Graphic granite
This fascinating looking rock is a type of igneous rock containing intergrowths of quartz and pale or white colored feldspar minerals. The pattern of intergrowths is so distinctive and reminiscent of writing that the texture is called “graphic” after the Latin word Graphus (or Greek Graphos) meaning “to write”.
The texture is defined by quartz grains grown throughout the feldspar, with the shape of the quartz defined by the crystal structure of the feldspar. The two crystal types would have been growing together, during the last stages of cooling of a large magma body. As the feldspar grains grew, they took components like aluminum and alkali elements out of the magma, enriching the magma right next to it in silica enough to cause quartz crystallization. The quartz crystals would start forming or “nucleate” along the edges of the feldspar grain because it is easier for new crystals to form at a place where there is a boundary for them to lock onto. The crystals kept growing outward until this pattern was locked in and preserved.
This pattern is most common in pegmatite rocks, which form fairly quickly in magmas with high water content. The high water enables the minerals to grow quickly enough for the crystal structure to lock into this repetitive pattern.
-JBB
Image credit: http://commons.wikimedia.org/wiki/File:Graphic_Pegmatite.jpg Read more: http://www.minsocam.org/ammin/AM71/AM71_325.pdf
