Cummingtonite

Hand Specimen identification: Prismatic habit, two perfect cleavages intersecting at near 60° when viewed in basal section, habit, color (if light brown), and association identify cummingtonite. It may be confused with other amphiboles, especially anthophyllite and gedrite.

Chemical Composition : (Mg,Fe) 7Si8O22(OH)2

Hardness : 51 2 to 6

Specific Gravity2.9 to 3.2

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

Luster/Transparency : vitreous, silky, fibrous/transparent to translucent

Color : light brown, whitish, or green

Streak : white

Optical Properties : Cummingtonite is colorless to pale green in thin section and exhibits weak pleochroism. Interference colors may be up to second order. Basal sections show typical amphibole cleavage displaying 56° and b = 18.01 angles. Extinction is inclined c = 5.33 to prismatic cleavage, polysynthetic twinning is common, and birefringence is greater than for antho-phyllite-gedrite. Biaxial 1+ 2, a = 1.644, b = 1.657,g = 1.674, d = 0.030, 2V = 80° to 90°.

Crystallography : Monoclinic, a = 9.51, b = 18.19, c = 5.33, b = 101.83°, Z = 2; space group C2/m; point group 2/m.

Habit : Cummingtonite forms prismatic, fibrous crystals; aggregates of radiating fibers or blades are common.

Structure and composition : Cummingtonite, like other amphiboles, has a double-chain structure. SiO4 tetrahedra are linked to make double chains that run parallel to the c-axis. Each tetrahedron shares two or three oxygen with neighbors, and has an unshared oxygen at the vertex pointing perpendicular to c. Chains are paired; unshared oxygens point toward each other and are bonded to the five octahedral cations occupying sites between them. A complete solid solution series exists between Mg-cummingtonite, Mg7Si8O22(OH)2 , and grunerite,(Fe7Si8O22(OH)2 . The name cummingtonite is given to intermediate compositions, (Mg,Fe)7Si8O22(OH)2 with Mg > Fe. Substantial Mn may replace Mg; Al and Ca may be present in small amounts.

Occurrence and associations : Cummingtonite occurs in mafic or marly medium-grade metamorphic rocks. Common associated miner-als include other amphiboles (hornblende, actinolite, or anthophyllite), garnet, plagioclase, and cordierite. Cummingtonite also occurs in a few rare kinds of igneous rocks.

Varieties : Amosite is an asbestiform amphibole similar to Fe-rich cummingtonite.

Related Minerals : Cummingtonite is closely related to the other am-phiboles and is polymorphic with members of the anthophyllite series.

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