tourmaline A member of the cyclosilicates and a borosilicate Na(Mg,Fe²⁺,Mn,Li, Al)₃Al₆(BO₃)₃[Si₆O₁₈](OH,F)₄. There are three important members of this family of minerals: dravite (NaMg₃Al₆(BO₃)₃[Si₆O₁₈](OH,F)₄); schorl (Na(Fe²⁺,Mn)₃Al₆(BO₃)₃ [Si₆O₁₈](OH,F)₄); and elbaite (Na(LiAl)₃Al₆ (BO₃)₃[Si₆O₁₈] (OH,F)₄); sp. gr. 2.9–3.2; hardness 7.0–7.5; trigonal; black, bluish, pink, or green, never colourless; elongate crystals common, also acicular needles and massive or radiating aggregates; cleavage good {1120} prismatic; occurs in granite pegmatites, pneumatolytic (see PNEUMATOLYSIS) veins, and granites as elbaite and schorl varieties; it may occur in the rock luxullianite formed by pneumatolytic action after boron has been introduced, where it will occur with topaz, spodumene, cassiterite, fluorite, and apatite. Dravite variety occurs in metamorphosed impure limestones and rarely in some basic igneous rocks; tourmaline is a common detrital ‘heavy’ mineral in sedimentary rocks. Good multicoloured crystals can be used as gemstones.

tour·ma·line / ˈtoŏrmələn; -ˌlēn/ • n. a brittle gray or black mineral that occurs as prismatic crystals in granitic and other rocks. It consists of a boron aluminosilicate and has pyroelectric and polarizing properties, and is used in electrical and optical instruments and as a gemstone.

tourmaline Silicate mineral, sodium or calcium aluminium borosilicate, found in igneous and metamorphic rocks. Its crystals are hexagonal system and glassy, either opaque or transparent. Some are prized as gems. Hardness 7.5; r.d. 3.1.

tourmaline brittle pyro-electric mineral orig. from Ceylon. XVIII. — F., ult. f. Sinhalese toramalli cornelian.