The tourmaline group is a large group of cyclosilicates (ring silicates) that are borosilicates.
The Tourmaline Supergroup
The generalised tourmaline structural formula is XY3Z6(T6O18)(BO3)3V3W
Tourmaline minerals can be assigned to groups depending on the dominant content of the X site, which may be
|Na1+ or K1+||alkali group|
Maruyamaite is the only known tourmaline mineral with
K at the X site.
Each of the groups can be divided into sub-groups, determined by the dominant content of the Y site, which may be Fe2+, Mg2+, Mn2+, Al2+, Li2+, Fe2+, or Cr2+.
For the alkali group, representative minerals for different values of Y are as follows:
For the calcic group, representative minerals for different values of Y are as follows:
For the X-vacant group, representative minerals for different values of Y are as follows:
Other sites are occupied as follows:
Z: Al3+, Fe3+, Mg2+, Cr3+
T: Si4+, Al3+, B3+
V: OH1–, O2–
W: OH1–, F1–, O2–
If F1– or O2– are dominant at the W site, the prefix fluor- or oxy- respectively is appropriate, as in fluor-buergerite and oxy-chromium-dravite
Colour Varieties of Tourmaline
An alternative way of identifying tourmaline minerals , which are often a mix of these different mineral species, is by their colour. Colour varieties include:
achroite, a colourless variety, usually, but not always, elbaite
blue cap tourmaline, a variety of elbaite with its top portion dark blue, and the rest of the crystal red or pink
indicolite, a blue gemmy variety, usually, but not always, elbaite or fluor-elbaite. Most blue Tourmalines are coloured by ferrous iron (Fe2+) as well as iron to iron charge transfer (Fe2+ - Fe3+) (https://www.gemstonemagnetism.com/tourmaline_black_blue_and_green.html).
rubellite, a red gemmy lithium-rich and more or less iron- and magnesium- free variety, primarily elbaite or fluor-elbaite, but may also be olenite or fluor-liddicoatite
verdelite, a green variety
watermelon tourmaline, a variety of concentrically colour-zoned tourmaline with red interiors and green exteriors
Properties of tourmaline
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Plutonic igneous environments
Pegmatites (common and characteristic)
The most common and characteristic occurrence of tourmaline is in
granite pegmatites with elevated boron content,
and in the rocks immediately
surrounding them. If there is sufficient boron and aluminium in the melt,
schorl is the first
tourmaline species to form, then elbaite. Tourmaline is found also as an
accessory mineral in igneous rocks
and metamorphic rocks.
Most pegmatitic tourmaline is black and associated with the common pegmatite minerals microcline, albite, quartz and muscovite.
Light lithium-bearing tourmaline also occurs in pegmatites, frequently associated with lepidolite, beryl, apatite and fluorite.
Hydrothermal tourmaline is found in sediment-hosted massive sulphide deposits, such as the Sullivan deposit in British Columbia, Canada, associated with chlorite, manganese garnet, pyrrhotite, quartz and muscovite variety sericite. Also intrusion-related, associated with albite, biotite, carbonates, muscovite and quartz (AofA).
Tourmaline may be found in granite, phyllite and limestone.
At the Golconda mine, Minas Gerais, Brazil, cookeite pseudomorphs after tourmaline have been found (KL p238).
At the Sapo mine, Minas Gerais, Brazil, fine tourmaline including elbaite, indicolite, "blue-cap" tourmaline and watermelon tourmaline is found in the pegmatite. Most of the crystals are loose, but some are resting on an albite or quartz matrix. Schorl is also abundant, associated with quartz, albite and microcline (Min Rec 40.4.290). Polylithionite pseudomorphs after tourmaline have also been found at Minas Gerais (KL p237).
At the Dafoe Property, Pierrepont, St Lawrence county, New York, USA, tourmaline crystals occur on walls and within open spaces in a tourmaline-bearing pegmatite vein (R&M 94.5.452-455). The black tourmaline crystals are complexly zoned with fluor-uvite cores and dravite rims (R&M 94.5.452-455).
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