Granite Reading Answers

Granite is a coarse grained igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the Earth's continental crust, where it is found in various kinds of igneous intrusions. These range in size and are found in watercourses only a few inches across to batholiths exposed over hundreds of square kilometres. Granite is typical of a larger family of granitic rocks that are composed mostly of coarse-grained quartz and feldspars in varying proportions.

These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though a few known as leucogranites contain almost no dark minerals.

Granite is nearly always massive, tough with beautiful fascia. These properties have made granite a widespread construction stone throughout human history. The word "granite" comes from the Latin granum, a grain, in reference to the coarse grained structure of such a completely crystalline rock. Granitic rocks mainly consist of

feldspar, quartz, mica, and amphibole minerals, which form an interlocking, somewhat equigranularity matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter colour minerals. 

Occasionally some individual crystals (phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a general, descriptive field term for lighter-coloured, coarse grained igneous rocks. Petrographic examination is required for identification of specific types of items can be predominantly white, pink, or grey in colour, depending on their granitoids. Granneralogy. The alkali feldspar in granites is typically orthoclase or microcline and is often perthitic. The plagioclase is typically sodium-rich oligoclase. Phenocrysts are usually alkali feldspar. Granitic rocks are classified according to the QAPF diagram for coarse grained plutonic rocks and are 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 between 20% and 60% quartz by volume, with 35% to 90% of the total feldspar consisting of alkali feldspar. Granitic rocks poorer in quartz are classified as syenites or monzonites, while granitic rocks subjugated by plagioclase are classified as granodiorites or tonalites. Granitic rocks with over 90% alkali feldspar are classified as alkali feldspar granites. Granitic rock with more than 60% quartz, which is uncommon, is classified simply as quartz-rich granitoid or, if composed almost entirely of quartz, as quartzolite. True granites are further classified by the percentage of their total feldspar that is alkali feldspar. Granites whose feldspar is 65% to 90% alkali feldspar are syenogranite, while the feldspar in monzogranite is 35% to 65% alkali

Telespar. 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, as described below.