Elbaite

Elbaite, a sodium, lithium, aluminium boro-silicate, with the chemical composition Na(Li1.5Al1.5)Al6Si6O18(BO3)3(OH)4,[3] is a mineral species belonging to the six-member ring cyclosilicate tourmaline group.

Elbaite
General
CategoryCyclosilicate
Formula
(repeating unit)		Na(Li1.5Al1.5)Al6Si6O18(BO3)3(OH)4
Strunz classification9.CK.05
Crystal systemTrigonal
Crystal classDitrigonal pyramidal (3m)
H-M symbol: (3m)
Space groupR3m
Identification
ColorGreen, red to pink, blue, orange, yellow, colorless, multicolored
Crystal habitPrismatic; striated
CleavagePoor/indistinct on {1120} and {1011}
FractureSub-conchoidal
Mohs scale hardness​7 1⁄2
LusterVitreous
StreakWhite
DiaphaneityTransparent to opaque
Density2.9–3.2
Optical propertiesUniaxial (-)
Refractive indexnω = 1.635–1.650, nε = 1.615–1.632
Birefringenceδ = 0.020
Other characteristicspiezoelectric and pyroelectric
References[1][2]
Major varieties
Achroitecolorless
Indicoliteblue
Rubellitered to pink
Verdelitegreen

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 the Greek "άχρωμος" meaning "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 section slices of prisms, often displaying curved sides.


  • Achroite
  • Rubellite
  • Indicolite
  • Verdelite
  • Watermelon tourmaline
Elbaite specimen at Yale's Peabody Museum of Natural History
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See also

References
  1. Elbaite. Mindat
  2. Elbaite. Webmineral
  3. http://rruff.info/ima/
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