Cronstedtite

Formula:  Fe₂Fe(SiFe)O₅(OH)₄

Origin of name:  For Axel Cronsted, Swedish mineralogist

Crystal system:  Monoclinic

Crystal class:  m

Twinning:  About [001] axis, penetration

Unit cell: a = 5.49 Å, b = 9.51 Å, c = 7.32 Å, β = 104.5°

Colour:  Greenish-black, brownish black, black

Diaphaneity:

Luster:  Brilliant

Habit:  Crystals three or six sided pyramids; fibres

Hardness:  3.5

Specific gravity:  3.34 to 3.45

Cleavage:  1; {001} perfect

Tenacity:  Somewhat elastic

Streak:  Dark olive green

Synonyms/varieties:

Comments:  Data are for cronstedtite-1M; there are also -2M, -1H, -2H, -3R, -6R and -9R polytypes