Graphic granite from a pegmatite body in the Devonian of New Hampshire. (8.1 centimeters across at its widest)

Dark gray = quartz (SiO2 - silica) Whitish = K-feldspar (KAlSi3O8 - potassium aluminosilicate)

Graphic granite (a.k.a. runic granite; a.k.a. runite) has an odd but distinctive type of crystalline texture. Graphic granites are typically bimineralic and consist of interpenetrating crystals (quartz in potassium feldspar), in a way that somewhat resembles ancient cuneiform writing (“graphic”). Some observed examples resemble Arabic writing. These rocks occur in pegmatitic granite intrusions.

This example is from the Ruggles Pegmatite Mine near Grafton, New Hampshire. The Ruggles Mine started off in the early 1800s as a muscovite mica mine, but it's now a tourist site and mineral collecting locality. Its walls have beautiful exposures of a mid-Paleozoic granite pegmatite, having unbelievably large crystals. Well over 100 minerals have been reported from this pegmatite, but the most common rock-forming minerals here are quartz, potassium feldspar, biotite mica, muscovite mica, and schorl tourmaline. The Devonian-aged pegmatite at Ruggles Mine is one of several in the Grafton Pegmatite Field, which were emplaced by magmatism during the ancient Acadian Orogeny.

Pegmatites are very coarsely-crystalline, intrusive igneous rocks. Pegmatitic texture refers all or almost all the crystals in the rock being larger than 1 centimeter. Most pegmatitic-textured igneous rocks have a granitic composition, and so are called pegmatitic granites or granite pegmatites, or just "pegmatites". Pegmatites tend to form in the margins of cooling batholiths, during the final stages of a crystallization. After most of the magma has crystallized, the residual magma is rich in gas & water & silica & incompatible ions (atoms too large or too small to fit in “normal” minerals that formed earlier). Cooling of such residual magma results in pegmatites. The water-rich nature of this residual magma allows rapid ion transport during crystallization, resulting in very large crystals. The incompatible ions tend to form unusual minerals and “garbage can” minerals (e.g., beryl, chrysoberyl, columbite/tantalite, uraninite, cryolite, monazite, apatite, lepidolite, spodumene, zoisite, topaz, zircon, molybdenite, etc.).

Geologic unit: Ruggles Pegmatite, Grafton Pegmatite Field, New Hampshire Plutonic Series, Devonian