What is granite, what makes it so versatile and how can we choose a good-quality stone?
Article: Jim Mann
Take a moment to look around and you are most likely to find granite. Inside you may find it on the floor, walls, vanity or bench top; outside it may be used as paving, external cladding or statuary. Granite is considered an enduring stone not only because of its hard-wearing and durable nature, but also because it’s always in fashion.
The word granite comes from the Latin granum, meaning “a grain” referring to the coarse-grained texture of this rock type. In geological terms, granite is a particular type of igneous rock formed from magma emplaced kilometres deep within the earths crust. The basic mineralogy of ‘true’ granite includes quartz, feldspar and mica but it may also contain traces of other minerals including pyrite.
In the commercial stone industry the term ‘granite’ is used more broadly to simplify a wide range of igneous rock types on the market. Although grouping different igneous intrusive rock types under one generic term may be convenient it can restrict the understanding of the variability in texture, properties and mineralogy of the various stone types on the market. As an example, the popular group of dimension stones marketed as ‘black granite’ include igneous rock types such as Gabbro and Norite which do not contain all of the basic minerals found in true granite. Another variation is granite-gneiss with the term indicating a variation in texture rather than mineral composition. Granite-gneiss is characterized by banding caused by segregation of different types of various minerals present within the rock producing light and dark veins or bands which produces an elongated structure which can cause the stone to split along the veins.
Granite is available in a wide range of colours including white, green, red, brown and even blue. The appearance is usually determined by the colour of the feldspar, the presence of alteration minerals and the type and distribution of the mica present. Yellowish granites usually gain their colour from a fine dusting of iron oxide present along grain boundaries. The concentration of minerals within the stone can present as dark, wispy (from biotite mica) or broad, light coloured veins (from quartz or feldspar).
Granite can be produced in a range of finishes such as polished, honed, sawn, flamed and wire-brushed making it suitable for a wide range of locations from hotel lobbies to pool surrounds. Granite can be brought to a high polish although the final appearance is affected by the mineralogy and grain size.
Occasionally, the polishing of coarse grain granite results in a crazed surface finish due to the highlighting of the grain boundaries. The development of fine pin-point pits can also be a problem with some types of granite as softer or flaky minerals such as mica are plucked from the surface during the polishing stage. Although these features may not affect the structural integrity of the stone they may detract from the appearance. When selecting granite with a polished finish it is good practice to view the slab at different light angles to ensure you are happy with the appearance.
Granite is generally considered the strongest and most durable commercial stone type. Most of the minerals within granite are hard giving the stone a high abrasion resistance. Granite is also highly acid resistant making it a great choice for kitchen bench tops, toilet floors and throughout entertaining areas. As granite can be prepared with a wide range of surface finishes it is also a highly versatile paving material especially for areas that require a very high slip resistance. The high strength of granite allows it to be prepared in large format for use as cladding as well as tiles with a thickness of less than 10mm. The use of granite for statuary and funerary works through the millennia is well documented and is a testament to its durability and popularity.
Granite is generally considered dimensionally stable and is therefore a good choice in wet-dry environments such as around a pool. Thermal expansion may be considered an issue for some granite installations especially when large format pavers are used and if the stone has a dark colour which tends to absorb more heat. In this situation it is important to install sufficient expansion joints to prevent pressure build up between tiles.
Unlike sedimentary rocks, igneous rocks such as granite do not usually lose significant strength when wet. It is not uncommon for very fine-grained igneous rock types such as ‘black granite’ to actually increase in strength slightly when they become wet. When granite does exhibit a noticeable strength loss when wet it is likely that the stone has undergone some weathering which leads to the development of clay and other unstable minerals along grain boundaries. It is important that both dried and soaked strength is taken into account when determining load bearing requirements.
Granite has a very high abrasion resistance compared to most other types of dimension stone making it a good choice for high traffic areas. Due to the high abrasion resistance textured finishes that have been applied to improve slip resistance are less likely to become polished over time. Although granite is resistant to abrasion the use of highly polished ‘black granite’ in high traffic or bottle neck areas may lead to the development of track marks which may require periodic repolishing to maintain a uniform finish.
Thermal finishes known as flaming or exfoliation are produced by shattering the mineral grains exposed at the surface. This surface treatment can extend into the stone to some depth which may vary the strength of the stone. Flaming of thin, large format tiles can lead to the development of shrinkage cracks as the unit cools during quenching.
Although granite is generally considered to have good resistance to staining, some mid-tone colours (e.g. light grey) tend to highlight oil and water stains by becoming darker. This effect is less evident on light coloured or very dark coloured stone and it can be reduced by the application of a good quality impregnating sealer.
Testing and Specification of Granite
Standard specification ASTM C615 provides a guide to the selection of granite dimension stone suitable for general building and structural purposes. The physical requirements for this specification are given in Table 1.
|Bulk Specific Gravity – min (kg.m-3)||2560|
|Water Absorption – max (% by weight)||0.4|
|Modulus of Rupture – min (MPa)||10.34|
|Flexural Strength – min (MPa)||8.3|
|Compressive Strength – min (MPa)||131|
|Abrasion Resistance – min (Ha)||25|
The specification of any dimension stone should be based on location, design and engineering considerations specific to the intended use. The specification states a minimum strength requirement which requires the determination of both dried and soaked strength. It is worth noting that the strength results may vary slightly with change of orientation due to the ‘onion peel’ cooling characteristic of igneous rocks.
Water absorption and flexural strength are the key performance indicators for this stone and should be evaluated closely throughout the project supply phase to ensure adequate performance in service.
Granite is a strong, durable and wear resistant stone type available in a wide range of colours and textures and suitable for nearly all locations. The following tips will assist you in the selection of the right stone for the job and in its maintenance well into the future:
– Ensure your selection is made on a representative sample that shows all likely variations in colour, shade, veining and grain size.
– Ensure strength tests are carried out on the proposed surface finish in both a dry and wet condition.
– View polished finishes in reflected light to ensure you are happy with the finish.
– Determine whether the stone needs to be sealed by performing a stain resistance test and by evaluating the change in appearance when wet.
– Ensure the slip resistance of the surface finish is appropriate for the location and an appropriate cleaning regime is specified.