Brewsterite

Formula:  (Sr,Ba,Ca)Al₂Si₆O₁₆ ^. 5H₂O

Origin of name:  For Sir David Brewster (1781-1868), Scottish physicist and mineralogist, who studied the optical properties of minerals and the element Strontium

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:  On {010}

Unit cell: a = 6.793 Ǻ, b = 17.573 Ǻ, c = 7.759 Ǻ, β = 94.54°

Colour:  White, colourless

Diaphaneity:  Transparent

Luster:  Vitreous

Habit:  Crystals short prismatic; granular or radial fibrous aggregates

Hardness:  5

Specific gravity:  2.543

Cleavage:  1; {010} perfect

Tenacity: Brittle, uneven fracture

Streak:

Synonyms/varieties:

Comments: Brewsterite-Ba is the most dominant form, member of the zeolite group. Hydrothermally deposited in druses lining cavities in basals and schists, more rarely in ore deposits.