Orbicular graniteWe recently shared a photo of a Rapakivi granite, which contains grains of pinkish

Orbicular graniteWe recently shared a photo of a Rapakivi granite, which contains grains of pinkish potassium feldspar surrounded by pale or white rims of plagioclase feldspar (http://tmblr.co/Zyv2Js1Zz1JTG). This is a different kind of granite called orbicular granite but it shows a similar feature – minerals with rims.Orbicular granites form when different minerals crystallize from a magma in alternating patterns. Here, the first mineral to form was probably plagioclase feldspar. A small grain of it formed, but then something about the magma changed, possibly a temperature increase, causing the edges of the grain to re-melt and rounding it into a spherical shape. Crystals don’t grow spherical on their own, so the shape must imply some sort of erosion of the grains.The magma then began crystallizing a different mineral, the dark layer, presumably in this case an amphibole mineral like hornblende, as is common in rocks that are crystallizing plagioclase. As the amphibole formed, the magma composition changed and the temperature decreased until again plagioclase was able to crystallize.From there, the alternating pattern continued, possibly due to arrival of new, hotter magma, or possibly due to movement of the crystal up and down inside a churning, convecting magma chamber. Finally, after this grain formed, it was broken and cross-cut by a dike of a different, later magma.The sample is likely a few centimeters across and is about 400 million years old. It comes from Sout Island on New Zealand’s west coast and is on display at the Auckland War Memorial Museum.To be completely accurate, this probably isn’t a true “granite”. True granites have a lot of potassium feldspar in them and this one doesn’t seem to, probably making it closer to a diorite or a quartz diorite. Commonly, the mining industry will call almost any igneous, crystalline rock “granite” even though geologists break these rocks apart into different groups depending on composition.-JBBImage credit: https://flic.kr/p/iQyXaRead more:http://www.aradon.com.au/orbicular_granite.htmlhttp://www.kristallin.de/orbiculite/orbicular_rocks1.htm -- source link

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