Granite Rock Formation, Composition, and Its Uses.
Granite is a light-colored, coarse-grained type of intrusive igneous rock that is composed mainly of quartz and feldspars with small-scale amounts of mica and amphibole minerals. Petrologists define granite rock as a type of rock in which quartz make up 10 to 50 percent of the folic components and 65 to 90 percent of the total feldspar content being an alkali feldspar. Granite rock is hard and tough, with a prominent lack of internal structures, thus, making it suitable for use as a construction stone.
Mineralogy.
Granite classification is done according to the presence and percentage of minerals such as quartz, alkali feldspar and plagioclase feldspar. True granite contains both the plagioclase and alkali feldspar as stated by the modern petrologic convention.
A granite rock with little or lacking plagioclase content is often referred to as alkali feldspar while a granitoid with less than 10% orthoclase content is called tonalite. Fig 1 below illustrates the mineralogical content of granite and its classification.
Figure 1 Granite mineralogical classification based on mineralogical content.
Geochemical formation and origin of Granite rock.
Granitoids are major components found almost everywhere in the earth crust. They crystalize from a near eutectic point magmas. These magmas will evolve to a near eutectic point because they represent low degrees of partial melting. Crystallization process reduces a melt in calcium, magnesium, iron, sodium, and titanium as well as enriching it with potassium, silicon and quartz (SiO2). Potassium and Silicon enrichment give rise to alkali feldspar as shown by Johannes, Wilhelm and François in Petrogenesis and experimental petrology of granitic rocks.[1]
The crystallization process takes place irrespective of the origin of the parental magma. However, the geochemical and mineral composition of the granite rock will exhibit some compositional elements of the original parent rock magma. In essence, the final composition of granite, as well as its physical properties are closely tied and distinct to its origin. The formation of large crystals, which is a common characteristic of granite rocks, is attributed to the slow cooling process.
Granitization.
This is an old theory, which states that granite is formed in a given location by extreme metasomatism. Metasomatism is the chemical alteration of metamorphic rocks by fluids creating greisen, skarns and may affect hornfels, which is found in the contact metamorphic aureole. Granite production through the process of metamorphic heat is always difficult to achieve, but, in some cases, it occurs in specific amphibolite and granulite terrains. Melting by metamorphism is difficult to recognize since a melted metamorphic rock is referred to as magma, and that the melting process requires high temperature and water to act as a catalyst in lowering the temperature of the rock, hence facilitating cooling and formation of granitic rock.
Granite ascent and emplacement.
The ascent and emplacement of granite on the upper continental crust is a phenomenon, which has not been properly defined by geologists. Two hypotheses stoke diaper, and fracture propagation is put forward to explain the phenomena. Stoke diaper basic idea states that the rising of magma as a single mass through the continental crust is attributed to buoyancy. Its rise transfers immense heat to wall rocks causing them to flow around the pluton giving it the freedom to pass rapidly with little heat loss.
Fracture propagation hypothesis states that magma rise occurs through small channels along dykes that are formed because of fault system or pre-existing fracture networks in the earth crust. The first magma solidifies as a result of the opening of the narrow conduits. This solidified magma then provides insulation to the later magma. In essence, granitic magma either makes room for itself or are intruded into other rocks to form an intrusion.
The emplacement of large batholiths is explained to occur through one of the following mechanism; stoping, assimilation, and inflation. Stoping is the mechanism where the granite cracks the wall rocks removes blocks of the overlying crust as it pushes its way up. In assimilation, the granite melts remove the overlying materials on its way up while inflation occurs when granite body is inflated under pressure causing it to be injected into position.
Radiation in Granitic rocks.
Granite rock is a natural source of radiation. Some of these rocks are known to emit a lot of radiation and are thus not safe to be used in the human environment. A common component of a granite rock known as alkali feldspar has potassium-40 radioactive isotope with a weak emission. However, the emission is found to be low when measured in Geiger counter. Uranium is also found in many granitic rocks, though in small quantities ranging from 10-20 parts per million. Thorium radioactive isotope has been found in Conway granite, making granite a natural radioactive source.
A study was done by the US. National Health and Engineering Inc to determine the level of radioactive emission by granite countertops. In this study, 39 full-size granite slabs were measured and shown to have low emission level, which was below the set safety standards. The study concluded that granite is safe for use in the human world.
Granite types.
Some of the typical granite types include the feldspar-phyric micro granite and rhyolite. Feldspar-phyric micro granite is light colored but with individual crystals in the groundmass averaging 2 mm in diameter. Its mineral composition is dominated by quartz, alkali feldspar and biotite. Rhyolite has individual crystals in the groundmass, which are too fine in nature, and cannot be distinguished by unaided eye. It has dark grey coloration when still fresh but changes color to purple, grey or pink when altered by external conditions as illustrated in Granite and other igneous rocks by Allen and Nancy.[2]
Weathering.
Physical weathering of granite rocks occurs in the form of exfoliation joints. This form of weathering occurs due to the expansion of the granite rock and the subsequent fracturing as pressure is relieved through the removal of overlying material by erosion and other external processes. Chemical weathering occurs when dilute carbonic acid present in rainwater alter the feldspar through the process of hydrolysis.
Common uses of Granite.
In the ancient times, granitic rocks were used in building structures. This is evidently shown by the red pyramid of Egypt, constructed in 26BC and is known to have been constructed of granite and limestone blocks. Chola dynasty in South India also used granite to build a temple in the year 11 century AD.
In the modern times, granite is used in making sculptures and memorials since it is hard and is capable of withstanding hammering forces without any breakages or fracture being experienced. It is also used as a dimensional stone and as flooring tiles in home apartments and public places. Other building applications of granite include cladding (interior/exterior), moulding, paving and landscaping among many others. Granite offers a wide variety of finishing to buildings that are classified as either textured finishing or smooth finishing. Buildings constructed of granite are highly durable.
Bibliography
Nancy Kelly, Allen. 2009. Granite and other igneous rocks. New York, NY: Rosen Pub. Group.
Wilhelm, Johannes and François Holtz. 1996. Petrogenesis and experimental petrology of granitic rocks. Berlin: Springer. http://books.google.com/books?id=C7UPAQAAMAAJ.
[1] Johannes Wilhelm and François Holtz. 1996. Petrogenesis and experimental petrology of granitic rocks. Berlin: Springer. http://books.google.com/books?id=C7UPAQAAMAAJ.
[2] Allen Nancy Kelly. 2009. Granite and other igneous rocks. New York, NY: Rosen Pub. Group.