Black terminated schorl crystal (6.5 × 5.0 × 4.5 cm) from Galiléia, Minas Gerais, Brazil — Photo: Ivar Leidus · CC BY-SA 4.0
Schorl
Also known as: Black tourmaline.
The black iron-rich tourmaline — the most common tourmaline species, forming striated prisms in granite and schist.
Intro adapted from Wikipedia (CC BY-SA).
Schorl is an IMA-recognized mineral species in the silicate class, with the chemical formula NaFe²⁺₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH).
It crystallizes in the hexagonal crystal system (space group R3m). It is a member of the Tourmaline structural group.
Key properties: Mohs hardness 7–7.5; specific gravity 3.18–3.22; typically black; White streak; Vitreous to resinous luster; optically uniaxial.
IMA symbol Srl, IMA status: approved, named in 1562.
Found in granites and granite pegmatites, high-temperature hydrothermal veins, and some metamorphic rocks; also detrital.
It is found associated with Quartz, albite, microcline, orthoclase, epidote, garnet, muscovite, topaz, cassiterite, scheelite, fluorite, beryl.
The name comes from the Old German SchÄurl, of uncertain derivation, perhaps meaning impurities.
Collections: Collector Favorites, Gemstones.