Annite

Formula:  KFe₃AlSi₃O₁₀(OH,F)₂

Origin of name:  From location, Cape Ann, Massachusetts, USA

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning: contact twins with composition surface {001} and twin axis [310]

Unit cell: a = 5.386 Å, b = 9.324 Å, c = 10.268 Å, b = 100.63°

Colour:  Black, greenish brown, red

Diaphaneity: Translucent to transparent

Luster:  Adamantine, vitreous

Habit:  Tabular crystals and foliated masses, cleavage fragments with pseudo-hexagonal outlines

Hardness:  2.5-3

Specific gravity:  3.3

Cleavage:  1; {001} perfect

Tenacity: Flexible

Streak: Brownish white

Synonyms/varieties:

Comments: 1M polytype. Member of mica group. Found in magnesium-poor igneous and metamorphic rocks.