Dravite

General Information: Chemical Formula:NaMg3Al6(BO3)3Si6O18(OH)4 Composition:(Molecular Weight = 1,135.17 gm)Sodium 2.03 % NaMagnesium 6.42 % MgAluminum 14.26 % AlSilicon 14.84 % SiBoron 5.71 % B Hydrogen 0.36 % H Oxygen 56.38 % O  Empirical Formula: NaMg3Al6(BO3)3Si6O18(OH)4 Environment:Metasomatically altered magnesian or dolomitic limestones in contact-metamorphic aureoles. Locality:Drava River, Austria. Name Origin:After its locality.Physical Properties: Cleavage:[???] Indistinct Color:black, green, red, blue, or white. Density:2.98 - 3.2, Average = 3.09 Diaphaniety:Transparent to translucent to opaque Habits:Massive - Uniformly indistinguishable crystals forming large masses., Columnar - Forms columns, Divergent - Crystals radiate from a center without producing stellar forms. Hardness:7-7.5 Luminescence:Non-fluorescent. Luster:Vitreous - Resinous Streak:colorlessMore details...

NaMg3Al6(BO3)3Si6O18(OH)4