Apophyllite

Formula: KCa₄Si₈O₂₀(F, OH).8H₂O

Origin of name:  From Greek for away from and leaf, in allusion to its manner of exfoliating on heating

Crystal system:  Tetragonal

Crystal class:  4/m 2/m 2/m

Twinning: Rare on {111}

Unit cell:  a = 8.96 Å, c = 15.77 Å

Colour:  Colourless, white, pink, yellow, green

Diaphaneity: Transparent to translucent

Luster:  Vitreous, pearly on {001}

Habit: Tabular to prismatic, commonly pseudo-cubic, granular

Hardness:  4.5 to 5

Specific gravity:  2.33 to 2.37

Cleavage:  2; {001} perfect, {110} imperfect

Tenacity:  Brittle

Streak: White

Synonyms/varieties:

Comments:  Specific data is for fluorapophyllite; most material is a solid solution somewhere between fluorapophyllite and hydroxyapophyllite. Formed as a secondary mineral in amygdules or druses in basalts; in cavities in granite; in tactite and other metamorphic rocks; a late-stage hydrothermal mineral in some mineral deposits.