Orinoco

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 grey, but is actually colourless and is reflecting and fusing the colours of the white and black minerals surrounding it.  The plagioclase feldspars are generally white with a porcelaneous lustre.  The K-feldspars are generally the ones that give granite its colour 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.

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