Quartzite: What is it; protection and maintenance

Quartzite is often described as a type of granite because it has seemingly similar physical characteristics and like granite is made up of predominantly silica. In fact many people sell quartzite as coming from the same family as granite. Unfortunately this is simply not true. Granite is an igneous rock where quartzite is a metamorphic rock. By definition this means that quartzite starts as another rock and then is transformed (or morphed) into quartzite. So exactly what is quartzite what is its origin and how is it formed?

Quartzite is a stone composed almost entirely of quartz grains (or silica) which is derived from sandstone or to a lesser extent chert. (Note: chert is a rock composed mostly of the mineral chalcedony. Chalcedony is the name for quartz with microscopically small crystals and hence is not clear like quartz). Sandstone is transformed into quartzite in two ways. The first is where sandstone is subjected to very high temperatures and pressure deep within the earth’s core. This process is related to tectonic compression within orogenic belts (the process of mountain building). During this process the original quartz (silica) grains and cementing compounds (these can be silicates or calcium based) are dissolved and then recrystallised on cooling forming an interlocking network of quartz crystals. All traces of the original sediments are erased. This type of quartzite is called Metaquartzite.

The second way quartzite is formed involves lower pressure and temperature. Heated silica based fluids circulate between the quartz grains cementing them together. This type of quartzite is called Orthoquartzite and is not strictly speaking a true metamorphic rock because the original grains of quartz are still present and bedding planes and other sedimentary structures are still evident. However orthoquartzite is the most pure form composed of well rounded quartz grains cemented by silica and hence is often 99% pure silica. Both types of quartzite have mineral impurities that create colour. Pure quartzite is usually white or grey however traces of iron, carbonates etc result in other colours such as the browns, pinks and reds.

Quartzite is a very chemically resistant stone as well as being very hard (approximately 7 on the Mohs hardness scale). Due to its chemical composition and the two processes involved in its formation it is also very stable as it comprises almost no other soluble minerals. However the crystal structure does have a very fine network of cracks that occur between crystals and this enables moisture to penetrate in both directions. Hence quartzite is still a porous stone with average water absorption of between 0.1 and 1.4%. Unlike granite the crystal structure of quartzite is on average larger making it very difficult for quartzite to be cut totally smooth or polished. In fact one of the main ways you can usually identify quartzite from granite is by the way quartzite fractures across or through the grains. Cutting granite is more like cutting butter where as quartzite is like cutting bread.

The protection and maintenance of quartzite is very similar to granite in that both have reasonable water absorption making them susceptible to below surface staining. However due to the larger crystal structure of quartzite and the purity of the impervious quartz crystal and silica cement, removing deep stains can be more challenging than granite. In most cases extraction or penetration is by way of the small network of fine cracks that exist between the crystals. The large crystal network also creates rough or textured surfaces that are much more prone to surface soiling. This means more attention has to be paid to routine maintenance than the smoother granite. The texture also impacts on installation where the texture makes it more difficult to remove cement or epoxy based grouts.

When selecting protection and maintenance products for quartzite all of the above characteristics need to be taken into account. The surface of quartzite is still a high energy surface and relatively dense making it difficult for water based chemicals to penetrate deep into the stone. Solvents with a much lower surface tension (15-20 dynes cm3 compared to water at approximately 35 dynescm3) are better equipped to break the high energy of the quartzite surface making them the best type of sealer for quartzite. (Note: current developments in nanotechnology wetting agents will eventually change this situation). Application of the sealer is best done prior to installation to ease grout clean up which otherwise is very difficult due to the rough surface texture. Routine maintenance is best done the same way as granite using a ph neutral cleaner. For any heavy duty cleaning quartzite is at least as chemical resistant as granite and therefore is relatively immune to alkaline and acid solutions. However whenever using any of these types of cleaners a small test area should be done to make sure there is no damage. Like many other commercial stones what is called or sold as quartzite is not always quartzite and hence the need to test treatments before total application. In summary quartzite does share many of the same characteristics as granite. It is a very hard, tough, chemically resistant natural stone. It is made up of predominantly silica (an even higher quantity than granite) has similar water absorption and hence is protected in the same way as granite. However it is not from the granite family. It is a metamorphic rock not igneous and starts out as sandstone (or chert). It is this origin and the resulting larger crystal structure that most differentiates it from granite. It means that it is hard to shape, difficult to polish and most importantly more difficult to maintain. It is in the area of maintenance where granite and quartzite differ the most with quartzite requiring more constant attention. However as long as this is understood quartzite makes for an incredibly long lasting durable (as well as slip resistant) finish especially ideal for commercial and high traffic exterior installations.

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