Dravite

Formula:   NaMg₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

Origin of name:   From locality; District of Drave, Austria

Crystal system:   Hexagonal

Crystal class:   3m

Twinning:   Rare on {10-11} or {40-41}

Unit cell: a = 15.96 Å, c = 7.21 Å

Colour:   Brown, black, greenish black, dark red, pale bluish green to blue

Diaphaneity:

Luster:   Vitreous to somewhat resinous

Habit:   Crystals short to long prismatic; sometimes nearly equant; single crystals or radiating groups; massive, compact, grains

Hardness:   7

Specific gravity:   3.03 to 3.

Cleavage:   2; {11-20} and {10-11} indistinct

Tenacity: 

Streak:   Colourless

Synonyms/varieties:   Gouverneurite, magnodravite (Mg-rich var.)

Comments:   Series with schorl and with elbaite