Heulandite

Formula:  (K,Na,Ca,Sr)_2-3Al₃(Al,Si)₂Si₁₃O₃₆

Origin of name:   For John Heuland, English collector

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning:  On {100}

Unit cell: a = 17.61 Ǻ, b = 17.80 Ǻ, c = 7.37 Ǻ, β = 116.3°

Colour:  Colourless, white, gray, yellow, pink, red, brown

Diaphaneity: 

Luster:  Vitreous, pearly on {010}

Habit:  Crystals trapezoidal, tabular; subparallel aggregates; massive, granular

Hardness:  3.5 to 4

Specific gravity:  2.1 to 2.2

Cleavage:  1; {010} perfect

Tenacity:  Uneven; brittle

Streak:  Colourless

Synonyms/varieties:

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