Elbaite

Elbaite
Elbaite from Male, Mogok, Sagaing, Mandalay, Myanmar (size: 61 x 60 x 52 mm, 198 g)
General
Category	Silicate mineral
Formula (repeating unit)	Na(Li1.5Al1.5)Al6Si6O18(BO3)3(OH)4
Strunz classification	09.CK.05
Identification
Color	Green, red to pink, blue, orange, yellow, colorless, multicolored
Crystal habit	Prismatic; striated
Crystal system	Trigonal
Cleavage	Poor/indistinct on {1120} and {1011}
Fracture	Sub-conchoidal
Mohs scale hardness	7½
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to opaque
Density	2.9–3.2
Optical properties	Uniaxial (-)
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, is a mineral species belonging to the six member ring cyclosilicate tourmaline group, with the following general composituion: Na(Li_1.5Al_1.5)Al₆Si₆O₁₈(BO₃)₃(OH)₄.

Elbaite forms three series, with dravite, with fluor-liddicoatite, and with schorl. Due to these series, specimens with the ideal end-member 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:
  • Red or pinkish-red; rubellite variety (from ruby)
  • Light blue to bluish green; Brazilian indicolite variety (from indigo)
  • Green—verdelite or Brazilian emerald variety
  • Colorless—achroite variety (from the Greek "άχρωμος" meaning "colorless")
  • 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.

References

Rubellite in lepidolite mica matrix, Pala, California.

Elbaite – Elba Island – Topotype deposit