Bertrandite

Bertrandite is a beryllium sorosilicate hydroxide mineral with composition: Be₄Si₂O₇(OH)₂. Bertrandite is a colorless to pale yellow orthorhombic mineral with a hardness of 6-7.

Bertrandite
Bertrandite from the Golconda pegmatite, Minas Gerais, Brazil
General
Category	Sorosilicate
Formula (repeating unit)	Be4Si2O7(OH)2
Strunz classification	9.BD.05
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) H-M symbol: (mm2)
Space group	Ccm21
Unit cell	a = 8.7135(4) Å, b = 15.268(1) Å, c = 4.5683(3) Å; Z = 4
Identification
Color	Colorless to pale yellow
Crystal habit	Thin tabular, prismatic to needle-like crystals commonly in radial clusters
Twinning	Common on {011} or {021} forming heart or V shaped twins
Cleavage	Perfect on {001}; distinct on {100}, {010} and {110}
Mohs scale hardness	6 - 7
Luster	Vitreous, pearly on cleavage surfaces
Diaphaneity	Transparent
Specific gravity	2.59 - 2.60
Optical properties	Biaxial (-)
Refractive index	nα = 1.591 nβ = 1.605 nγ = 1.614
Birefringence	δ = 0.023
2V angle	Measured: 73° to 81°
References	[1][2][3]

It is commonly found in beryllium rich pegmatites and is in part an alteration of beryl. Bertrandite often occurs as a pseudomorphic replacement of beryl. Associated minerals include beryl, phenakite, herderite, tourmaline, muscovite, fluorite and quartz.[1]

It, with beryl, are ores of beryllium.

It was discovered near Nantes, France in 1883 and named after French mineralogist, Emile Bertrand (1844–1909).[1][2][3]

One of the world's largest deposits of bertrandite is Spor Mountain, Thomas Range, Utah which is currently the source of most of the world's beryllium production.[4]