Barra do Salinas district, Barra do Salinas, Coronel Murta, Minas Gerais, Brazil

 XY₃Z₆T₆O₁₈(BO₃)₃V₃W

The term tourmalines indicates a supergroup of cyclosilicates having the general formula reported above. More than 30 different species are currently known. For further details see Henry et al. (2011). Am. Mineral., 96, 895-913

Crystal System: trigonal
Colour: tourmalines display a wide range of colours, from colourless to black
Habitus: prismatic to acicular crystals. Also as radial, fibrous and massive aggregates
Hardness: 7
Lustre: vitreous, resinous
Fracture: uneven to conchoidal
Cleavage:  very poor on {11-20} and {10-11}
Density: 2.9 - 3.1 g/cm³
Origin and geological occurrence: it is a mineral widespread in several geological enviroments, from magmatic to metamorphic. Also as a detrital or authigenic mineral in sedimentary rocks.

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Samples:

9C.1 Tourmaline  Black crystal with quartz, garnet, and mica #neso soro silicates

9C.2 Tourmaline   Black prismatic crystals with quartz and mica #neso soro silicates