Granites usually have a coarse texture individual minerals are visible without magnification , because the magma cools slowly underground, allowing larger crystal growth. Granites are most easily characterized as light colored and coarse grained as a result of cooling slowly below the surface. Color variation is a response to the percent of each mineral found in the sample. The crystals in granite provide a variety of mixed colors — feldspar pink or red , mica dark brown or black , quartz clear pink, white, or black and amphibole black.
It is widely used for architectural facades, construction materials, ornamental stone and monuments. Crushed granite is used as a durable construction material in asphalt and concrete used in highway and infrastructure projects.
No significant quantities of granite are quarried in US for the purpose of granite countertops. The inexpensive blast extraction method involves drilling a hole into rock surface and blasting the area to extract blocks of granite. Air bag extraction uses air instead of an explosion to loosen granite boulders.
Stone-cut extraction requires cutting out granite from the earth using precision machinery. Stone-cut extraction is the most expensive, but the most accurate extraction method. It is obvious that the natural stone fabricator needs to be well versed with various rock types, so that the home owners may be advised about the right selection of stone for their projects.
Reputed stone fabricators tend to become members of the Marble Institute of America to get a thorough education on different natural stone materials. Our stone professionals will be able to advise you about the right choice of stone for your project. Granite Mineral Proportions A large portion of granite consists of feldspar. Schist stone They are not igneous rocks like granite, and are actually metamorphic rocks.
Gneiss Stone A good part of the stones used as countertop materials in the United States are commonly called granite slabs , but are actually Gneiss rocks. Cooling The slow cooling period is instrumental in the formation of granite. The edges of a "stack of sheets" will look similar to the edge of a stack of playing cards. Amphibole minerals such as hornblende are dark in color and will often have a prismatic habit. The best way to learn about rocks is to have specimens available for testing and examination.
Granite is a plutonic rock in which quartz makes up between 10 and 50 percent of the felsic components. Alkali feldspar accounts for 65 to 90 percent of the total feldspar content. Applying this definition requires the mineral identification and quantification abilities of a competent geologist. This type of analysis cannot be done precisely by a student in a classroom or a geologist in the field.
This is an example of the complexities that can be involved in assigning a formal name to an igneous rock. Many rocks identified as "granite" using the introductory course definition will not be called "granite" by the petrologist. They might instead be alkali granites, granodiorites, pegmatites , or aplites. These names are for specific types of granite. These names require a consideration of the grain size and the mineral composition of the rock - beyond determining that the rock is a granite.
A petrologist might call these "granitoid rocks" rather than granites. There are many types of granite based upon mineral composition and texture. The accompanying chart Generalized Composition Ranges of Common Igneous Rocks illustrates the range of granite compositions. From the chart you can see that orthoclase feldspar , quartz , plagioclase feldspar , micas, and amphiboles can each have a range of abundances.
Using the terminology of geologists, they would be clockwise from top left : granite , gneiss , pegmatite , and labradorite. Click on any of their names above for an enlarged view. Each of the images above represents a slab of polished rock about eight inches across. Use of the word "granite" in the dimension stone and crushed stone industries is different from how the word is used by geologists. In these industries, the name "granite" refers to an igneous rock that meets the following criteria:.
Using these criteria, gabbro , basalt , pegmatite , schist , gneiss , diabase , diorite , and many other igneous rocks will be called "granite. These "granites" are used to make crushed stone that is used for highway construction, concrete, building construction, fill, railroad ballast, and many other purposes. They are used in the dimension stone industry to make countertops, floor tiles, curbing, building veneer, monuments, paving stones, and many other products.
These granites might be used with sawn, sheared, or polished surfaces. Pegmatite: Photograph of a granite with very large crystals of orthoclase feldspar. Granites composed mainly of crystals over one centimeter in diameter are known as "pegmatites. Granitic rocks: This triangular diagram is a classification method for granitic rocks.
It is based upon the relative abundance of feldspars K-Na-Ca and quartz. Mafic elements are not considered. It is modified after a classification chart prepared by the International Union of Geological Sciences. Image and modification by the United States Geological Survey. 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.
Both of these types of granite are formed by melting of high grade metamorphic rocks, either other granite or intrusive mafic rocks, or buried sediment, respectively. M-type or mantle derived granite was proposed later, to cover those granites which were clearly sourced from crystallised mafic magmas, generally sourced from the mantle. These are rare, because it is difficult to turn basalt into granite via fractional crystallisation. A-type or anorogenic granites are formed above volcanic "hot spot" activity and have peculiar mineralogy and geochemistry.
These granites are formed by melting of the lower crust under conditions that are usually extremely dry. The rhyolites of the Yellowstone caldera are examples of volcanic equivalents of A-type granite. Granite is a normal, geological source of radiation in the natural environment. Granite contains 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 formed over or from particularly uraniferous granites can become a trap for radon gas, which is heavier than air.
However, in the majority of cases, although granite is a significant source of natural radiation as compared to other rocks it is not often an acute health threat or significant risk factor. Various resources from national geological survey organisations 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.
The Red Pyramid of Egypt c. Menkaure's Pyramid, likely dating to the same era, was constructed of limestone and granite blocks. The Great Pyramid of Giza c. Other uses in Ancient Egypt, include columns, door lintels, sills, jambs, and wall and floor veneer. How the Egyptians worked the solid granite is still a matter of debate. Patrick Hunt has postulated that the Egyptians used emery shown to have higher hardness on the Mohs scale.
Many large Hindu temples in southern India, particularly those built by the 11th century king Rajaraja Chola I, were made of granite. There is a large amount of granite in these structures.
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