Topazolite garnet The garnets are a pair of related but not interchangeable mineral families that sh

Topazolite garnetThe garnets are a pair of related but not interchangeable mineral families that share a common structure, with different elements occupying crucial points in the lattice forming the various members. The most common family is called pyralspite, from the combination of pyrope (often found in the mantle, coloured by chromium), almandine (brownish red to pale pink, coloured by iron) and spessartine )orange, coloured by manganese. Within the family these elements substitute for each other, and the combination of the three elements in an individual garnet will tell us something about its geological origin. Pyropes are often igneous in origin, carried up from the mantle by eruptions such as kimberlites, while almandines tend to be metamorphic in origin and spessartines can form either in granite pegmatites or metamorphically.The second family is called the ugrandite series, after uvarovite (the rarest, see http://bit.ly/1WNilPR), grossular (including the iron coloured orange hessonite, and the bright green chromian Tsavorite, see http://bit.ly/1ygBzM5) and andradite (like this specimen and coloured by iron, but also including the chromian king of garnets, demantoid, see http://bit.ly/1oE5O31). So far, all the deposits found in this family result from one flavour or another of metamorphism.They have a very high lustre (how much light they reflect), called adamantine by mineralogists, and caused by the gem’s high refractive index (how much it bends light rays passing through). They also have a very high dispersion (aka fire). This is what causes the flashes of colour in diamonds as they are moved, and is due to fact that the different wavelengths of colour in white light have different energies, and are bent in varying amounts by the crystal structure as the light rays pass through.Andradite’s dispersion is higher than diamond’s and pale yellow specimens rival fancy coloured carbon in their beauty. Its hardness is high enough for use in all kinds of jewellery. Yellow stones display more fire, while the deep green ones have a lovely glow, but the fire is usually masked by the saturation of the body colour.The 6.5 x 3.5 x 3.5 cm brandy hued crystal cluster illustrating this post was mined in Madagascar, and sits on a metamorphic garnetiferous matrixLozImage credit: Rob Lavinsky/irocks.comhttp://www.mindat.org/min-7535.htmlhttp://www.gemstone.org/index.php…http://www.palagems.com/gem_spectrum4.1.htmA gem buying trip to Russia: http://www.palagems.com/demantoid_garnet.htmhttp://www.gemdat.org/gem-1258.htmlhttp://www.mindat.org/min-1258.html -- source link

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