Granite

Captured at Khos Pich, Bench along the river 

- How does granite form on the Earth? 

- Why does granite have various colours? 

- What happens when granite is weathered?

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.

Pegmatite (Amethyst)

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.

What happens when granite is weathered?

Na-feldspar and K-feldspar undergo hydrolysis to form kaolinite (clay) and Na+ and K+ ions.

The quartz (and muscovite if present) remains as residual minerals due to their high weathering resistance.

Biotite and/or amphibole undergo hydrolysis to form clay, and oxidation to form iron oxides.

The weathered rock fragments become constituents of the soil.

What happens next?

Quartz grains may then be eroded and become sediment. The quartz in granite is sand-sized and therefore forms quartz sand. This quartz sand will ultimately be transported to the sea (as bed load in a river) where it will accumulate to form beaches.

Clays will ultimately be eroded and washed out to sea by rivers. Clay is fine-grained and remains as suspended load in the water column so it may be deposited in quiet water.

Dissolved ions will be transported by rivers to the sea in the dissolved load, and will become part of the salts in the sea.