Buergerite

Formula:  NaFe₃Al₆(BO₃)₃Si₆O₂₁F

Origin of name:  For Martin Julian Buerger (1903-1986, American crystallographer, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A., inventor of the X-ray precession camera permitting undistorted photography of the reciprocal lattice.

Crystal system:  Hexagonal

Crystal class:  3m

Twinning:

Unit cell: a = 15.869 Å, c = 7.188 Å

Colour:  Dark brown, black; bronze schiller

Diaphaneity: Translucent

Luster:  Vitreous

Habit:  Crystals short prismatic

Hardness:  7

Specific gravity:  3.31

Cleavage:  Distinct prismatic

Tenacity: Brittle

Streak:  Yellow-brown

Synonyms/varieties:

Comments: Pyroelectric and piezoelectric, member of the tourmaline group. Probably of pneumatolytic origin, in cavitiesin rhyolite.