The beauty of metamorphism Most garnets (barring the chromian Pyropes and a few of the iron bearing

The beauty of metamorphismMost garnets (barring the chromian Pyropes and a few of the iron bearing Almandines which are mantle minerals) are born in metamorphic events, either the baking and stewing in the resulting expelled juices incurred when rocks are intruded by hot molten rock, or more often in the slower and more spectacular mountain building events, where bands of different minerals record bands of different pressure temperature conditions at different depths as the crust stacked up in layers atop itself. Calcium grossular garnets can form in skarns, which are limestones stewed by granites, though the emerald green Tsavorites (see https://bit.ly/1ygBzM5) formed in a mountain building event, as does the king of green garnets, Demantoid (see https://bit.ly/1oE5O31).In the red garnets there is a solid solution series, with the mantle sourced pyrope garnets (along with chrome diopside a diamond indicator mineral that erupts in the same kimberlite that carries the diamonds up from the mantle), at one end, the mostly metamorphic iron Almandine garnets in the middle and these that form mostly in Earth distilled granitic rocks or metamorphosed sediments (which tend to be aluminium rich enough for garnet formation) . Here we have orange Spessartite/ine garnets(aka Mandarin or Malaya garnet in the gem trade), named after their type location, the town of Spessart in Germany. The colour is caused by manganese, and it can also be recognised by its inclusions in a hand lens, which consist of veil patterns of fluid and crystalline inclusions. Major sources include the Umba valley in Tanzania and Kenya (also famous for corundums), Nigeria and Namibia. Colours are natural as these gems are not treated.Garnets in a rock are used by geologists as both geobarometers and geothermometers to gauge the ambient temperature and pressure conditions in which the rock formed, and hence the grade of the metamorphic event. In the 8x4x2cm specimen in the photo mined in Namibia, multiple crystals of gemmy garnet dot a mica schist, in which the many flakes of mica that characterise rocks at this metamorphic range of pressure temperature conditions are also clearly visible. The full mineralogy of the rock is a function of the conditions and the geochemistry of the original rock, known as the protolith.LozImage credit: Marco Frigerio‎http://www.minerals.net/mineral/spessartine.aspxhttp://www.minerals.net/gemstone/spessartite_gemstone.aspx -- source link

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