Chemistry:Elbaite

Short description: Cyclosilicate, mineral

Elbaite
General
Category	Cyclosilicate
Formula (repeating unit)	Na(Li1.5Al1.5)Al6Si6O18(BO3)3(OH)4
Strunz classification	9.CK.05
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) H–M symbol: (3m)
Space group	R3m
Identification
Color	Green, red to pink, blue, orange, yellow, colorless, multicolored
Crystal habit	Prismatic; striated
Cleavage	Poor/indistinct on {1120} and {1011}
Fracture	Sub-conchoidal
Mohs scale hardness	​7 1⁄2
|re|er}}	Vitreous
Streak	White
Diaphaneity	Transparent to opaque
Density	2.9–3.2
Optical properties	Uniaxial (−); moderate relief
Refractive index	nω = 1.635–1.650, nε = 1.615–1.632
Birefringence	δ = 0.020
Other characteristics	piezoelectric and pyroelectric
References
Major varieties
Achroite	colorless
Indicolite	blue
Rubellite	red to pink
Verdelite	green

Elbaite, a sodium, lithium, aluminium boro-silicate, with the chemical composition Na(Li_1.5Al_1.5)Al₆Si₆O₁₈(BO₃)₃(OH)₄, is a mineral species belonging to the six-member ring cyclosilicate tourmaline group.

Elbaite forms three series, with dravite, with fluor-liddicoatite, and with schorl. Due to these series, specimens with the ideal endmember formula are not found occurring naturally.

As a gemstone, elbaite is a desirable member of the tourmaline group because of the variety and depth of its colours and quality of the crystals. Originally discovered on the island of Elba, Italy in 1913, it has since been found in many parts of the world. In 1994, a major locality was discovered in Canada, at O'Grady Lakes in the Yukon.

Elbaite forms in igneous and metamorphic rocks and veins in association with lepidolite, microcline, and spodumene in granite pegmatites; with andalusite and biotite in schist; and with molybdenite and cassiterite in massive hydrothermal replacement deposits.

Elbaite is allochromatic, meaning trace amounts of impurities can tint crystals, and it can be strongly pleochroic. Every color of the rainbow may be represented by elbaite, some exhibiting multicolor zonation. Microscopic acicular inclusions in some elbaite crystals show the cat's eye effect in polished cabochons.

Elbaite varieties

• Colorless: achroite variety (from grc άχρωμος (ákhrōmos) 'colorless')
• Red or pinkish-red: Rubellite variety (from ruby)
• Light blue to bluish green: Brazilian indicolite variety (from indigo)
• Green: Brazilian verdelite variety (from emerald)
• Watermelon tourmaline is a zoned variety with a reddish center surrounded by a green outer zone resembling watermelon rind, evident in cross-sectional slices of prisms, often displaying curved sides.

• 

  Achroite

• 

  Rubellite

• 

  Indicolite

• 

  Verdelite

• 

  Watermelon tourmaline

Elbaite specimen at Yale's Peabody Museum of Natural History

See also

• List of minerals