is similar to other members of the cum-mingtonite-grunerite series (see cummingtonite), but exhibits less pleochroism than cummingtonite, has an extinction angle of 10° to 15° prismatic cleav-age, and may show interference colors up to third order. Biaxial 1- 2, a = 1.69, b = 1.71, g = 1.73, d = 0.040, 2V = 80° to 90°.

Crystallography : Monoclinic, a = 9.6, b = 18.3, c = 5.3, b = 101.8°,Z = 2; space group C2/m; point group 2/m.

Habit : Grunerite typically forms fibrous, bladed, or colum-nar crystals, often radiating

Structure and composition : Grunerite is an end member of the cummingtonite-grunerite series. Structure and composition are anal-ogous to cummingtonite. The name grunerite is by definition restricted to compositions close to end member Fe7Si8O22(OH)2.

Occurrence and associations : Grunerite is found with Fe-rich minerals such as magnetite, hematite, minnesotaite, hedenbergite, fay-alite, or garnet in metamorphosed Fe-rich sediments.

Varieties :

Related Minerals : Grunerite is closely related to the other amphiboles, especially cummingtonite.

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