Graphic granite is a term used to describe a very distinctive
igneous texture seen in some samples of
granite. The
texture comprises
angular crystals of grey
quartz (ranging in length from 1 to 12 mm) embedded within coarse-grained, light-grey to pink alkali
feldspar. The size, shape and orientation of the quartz grains (in relation to each other) gives the outward appearance of
cuneiform characters, which were used in the
alphabets of several
Mesopotamian cultures. The name "graphic granite" is derived from the Greek
graphos, meaning "writing" (not because the rock is fond of excessive violence or sexual situations). The quartz crystals are actually rod-shaped, and the
wedge pattern is best observed in
cross-section.
Contrary to outward appearances, graphic granite is not an example of a porphyritic texture. "Porphyritic" is a genetic term, implying that the individual minerals crystalized from the magma at different times and temperatures. In fact, graphic granite is only formed when alkali feldspar and quartz are crystallizing at the same time and at constant rates. These conditions describe a point where temperature and the bulk composition of the magma are in equilibrium; when these factors are plotted in a phase diagram, this point (the eutectic) is readily identifiable.
Other terms used to describe graphic granite include: Hebrew stone, Hebraic pegmatite, corduroy rock and runite (from the Norse runes, which the wedges also resemble).
Sources:
Principles of Igneous and Metamorphic Petrology (1990), Anthony R. Philpotts, 498 pp., Prentice Hall
GemRocks: Graphic Granite -
http://www.cst.cmich.edu/users/dietr1rv/graphic.htm