|Composition of a granite (Credit: National Park Service)|
Granite is an igneous rock made up of primarily quartz, feldspar, micas, amphiboles, and a mixture of additional trace minerals. These minerals and their variation in abundance and alteration give granite the numerous colors and textures we see in granite countertops. Formally, granite is a plutonic rock that is composed of between 10 to 50% quartz (typically semi-transparent white) and 65 to 90% total feldspar (typically a pinkish or white hue).
Granite is an intrusive igneous rock, which means it was formed in place during the cooling of molten rock. Generally, the slower the molten rock cooled, the larger it’s mineral crystals with K-Feldspar megacrysts forming in special circumstances greater than 5cm. During formation of granite it is buried below kilometers of rock and sediment necessary to produce enough heat to melt rock. Of course, the granite we see today is near surface, and thus at some point was uplifted, causing overlying sediment to be shed via erosion. This transition from high pressure and temperature to atmospheric temperature and pressure can cause the granite to slightly expand and crack. This, in addition to seasonal variations in temperature can leave you with a weakened and less desirable granite to use for countertops.
By Trevor on Nace forbes.com