Granite is a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz; it also contains minor amounts of mica and amphibole minerals see the accompanying chart titled Generalized Composition Ranges of Common Igneous Rocks.
Once students know how to identify the minerals in granite, this simple description enables them to identify the rock based upon a visual inspection. During that visual inspection, students should use a hand lens to confirm that the minerals of granite are present in the rock.
That inspection would involve confirming that each of the minerals expected in granite is physically present in the rock - and present in the proper proportion. Feldspar minerals are abundant in granite. They are usually white, gray, pink or reddish in color. Many grains will exhibit two directions of cleavage that intersect at right angles. You should be able to observe this cleavage pattern in granite with a hand lens.
Quartz will usually be a transparent mineral that is colorless or gray in color. Many grains will exhibit a conchoidal fracture - with a vitreous luster on the conchoidal fracture surfaces. The mica minerals expected in granites include muscovite or biotite. Micas occur in very thin sheets. They will often be in "books" of numerous sheets stacked upon one another. The surfaces of these sheets will have a highly reflective vitreous luster. 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. So, the name "granite" is a name used for igneous rocks that are composed of orthoclase feldspar, quartz, plagioclase feldspar, micas, and amphiboles that are present in crystals large enough to be visible with the unaided eye.
Special names are used for granitic rocks based upon their grain crystal size. If a granitic rock has especially large grains over one centimeter across , it is often called "pegmatite. Granitic rocks that have a mineral composition that borders upon diorite might be called "granodiorite. The accompanying triangular diagram displays a classification method used for granitic rocks based upon the relative abundances of quartz, alkali feldspars, and plagioclase feldspars.
This is not a chart for use by the beginning student of igneous rocks. It is a classification used by experts who have the skills and equipment needed to quantify the mineral composition of the rock. Granite Countertops: Granite countertops in a new kitchen. Granite is the rock most often quarried as a "dimension stone" a natural rock material that will be cut into blocks or slabs of specific length, width, and thickness.
Granite is hard enough to resist abrasion, strong enough to bear significant weight, inert enough to resist weathering, and it accepts a brilliant polish. These characteristics make it a very desirable and useful dimension stone. Most of the granite dimension stone produced in the United States comes from high-quality deposits in five states: Massachusetts, Georgia, New Hampshire, South Dakota, and Idaho.
Granite has been used for thousands of years in both interior and exterior applications. Rough-cut and polished granite is used in buildings, bridges, paving, monuments, and many other exterior projects. Indoors, polished granite slabs and tiles are used as countertops, floor tiles, stair treads, and many other practical and decorative features. High price often reduces the popularity of a construction material. Granite often costs significantly more than man-made materials.
However, granite is frequently selected because it is a prestige material, used in projects to produce impressions of elegance, durability, and lasting quality. Granite is also used as a crushed stone or aggregate. Crushed limestone is by far the most commonly used crushed rock in the U. Crushed granite is used in road construction and railroad beds. Larger pieces of granite are used to stabilize the land around roadways to minimize and even eliminate soil erosion. Please see either crushed stone or dimension stone for information about granite mining.
There is an enormous abundance of granite throughout the United States, so it is not a surprise that a significant amount of granite is used in crushed stone applications. Granite is used extensively as dimension stone. It is used in the construction of buildings, both as building blocks and as veneers on frame structures. Because it can be smoothed to a very high polish, granite has found extensive use in memorials, headstones, monuments, carved decorations on buildings, statues and the like.
Approximately 1. Skip to content Return to Minerals Database. Type Rock. Related topics: Mica. Volcano Coloring Page.
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