Tourmaline

schorl

elbaite

The tourmaline group is a large group of minerals which includes
schorl, dravite, elbaite, fluor-elbaite, uvite, olenite and fluor-liddicoatite.
Varieties, which are often a mix of these different mineral species, include
achroite, a colourless variety, usually, but not always, elbaite
indicolite, a blue gemmy variety, usually, but not always, elbaite or fluor-elbaite
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
blue cap tourmaline, a variety of elbaite with its top portion dark blue, and the rest of the crystal red or pink
watermelon tourmaline, a variety of concentrically colour-zoned tourmaline with red interiors and green exteriors
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments (tourmaline):

Plutonic igneous environments
Pegmatites (common and characteristic)
Metamorphic environments
Hydrothermal environments

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 sericite. Also intrusion-related, associated with albite, biotite, carbonates, muscovite and quartz.

Tourmaline maybe found in granite, phyllite and limestone.

At the Sapo mine, Minas Gerais, Brazil, fine tourmaline including elbaite, indicolite, "blue-cap" and watermelon 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.

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