Elbaite - Encyclopedia

    Class : Silicates
    Subclass : Cyclosilicates
    Crystal system : Trigonal
    Chemistry : Na(Al,Li)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
    Rarity : Uncommon

Elbaite is a sodium and lithium tourmaline which forms a series with the magnesium dravite. It is a tourmaline specific to sodolithic pegmatites where it rubs shoulders with lepidolite, sometimes beryl and topaz. It owes its name to its place of discovery : the Island of Elba. Like the majority of minerals in the tourmaline group, elbaite is almost always well crystallized. Its crystals are usually elongated prisms strongly striated according to elongation, with a triangular section with curved edges and pyramidal endings. The fracture is uneven and the crystals, often broken, are re-cemented by later minerals (especially quartz). The crystals are commonly associated in parallel or radiate groupings. Elbaite is variously colored, which results in the distinction of numerous varieties : rubellite (pink to dark red), indicolite (blue), verdelite (green) and achroite (colorless), but the coloring frequently varies from one end of the crystal to the other (tsilaïsite variety, multi-colored). The light green color is due to the presence of Fe²⁺ ions, the pink color to Mn³⁺ ions (and not to lithium as is often thought), the colorless varieties being rich in Mg²⁺ or Mn²⁺ ions. The “neon” blue color of Paraíba tourmaline is attributed to Cu²⁺ and the yellow color could be linked to Ti⁴⁺ and Fe²⁺. The coexistence of several varieties within the same deposit is commonplace. It is a stone widely used in jewelry since the 16^th century, it is used in the manufacture of pressure gauges thanks to its piezoelectric properties.

Main photo : Elbaite from Stak Nala, Haramosh Mts., Roundu District, Gilgit-Baltistan, Pakistan

Twins are known on {10-11} and {40-41} but are very rare.

As this mineral is often naturally fractured, it is not uncommon for the crystals to be restored and glued back together. Numerous assemblies also exist, particularly from Pakistan and Afghanistan.

Some rubellites from Pakistan showing a beautiful pink color are also irradiated. This treatment is very difficult to detect unless the specimens present matrix with quartz which is then smoky in color.

Certain isolated tourmaline crystals may also have been heated, the inclusions of which the tension discs can make it possible to highlight this treatment under magnification.

Tourmaline has not yet been synthesized due to its complex chemistry, but some gemstones are diffused with copper to give them a beautiful Paraiba blue-green color (difficult to highlight without laboratory equipment).