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Tourmaline Group of mineral species

Tourmaline group consists of 30 specimens occurring in a variety of colors. Usually, iron-rich tourmalines are black to bluish-black to deep brown, while magnesium-rich varieties are brown to yellow, and lithium-rich tourmalines are almost any color: blue, green, red, yellow, pink, etc. Rarely, it is colorless. Bi-colored and multicolored crystals are common, reflecting variations of fluid chemistry during crystallization. Crystals may be green at one end and pink at the other, or green on the outside and pink inside; this type is called watermelon tourmaline. Some forms of tourmaline are dichroic, in that they change color when viewed from different directions.

Tourmaline (synonymous with tourmaline group) comprises trigonal borosilicates of the cyclosilicate superclass.The structure for the group is one in which SiO4 tetrahedra are linked into six-membered rings having a hexagonal pattern and are stacked up with intervening distorted triangular BO3 groups, linked by D site cations; the SiO4 tetrahedra are linked vertically by G site cations and X site anions, while the columns are linked horizontally by both D site cations and X site anions. The A site cations and the Z site anions occupy the channels down the centre of the columns (or the A site may be vacant.)

The Following are recognized distinct Tourmaline group species

Adachiite CaFe2+3Al6(Si5AlO18)(BO3)3(OH)3OH
Chromium-dravite NaMg3Cr6Si6O18(BO3)3(OH)3OH
Chromo-alumino-povondraite NaCr3(Al4Mg2)Si6O18(BO3)3(OH)3O
Darrellhenryite NaLiAl2Al6Si6O18(BO3)3(OH)3O
Dravite NaMg3Al6Si6O18(BO3)3(OH)3OH
Elbaite Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)3OH
Feruvite CaFe2+3(MgAl5)Si6O18(BO3)3(OH)3OH
Fluor-buergerite NaFe3+3Al6Si6O18(BO3)3O3F
Fluor-dravite NaMg3Al6Si6O18(BO3)3(OH)3F
Fluor-elbaite Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)3F
Fluor-liddicoatite Ca(Li2Al)Al6Si6O18(BO3)3(OH)3F
Fluor-schorl NaFe2+3Al6Si6O18(BO3)3(OH)3F
Fluor-tsilaisite NaMn2+3Al6Si6O18(BO3)3(OH)3F
Fluor-uvite CaMg3(Al5Mg)Si6O18(BO3)3(OH)3F
Foitite ៛(Fe2+2Al)Al6Si6O18(BO3)3(OH)3OH
Luinaite-(OH) (Na,៛)(Fe2+,Mg)3Al6Si6O18(BO3)3(OH)3OH
Magnesio-foitite ៛(Mg2Al)Al6Si6O18(BO3)3(OH)3OH
Olenite NaAl3Al6Si6O18(BO3)3O3OH
Oxy-chromium-dravite NaCr3(Mg2Cr4)Si6O18(BO3)3(OH)3O
Oxy-dravite Na(Al2Mg)(Al5Mg)Si6O18(BO3)3(OH)3O
Oxy-schorl Na(Fe2+2Al)Al6Si6O18(BO3)3(OH)3O
Oxy-vanadium-dravite NaV3(V4Mg2)Si6O18(BO3)3(OH)3O
Povondraite NaFe3+3(Fe3+4Mg2)Si6O18(BO3)3(OH)3O
Rossmanite ៛(LiAl2)Al6Si6O18(BO3)3(OH)3OH
Schorl NaFe2+3Al6Si6O18(BO3)3(OH)3OH
Tsilaisite NaMn2+3Al6Si6O18(BO3)3(OH)3OH
Uvite CaMg3(Al5Mg)Si6O18(BO3)3(OH)3OH
Vanadio-oxy-chromium-dravite NaV3(Cr4Mg2)Si6O18(BO3)3(OH)3O
Vanadio-oxy-dravite NaV3(Al4Mg2)Si6O18(BO3)3(OH)3O

A revised nomenclature for the tourmaline group was published in 2011

For more reading about Tourmaline group minerals: