Friedelite

Formula:  Mn₈Si₆O₁₅(OH,Cl)₁₀

Origin of name:  For C. Friedel, French chemist

Crystal system:  Monoclinic pseudohexagonal

Crystal class:  2/m

Twinning:  Universal on {001} about [100] axis or [-110] axis, giving pseudo-rhombohedral symmetry

Unit cell: a = 23.33 Ǻ, b = 13.40 Ǻ, c = 7.45 Ǻ, β = 105.1°

Colour:  Pale pink to dark red, brownish red to brown, light to dark yellow

Diaphaneity:

Luster:  Vitreous to somewhat pearly

Habit:  Crystals tabular, hemimorphic; acicular; massive, cryptocrystalline, fibrous or lamellar

Hardness:  4 to 5

Specific gravity:  3.04 to 3.06

Cleavage:  1; {001} perfect

Tenacity:  Uneven; brittle

Streak:  Pale rose

Synonyms/varieties:

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