Granite: Its physical, geochemical general properties

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Alister Cruickshanks (UK)

Granite rocks are igneous rocks that were formed by slowly cooling pockets of magma that were trapped beneath the earth’s surface. Granite is used all over the world in the construction industry due to its unique properties and versatile range of colours and textures. One common characteristic of granite is it has a visible crystalline texture formed of quartz and orthoclase or microcline. Quartz makes up 10% to 60% of the composition, with feldspar accounting for between 65% and 90% and biotite between 10% and 15%. As a result, its hardness makes it not only ideal for building construction, but also for sea and river defences.

Fig. 1. Adamellite granite.

As a building material, granite is often used for cladding walls, and for roofing and flooring, with a range of other interior and exterior applications. In this respect, one of the biggest advantages it has is its hardness. In fact, granite is actually the hardest building stone available and leads to excellent wear. On the Mohs hardness scale, granite is between 5 and 8, with an abrasive hardness of 37 to 88. Its density can range between 2.54 to 2.66g/cc.

Fig. 2. A beautiful grey granite.

It is also impermeable with a porosity ranging from 1e-009 and 1e-006. Another important property is it is highly thermally stable, showing no alteration with changes in temperature, which provides excellent fire protection. It is also often used in the construction of tanks for storing caustic material simply because it is highly resistant to chemical erosion.

Fig. 3. A granite displaying pink feldspar.

Granites are classified in two groups according to their magmatic origin. These are:

  • S-types, where granites form from the partial melting of meta-sedimentary source rocks.
  • I-types where granites have derived from source rocks of igneous composition which have not gone through the surface weathering process or from crystal fractionation of magmas.

However, determining the origin is often extremely difficult, but many various tests and analyses have been developed over the years. One of the first tests Chappell and White (1974) discovered was that S-type granites are compositionally restricted whereas I-types are compositionally expanded.

Fig. 4. Graphic granite, a leucocratic granitic rock consisting of alkali feldspar with exsolved quartz typically forming a distinctive repetitive pattern sometimes resembling cuneiform writing, hence its name.

These differences were discovered while studying granites from the Tasman Orogenic Zone of Eastern Australia. They conclude that the two types have marked differences in geochemical parameters. Their research was based on evidence that I-types have relatively high sodium greater than 3.2% for felsic varieties and more than 2.2% in mafic types compared with low sodium levels of less than 3.2% in S-type felsic varieties and less than 2.2% in S-type mafic varieties. Chappell and White also used a range of other tests, including the Aol A1203 ratio and normative corundum to determine the geochemical differences in the two types.

Fig. 5. Biotite granite, although it actually contains both muscovite and biotite.

More importantly Hine et al.(1978) discovered the main differences between the origins of granites can be determined by Petrographic methods to access the differences in the geochemical aspects reflected in the mineralogy. The most common difference is that hornblende is common in the mafic I-types and is often present in the felsic I-types, but is never found in the felsic S-types. Instead, in these types, muscovite is common while, in S-type mafic varieties, biotite is very abundant.

Fig. 6. Rapakivi granite from Finland, a hornblende-biotite granite containing large round crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase).

Only by combining the Petrographic methods with the Geochemical methods of Chappel and White can a true conclusion be given to the magmatic origin of a particular granite specimen.

Further reading

Rocks and minerals: The definitive visual guide, by Ronald Louis Bonewitz, Dorling Kindersley (2008), 356 pages (hardback), ISBN: 978-14-05328-31-9

References

Chappell, B.J. and White, A.J.R., 1974, “Two Contrasting Granite Types”. Pac. Geol., v8, pp.173-174.

Hine, R., Williams, I.S., Chappell, B.W. and White, A.J.R., 1978, “Contrasts Between I- and S-Type granitoids of the Kosciusco Batholith”, J. Geol. Soc. Aust., V25, pp.219-234.

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