Marble has been synonymous with prestige and elegance for thousands of years and Australia is now witnessing a resurgence in its use, both inside and out. The subtle colours and delicate figuring allow marble to be used as a minimalist backdrop or as a high-class statement. So what is this timeless stone?
The Relationship Between Marble and limestone
As discussed in an earlier ‘Back to Basics’ article regarding limestone, there is a complex (if not ambiguous) relationship between marble and limestone. Both stone types are composed predominantly of calcite and/or dolomite but their structure and properties are quite different. Limestone is a sedimentary stone type formed by the deposition of small fragments of coral, shells and other marine organisms which undergo de-watering, compaction and cementation through pressure applied by the overlying sediment.
Marble could be called a ’next generation’ rock as it is formed by exposing the sedimentary limestone to high pressure due to deep burial within the earth, applied forces through tectonic movement (e.g. Carrara marble deposit) or high temperatures from contact with molten rock intrusions (e.g. Chillagoe marble). This process is known as metamorphosis and results in the recrystallisation of the minerals present. The recrystallisation also obscures most of the previous features (e.g. shell fragments) and textures (e.g. bedding) producing a product that does not have a noticeable rift or ‘grain’. Crystal size varies from very fine-grained material such as Statuario where the grains can be less than 1mm in size to coarse-grained (>10mm) such as Crushed Ice from northern Queensland.
Compared to its parent rock, marble is less porous enabling it to be polished. In a commercial sense, limestone and marble appear to be differentiated by the level of polish which can be achieved. If the stone can achieve a high polish then it is usually sold as a marble, if not, it is likely to be sold as limestone.
In trying to determine if a stone is a limestone or marble it is important to remember that marble is a metamorphic rock and fossil fragments are converted into calcite crystals which removes any trace of the original structure. The fracture face of a marble tile will also be ‘sugary’ with uniform grain size while a limestone may show remnants of the original sediment.
Marble typically has a white background colour with varying bands, streaks or swirls of other colours depending on the type of trace minerals present. White marble such as Calacatta is composed of nearly 100% calcite while other popular marble types such as Bianco Carrara contain a significant proportion of dolomite which produces the wispy grey bands through the stone. The variable chemistry of dolomite can also lead to the formation of marble with colours ranging from yellow to red to dark brown. Organic impurities can produce marble that is black while the presence of chlorite can impart a green colour.
Brecciated marble types such as Arabescato are striking in appearance and are formed after the rock has been subjected to a second form of metamorphism that has broken up the marble into angular fragments (breccia) and re-cemented together by finer calcite-rich material.
Marble is a highly versatile stone type as it is able to be cut and ground to a high polish making it suitable for processing into cubic form as well as panels for external wall cladding and calibrating into thin tiles for walls and floors. The low hardness and characteristically fine grain size makes the stone suitable for carving into intricate statuary.
Marble is predominantly composed of calcium carbonate which can be etched by acidic substances such as wine, soft drinks and some liquid soap. Pure fine-grained calcite marbles are particularly vulnerable to etching as the acid attack occurs at the grain boundaries. Dolomite is less soluble in acid than calcite therefore dolomitic marble is slightly less prone to etching. If the stone is likely to be exposed to acids the use of a honed or matt surface will make etching less conspicuous. It is important to note that the application of an impregnating sealer will not prevent etching as they do not protect the surface of the stone.
Occasionally it is known to develop irregular pale yellow brown stains due to the deposition of soluble iron from the natural (quarried) moisture in the stone. This discolouration can seriously detract from the appearance of the stone but is difficult to predict. The best way to try and avoid this distressing phenomenon is to ensure the stone comes from a quarry and bench that is known to produce quality stain-free stone.
Marble has a relatively low abrasion resistance which can lead to loss of polish in high traffic areas. Although marble can be prepared in a wide range of surface finishes from polished to grit blasted, the low hardness and fine-grained nature of marble can present some limitations with regards to slip resistance as the calcite grains tend to become rounded instead of presenting a rough textured edge which would otherwise provide mechanical grip.
Most ‘true marble’ is not moisture sensitive and is suitable for use in bathrooms. The stone type marketed as Green Marble contains a mineral known as serpentine and some types are known to be dimensionally unstable and unsuitable for wet areas and need to be installed with specialty adhesives.
The reputation of marble for use as external veneer cladding has been dealt a blow in the past due to the warping of panels on some prominent projects such as the Amoco Building in Chicago. This permanent warping phenomenon is known as thermal hysteresis. Hysteresis is defined as “a lag in the return of an elastically deformed body to its original shape after the load has been removed”. For marble the deformation is primarily due to cyclic heating and cooling from the sun exacerbated by cyclic wetting and drying.
When a marble panel is fixed to a façade the external face is heated and expands relative to the internal face leading to warping. As the outer face cools, the panel should return to its original shape but in some types of marble the dislocated grains become interlocked causing permanent deformation. The cyclic heating and cooling leads to an incremental increase in the dislocation which causes an increase in porosity and subsequent loss of strength which can eventually lead to a catastrophic failure.
It is important to note that not all marble is sensitive to thermal hysteresis. The exact cause of hysteresis is a complex issue and the likelihood of it occurring depends on a broad range of issues. Fine-grained marble with irregular interlocking grains have been found to be more sensitive to the development of hysteresis. Locations with extreme temperature variations in conjunction with regular wetting and drying are considered high risk areas for marble veneer. In general the risk of panels warping can be reduced by increasing the panel thickness (thereby making the panel stiffer) and minimizing the length to width ratio to reduce the apparent warping within the panel.
Testing and Specification of Marble
Standard specification ASTM C503 provides a guide to the selection of marble dimension stone suitable for general building and structural purposes. The standard provides some separate requirements for calcite and dolomitic marble. The physical requirements for this specification are given in Table 1.
|Bulk Specific Gravity – min (kg.m-3)|
|Water Absorption – max (% by weight)||0.20|
|Modulus of Rupture – min (MPa)||6.9|
|Flexural Strength – min (MPa)||6.0|
|Compressive Strength – min (MPa)||5.2|
|Abrasion Resistance – min (Ha)||10|
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.
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.
Marble is a versatile stone type suitable for nearly all locations. Taking note of the following points will assist you in the selection of the right stone for the job and its maintenance well into the future:
– Use water absorption and flexural strength tests as key performance indicators.
– If the stone is to be used as external cladding, evaluate the risk of thermal hysteresis on a project by project basis.
– Some loss of polish and change in slip resistance properties may occur in high traffic areas and needs to be taken into account.
– As marble is acid sensitive it is important to clean up spills immediately to avoid permanent staining and etching.
– If you are using a ‘green marble’ ensure the stone is dimensionally stable and install accordingly.