Tuesday, June 13, 2017

Graphic granite

Graphic granite is relatively common rock consisting of alkali feldspar (i.e., rich in potassium, in some cases in combination with sodium) and quartz, but the rock has a very interesting texture, consisting of a distinctive repetitive pattern that resembles cuneiform writing.

The above picture and the following two pictures are of the same piece of graphic granite, which is about 7 inches long (= 18 cm; the scale is in centimeters).

The origin of graphic granite was debated for over a century. It is now known to be the result of simultaneous growth of quartz (gray color in the rock above) and feldspar (white color) under conditions that favor the planar growth of the feldspar host. 

The next two pictures are different views of the same piece of rock, but you can notice how the texture differs, depending on the view.

Graphic granite occurs in pegmatites, which form during the final stage of a magma's crystallization. The graphic granite illustrated in these three pictures came from the pegmatite at the Stewart Mine in San Diego County, Southern California (see my archived post for  September 30, 2016 which focuses on the mineral rubellite from the Stewart Mine).

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