Tourmaline group - cyclosilicates

Formule chimique : Na(Li,Al)3Al6(BO3)3(Si6O18)(OH)4.
Système cristallin : Rhombohedral
Etymologie : From the island of Elba, Italy.
Dureté : 7
Densité : 3 – 3.1
Propriétés physiques : Vitreous luster, colorless (achroite), pink (rubellite), green (verdelite) or blue (indicolite). Transparent to translucent. Conchoidal fracture. Indistinct cleavage.
Propriétés chimiques : Acid-proof.
Formule chimique : Na(Li,Al)3Al6(BO3)3(Si6O18)(OH)4.
Habitus : In well-formed elongated prismatic crystals, striated in the elongation.
Gîtologie : Elbaite belongs to the tourmaline group. It is a mineral of the sodolite pegmatites.
Determination : Easy because of its colours and habitus.
Utilisation : Because of its piezoelectric properties, tourmaline is used in electrochemistry. When it is a gem, it can be used in jewellery with its diversity of colours.

Tourmalines are common in pegmatites. The largest and most beautiful deposits are in Brazil (Minas Gerais), Elba Island (Italy), the Urals (Russia), California and Maine (USA), Namibia, Madagascar,…
In France, it is found in the Massif Central, the Massif Armoricain, the Maures, etc.

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