Acuminite

Formula:  SrAlF₄(OH)^.H₂0

Origin of name:  from the Latin "acumen" for spearhead, characteristic shape of the crystals

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning: On {100} as contact twins

Unit cell: a = 13.223 Å, b = 5.175 Å, c = 14.251 Å, b = 111.61°

Colour:  Colourless

Diaphaneity: Transparent

Luster: Vitreous

Habit:  Spear point-shaped crystals 1 mm long, in aggregates

Hardness:  3.5

Specific gravity:  3.29

Cleavage:  1;  (001) perfect

Tenacity:

Streak: White

Synonyms/varieties:

Comments:  Dimorphous with tikhonenkovite. Found in a small cavity in a specimen from a strontium-rich portion of a cryolite deposit