Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar on the A-Q-P half of the diagram. True granite according to modern petrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase the rock is referred to as alkali granite. When a granitoid contains <10% orthoclase it is called tonalite; pyroxene and amphibole are common in tonalite. A granite containing both muscovite and biotite micas is called a binary or two-mica granite. Two-mica granites are typically high in potassium and low in plagioclase, and are usually S-type granites or A-type granites. The volcanic equivalent of plutonic granite is rhyolite. Granite has poor primary permeability but strong secondary permeability.
Granite is a natural source of radiation, like most natural stones. However, some granites have been reported to have higher radioactivity thereby raising some concerns about their safety.
Some granites contain around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite, gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts. Many large granite plutons are the sources for palaeochannel-hosted or roll front uranium ore deposits, where the uranium washes into the sediments from the granite uplands and associated, often highly radioactive, pegmatites. Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite may be susceptible to higher doses of radiation than other communities. Cellars and basements sunk into soils over granite can become a trap for radon gas, which is formed by the decay of uranium. Radon can also be introduced into houses by wells drilled into granite. Radon gas poses significant health concerns, and is the #2 cause of lung cancer in the US behind smoking.
There is some concern that materials sold as granite countertops or as building material may be hazardous to health. One expert, Dr. Dan Steck of St. Johns University, has stated that approximately 5% of all granites will be of concern, with the caveat that only a tiny percentage of the tens of thousands of granite slabs have been actually tested. Various resources from national geological survey organizations are accessible online to assist in assessing the risk factors in granite country and design rules relating, in particular, to preventing accumulation of radon gas in enclosed basements and dwellings.
A study of granite countertops was done (initiated and paid for by the Marble Institute of America) in November 2008 by National Health and Engineering Inc of USA, and found that all of the 39 full size granite slabs that were measured for the study showed radiation levels well below the European Union safety standards (section 220.127.116.11 of the National Health and Engineering study) and radon emission levels well below the average outdoor radon concentrations in the US.
Other researchers and organizations do not agree with the Marble Institute's stated position on granite safety, including AARST (American Association of Radon Scientists and Technicians) and the CRCPD (Conference of Radiation Control Program Directors, an organization of state radiation protection officials). Both organizations have committees currently setting maximum allowed levels of radiation/radon as well as protocols for measuring radiation/radon from granite countertops. The European Union regulations will likely serve as the basis for new EPA based regulations for granite building materials in the U.S.
For more info, please visit
Taken by: Wolf
This work is licensed under a Creative Commons Attribution-NoDerivs 3.0 Unported License.