Geological Classification of Granite
Granite is an intrusive igneous rock which means it is cooled slowly deep upper the Earth’s crust.
It is composed of 25% to 35% quartz and over 50% potassium- and sodium rich feldspars.
The commercial stone industry, depending on the supplier or organization, loosely accepts various granite-like stones under the label of “granite.” Some include, banded or massive, non banded gneiss, and a few quartz-based stones. Such stones are marketed as “granite-like” or “granitoid,” though they are not true granite.
Gneisses is a high level metamorphic stone which are included in the granite category by commercial interests. However, the quartz-based stones are definitely not granite, but rather the silica-cemented sedimentary stone quartzite or its metamorphic equivalent, orthoquartzite.
Defining Granite and It’s Components
Granite usually has three to four basic mineral components:
The first being, Quartz; it appears as irregular, watery-looking, or translucent grains.
Next, Orthoclase Feldspar; in most, but not all, commercial, geologic granites, the light-colored minerals, white- to flesh-colored are orthoclase feldspar.
Plagioclase Feldspar features darker gray to bluish grains, with some grains exhibiting fine, grooved lines and/or an iridescent.
Geologic granite, in terms of mineral content, can be formed by granitization, a true metamorphic process by which a solid rock is converted to granite by entry and/or exit of material or change of chemical components without going through a liquid or molten state.