garnet

A cut and polished Garnet

So on Saturday (18th January) Geobus reached the grand old age of 2 and to celebrate we thought we would tell you a little bit about our birthstone – the mineral Garnet!

All garnets have the basic chemical formula X3Y2(SiO4)3 and X is usually calcium, ferrous iron (Fe2+), magnesium or manganese, and Y is aluminium, ferric iron (Fe3+), or chromium. The different compositions can have many colours and the common crystal habit is dodecahedron. On the Mohs hardness scale, garnets range from 6.5–7.5 and so the harder ones are used as abrasives (such as in sand blasting and sandpaper). It is a heavy mineral so it often gets concentrated into dark placer deposits on beaches and in sandy river systems.

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Large Alamandine crystals as seen within a garnet-mica schist

microscope garnet

Garnet as seen under a polarising microscope – it appears black in colour due to being an isotropic mineral

A common garnet seen in metamorphic rocks such as mica schists (top image) is almandine whose formula is Fe3Al2(SiO4)3. Almandines are deep red and can grow to large sizes and are therefore often used as gemstones. If we looked at almandine (or any garnet) under the polarising microscope (bottom image), the garnet is always dark even if you rotate the stage of the microscope. This is because the light is transmitted at the same velocity in all directions – such minerals are called isotropic. The very brightly coloured minerals are micas.

The chemistry of garnets tell us about their origins. By measuring the concentration of elements within garnet, and other minerals growing with it, it is possible to calculate the pressure and temperature of the garnet’s formation and work out the depth of its formation in the crust. COOL!