SIDNEY MOULDS / SCIENCE PHOTO LIBRARY SIDNEY MOULDS / SCIENCE PHOTO LIBRARY
Macrophotograph of crystals of tourmaline in a matrix of quartz. The tourmaline is seen as the black, needle-like objects. Tourmaline is a complex silicate of boron and aluminium. It is classed as a cyclosilicate, as the silicon appears in a six-membered oxidised ring, with three oxygen atoms for each silicon. The rest of the component groups (aluminium tetroxide and fluorohydroxide) are linked to the chain by various cations. In this black form, the cations are iron (II) and manganese. Tourmaline may also be found in muscovite and feldspars, or as a crystallization product in granite pegmatites. Magnification x0.7 at 35mm size.
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