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What rock is my granite countertop or tile, really?

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granite

This commercial granite is true granite.

Photo (c) Andrew Alden, licensed to About.com (fair use policy)
Question: What rock is my granite countertop or tile, really?
Answer: When they speak of granite, stone dealers and geologists mean rather different things.

Stone dealers lump a wide variety of rock types under the name "granite." Commercial granite is any (1) crystalline rock that is (2) harder than marble (3) with large mineral grains. Let's unpack that statement:

  • Crystalline rock is rock that consists of mineral grains that are tightly intergrown and locked together, making a tough, impervious surface. Crystalline rocks are made of grains that have grown together at high temperature and pressure, rather than being made of existing sediment grains that have been cemented together under gentler conditions. That is, they are igneous or metamorphic rocks rather than sedimentary rocks. This differentiates commercial granite from commercial sandstone and limestone.
  • Marble is crystalline and metamorphic, but it consists largely of the soft mineral calcite (hardness 3 on the Mohs scale). Granite instead consists of much harder minerals, mostly feldspar and quartz (Mohs hardness 6 and 7 respectively). This differentiates commercial granite from commercial marble and travertine.
  • Commercial granite has its minerals in large, visible grains (hence the name "granite"). This differentiates it from commercial slate, greenstone and basalt in which the mineral grains are microscopic.

To geologists, true granite is a far more specific rock type. Yes, it is crystalline, hard, and has visible grains. But beyond that, it is a plutonic igneous rock, formed at great depths from an original fluid and not from the metamorphism of another rock. Its light-colored minerals consist of 20 to 60 percent quartz, and its feldspar content is no less than 35 percent alkali feldspar and no more than 65 percent plagioclase feldspar (see granite in the QAP classification diagram). Other than that it can contain any amount (up to 90 percent) of dark minerals such as biotite, hornblende and pyroxene. This differentiates granite from diorite, gabbro, granodiorite, anorthosite, andesite, pyroxenite, syenite, gneiss and schist—but all of these excluded rock types can be sold as commercial granite.

The important thing about commercial granite is that, whatever its actual mineral composition, it is (1) rugged—suitable for hard use, takes a good polish and resists scratches and acids—and (2) attractive with its granular texture. You really do know it when you see it.

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