Bertrandite

Formula:  Be₄Si₂O₇(OH)₂

Origin of name:  For Emile Bertrand, French mineralogist (1844-1909)

Crystal system:  Orthorhombic

Crystal class:  mm2

Twinning:  Common on {011} or {021}

Unit cell: a = 8.71 Ǻ, b = 15.26 Ǻ, c = 4.56 Ǻ

Colour:  Colourless, pale yellow

Diaphaneity: Transparent

Luster:  Vitreous, pearly

Habit:  Crystals prismatic or thin tabular; pseudomorphous after beryl

Hardness:  6 to 7

Specific gravity:  2.60

Cleavage:  4; {001} perfect, {110}, {010} and {100} distinct

Tenacity:  Brittle

Streak:

Synonyms/varieties:

Comments: Found in fissures in granites and associated pegmatites and in miarolitic cavities in greisens, commonly found as an alteration product of beryl, more rarely as a primary mineral