Tounkite

Formula: (Na,Ca,K)₈(Al₆Si₆O₂₄)(SO₄)₂Cl.H₂O

Origin of name: after locality Tunka (Tounka) Valley, Russia, near the deposits

Crystal system: Hexagonal

Crystal class: 622

Twinning:

Unit cell: a = 12.843, c = 32.239

Colour: Yellowish bottle-green, may be tinted blue by included lazurite

Diaphaneity: Transparent

Luster: Vitreous

Habit: columnar crystals or replacements of lazurite

Hardness: 5 to 5.5

Specific gravity: 2.56

Cleavage: 1; {1010} fair

Tenacity: 

Streak: White

Synonyms/varieties:

Comments: readily soluble in dilute HCl.  Replaces lazurite in diopside-lazurite rocks.