The American Geological Institute (AGI) defines granite as an Intrusive Igneous Rock (cooled slowly in the deep upper part of the Earth’s crust) composed of 25% to 35% quartz and over 50% potassium- and sodium- rich feldspars, with a common accessory mineral of less than 20%, usually muscovite (clear mica), biotite (dark, iron-bearing mica), or hornblende (amphibole). However, the commercial stone industry loosely accepts various granite-like stones under the label of “granite.” Such stones are marketed as “granite like” or “granitoid,” though they are not true granite. Granite, as defined by the AGI, usually has three to four basic mineral components:
Appears as irregular, watery-looking, or translucent grains.
In most, but not all, commercial, geologic granites, the light-colored minerals, white- to flesh-colored are orthoclase feldspar.
(NaAlSi3O8 to CaAl2Si3O8)
Features darker gray to bluish grains, with some grains exhibiting fine, grooved lines and/or an iridescent sheen. Sometimes the mineral is a light, creamy color, nearly indistinguishable from orthoclase, except for the occasional presence of the fine lines.
DARK ACCESSORY MINERALS
Biotite and Hornblende are complex silicates with all the elements in quartz, orthoclase feldspar, and plagioclase feldspar, plus iron and magnesium, which gives these minerals their dark color.
Magnetite (Fe3O2) is often a trace mineral (±2-4%) in granite, and easily recognized by its strongly magnetic property. It, too, is black and opaque. It has a hardness rating of H=6, polishes well, has a metallic appearance, and is opaque (i.e., it does not transmit light even when very thin).
All of these granitic minerals have a Mohs Scale hardness rating of H=7 or H=6 with the exception of biotite, which has a variable hardness of 2.5 to 4.5, depending on the angle of the polished surface in relation to the edges of the “books” of sheets that biotite mica crystals exhibit. Thus, biotite does not polish well and appears as small, rough spots on a polished surface. This is not a flaw in either the stone or polishing, but simply the way biotite is—softer than all other minerals in granite. Biotite hardness makes little difference in most applications, as the overwhelmingly dominant feldspar and quartz minerals are much harder.
The ultimate test of any building material is its ability to have and maintain the necessary structural strength, as well as beauty of appearance and low cost of maintenance, over the useful life of the structure. Experience has proven that stone meets this test as few other building materials can. Studies have shown that the durability of most stones is little affected by cycles of weather. This is because most granites have a low rate of moisture absorption.
Granite is not combustible and therefore is considered a fire-resistant material. Because of its thermal conductivity, heat transfer is fairly rapid. Most stone is not considered a highly rated thermal insulator.
The color of a granite is governed largely by that of the feldspar, usually the most abundant mineral. Lighter-colored granites are the average composition of continental crust, while darker granites are more likely associated with or influenced by nearby oceanic composition stone. Dark, granular igneous rocks, classified petrographically as anorthosite, basalt, diabase, diorite, and gabbro, are also used as dimension stone, and are classed commercially as “black granite.” Under ordinary atmospheric conditions, granites will endure for years without any change in color or durability.
Bacteria requires several things in order to thrive and grow: oxygen, water, sunlight, nutrients, and a substrate to form on. The minerals in granite are toxic to bacteria. As a result, there is no habitable environment for the bacteria to live and grow on a granite surface.
Radon is a naturally occurring gas generated by the decay of trace amounts of uranium found in the Earth’s crust. It is an unstable gas that quickly breaks down and dissipates in the air. Radon is measured in units called picocuriesperliter (pCi/l). A picocurieisone trillionth (10-12) of a curie, which is the amount of radioactivity emitted by a gram of radium. The U.S. Environmental Protection Agency has established 4 pCi/l as the standard for indoor air; 20 pCi/l represents the maximum amount of exposure to radium that is now allowed by U.S. regulations. Over 500 measurements of radon emissions from granite have been published in the peer reviewed scientific literature. This study and other radon and radiation information and test results may be found at the Natural Stone Institute. None of the research found a single stone that would be a health risk to homeowners.