Granite is possibly the most common igneous rock type known to the general public. Granite, which is named for its “granular” or phaneritic texture, has crystals that tend to be easily seen, although they are generally small. It is a rock that has been used for centuries for many different purposes such as building material. Granite was used with limestone as a building material for the pyramids of Egypt. Its durability, beauty and abundance make it a preferred choice of stone over most others. Granite is also a source of many mineral specimens. Unfortunately, most of the crystals in a granite form anhedral crystals or crystals that lack their outward crystal shape. This is due to the way that the crystals grow into each other to form interlocking crystal frameworks. Although this gives granite its great durability, it limits its desirability as a source of mineral specimens. Occasionally there are pockets within a granite where crystals can form very nice specimens.
The crystals of granite form while the molten material inside the Earth’s crust cools relatively slowly . Molten rock or magma that would have formed granite had it stayed in the Earth’s crust, but instead managed to erupt onto the surface of the Earth, forms a rock called rhyolite. The two rock types have the same chemistry. Rhyolite however does not generally have the same texture and crystals are generally too small to see. If granite type rock has crystals that grow larger than a large pebble (roughly 3 cm or about 1 inch across) then it is called a pegmatite.
The minerals that are found in granite are primarily quartz, plagioclase feldspars, potassium or K-feldspars, hornblende and micas. Quartz is usually the last mineral to crystallize and fills in the extra space between the other minerals. Quartz’s hardness, lack of chemical reactivity and near lack of cleavage give granite a significant amount of its desirable durable properties. The quartz will appear gray, but is actually colorless and is reflecting and fusing the colors of the white and black minerals surrounding it. The plagioclase feldspars are generally white with a porcelaneous luster. The K-feldspars are generally the ones that give granite its color variations from yellow to orange to pink or blue. Dark K-feldspars can give granite its black varieties as well. The micas are generally muscovite (silver), biotite (black or brown) or lepidolite (violet or pink) and provide the sparkle that some granites possess. The hornblende and biotite provide granite with the black pepper portion of the famous and distinctive “salt and pepper” look to classic granite.
Some accessory minerals include gemstones such as tourmaline, beryl, topaz, zircons and apatite. These minerals are generally scattered in the groundmass and generally do not affect the overall appearance of the stone. Other accessory minerals are important economically such as phosphates and rare earth oxides. Related to the rare earth elements is a significant concentration in granite of the element uranium. Granite is actually rather radioactive and has 5 to 20 times the concentration of uranium compared to other common rock types. Some health concern exists in areas that are rich in granitic terrain, as background radiation is enhanced by the presence of large granite bodies. Although the uranium is generally not concentrated enough to make granite a uranium ore, the leaching and erosion of granite has helped produce most of the uranium ore deposits around the world.
Granite is a very general term and is applied to a whole host of different rocks. Many different granites have been identified based on their varied chemical and mineralogical compositions. Generally the term granite is used as a suffix to indicate its textural and general composition. If a granite is rich in lithium then the rock might be referred to as lithium granite. Also, petrologists may choose to classify different but similar granitic rock types by using the terms such as granodiorite or syeno-granite. Some granites are named according to their unusual characteristics. In some granites the feldspars had time to form rectangular crystals before quartz and the other minerals crystallized. The resulting rock appears to be a collection of unintelligible letters, numbers or figures and the rock is called “Graphic Granite”.
There are hundreds if not thousands of granites that are given local or marketable names. Some of these name are descriptive such as indicating an unusual color or indicate the source of the rock such as Pikes Peak Granite. Many of these granites are distinctive and many rocksmiths can easily distinguish where a sample came from.
Some rocks are incorrectly called granites. Several course grained metamorphic rocks are incorrectly referred to as granites. Generally granite does not exhibit signs of metamorphism such as a layered pattern, metamorphic minerals present such as garnet or crystal orientation. Rocks showing these characteristics are probably gneisses or schists, some of which can be very attractive and durable like a granite. But really, who wants a schistic countertop? Since the general public trusts a term like granite, the name granite is often applied to these stones.
Many other igneous rocks are also incorrectly referred to as granite. These rocks have granite’s “salt and pepper” look, but have different mineral assemblages that preclude them from being classified as a granite. Syenites, diorites, gabbros, monzonites and anorthosites have little or no quartz. If the rock has little or no K-feldspar but generally equal amounts of plagioclase feldspar and quartz, then it is a tonolite. A very rich quartz rock (>90% quartz) of igneous intrusive origin is called a quartzolite, but will have a very different look from granite. Rocks that have so little silica content that they can not produce decent feldspar let alone quartz are classified as feldspathoid rocks and they also can look like granite. Quartz is always a must and identification of quartz in the rock is key. Basically, if there are randomly scattered crystals of nearly equal amounts of quartz and plagioclase feldspar with some K-feldspar and hornblende or mica crystals that are melted into each other (an intrusive igneous rock), then it is granite.
As an igneous rock, granite forms from melted or molten rock called magma. As an intrusive rock, granite forms from molten rock that never reaches the surface of the Earth. Granite forms from the melting of lighter materials than is found in the deep crust or mantle. Where did this magma come from? There are scientists that disagree on this subject. In general there is agreement that most granite is derived from the melting of subducted crustal rock (lighter weighted rock) that slipped into the mantle in subduction zones such as those that are found ringing the Pacific Ocean today. If true then granite is a “newer” rock type as it required the plate tectonic process to have proceeded along before the first granites formed. It could have formed from some other process that segregates the lighter aluminum/silica material from more dense magnesium/iron material. It could also have been produced from a process called granitization or the melting of a chemically similar rock from intense metamorphism into a completely melted magma. However most granite crystallization models require water to be in the magma and the intense metamorphism scenario would not allow a lot of water to be present. The great variation and abundance of granite suggest that there could be many various formation models.
Granite is found in all continents around the world and is generally the foundation of many orogenic belts or mountain chains. Most often granite is the underlying rock upon which sedimentary and other continental rocks rest. Granite is found in batholiths or large magma plumes that rose into the continental rocks. But it can be seen in lots of other intrusive features such as dikes, sills and lacoliths.