All Minerals

Quartz

Quartz is the most abundant mineral on earth, it is the main crystalline mineral form of silica (SiO₂). Natural deposits of relatively pure quartz are plentiful and grains of it are found in many other types of rocks and in most silts, soils and clay. White or milky quartz is an opaque white, greasy-looking, sharply angular very hard rock. Pure quartz melts at 1713C, far beyond the range of an average pottery or industrial production kiln.

Quartz is a fascinating study in the importance of considering the mineralogy of materials (as opposed to their chemistry). For example, quartz and fused silica glass have the same chemistry, 100% SiO₂. But they have dramatically different physical properties (e.g. thermal expansion, hardness, specific gravity, melting temperature). To illustrate, consider diamond and graphite: they have the same chemistries. But they have radically different physical presences.

From a theoretical perspective, quartz is a mineral. However, in ceramics, it is also considered a material, often simply called "silica" (however original container bags often label it as "ground quartz"). Huge quantities of quartz powder are used in ceramic bodies and glazes. In bodies the quartz particles most often remain unchanged in the fired matrix, behaving as an embedded aggregate. They act as a "skeleton" in porcelain bodies. They raise the thermal expansion and make it easier to fit glazes. However, in glazes, the quartz particles dissolve in the glass and become a part of its chemical structure (thus the original properties of the quartz are gone, giving way to the new physical presence of the glass). The term "silicate" refers to minerals or fired materials where SiO₂ predominates in the inventory or molecules that make up its structure. Kaolin is a silicate. In kaolin crystals, there are 2 molecules of SiO₂ for every one of Al₂O₃. They combine chemically to form the familiar flat, stop-sign shape.

Quartz is the mineral form of silica powder used in ceramic glazes and clay bodies. It is the mineral that silica "wants to be" when cooled from molten to solid (it can be other things depending on the speed of cooling). Unlike silica glass, the quartz phase of silica is subject to inversion and accompanying volume and form change when fired through 573C. Room temperature quartz is called alpha quartz, beta quartz exists only above 573C.

Quartz sand is often used in bodies as grog for texture and to increase thermal expansion. Quartz of very fine particle size (-400 mesh) will typically enter the feldspathic melt or convert to cristobalite during firing if fluxes are lacking, coarse powdered grades help to 'squeeze' glazes into fit. Intermediate sizes (200-300 mesh) seem to be best however, since their greater surface area exerts more compressive squeeze per unit.

Quartz is also used in agriculture, paving, brick and tile, concrete, cleansers, foundry casting, paint, glass, soaps, fiber glass, electronics, plaster, sandblasting, industrial effluent filtration, drinking water filtration, hazardous waste control.

Related Information

Substituting alumina in a clay body dramatically lowered thermal expansion

These are glazed test bars of two fritted white clay bodies fired at cone 03. The difference: The one on the right contains 13% 200 mesh quartz, the one on the left substitutes that for 13% 200 mesh calcined alumina. Quartz has the highest thermal expansion of any traditional ceramic material. As a result the alumina body does not "squeeze" the glaze (put it under some compression). The result is crazing. There is one other big difference: The silica body has 3% porosity at cone 03, the alumina one has 10%!

The same liner glaze crazes on the porcelain but not the stoneware

The stoneware is made from sedimentary clays mined in a in southwestern Saskatchewan quarry, they have a higher quartz grain content and enough natural feldspar to produce functional density (the practical porosity is about 2.5%). The porcelain on the right is a 45:35:20 kaolin:feldspar:silica blend (there is enough feldspar for about 0.5% porosity). The stoneware has more quartz particles to impose their high thermal expansion because fewer are taken into solution by the feldspar. That means that the body has a whole can put the squeeze on the glaze to prevent it from crazing.

Lazulite Siderite Quartz

Quartz Rock Rose

Rutilated quartz

Cassiterite W Quartz

Cordierite In Quartz

Closeup of in-situ quartz mineral at the MGK quarry site in India

Health warning phrases on a bag of ground silica

Micro photograph taken by an ordinary iPhone is still very useful

These are 40 mesh particles, about 400 microns in size. They are clearly visible as rounded, not angular. The carbon and high iron mineral particles are easy to spot. The rest appear to be quartz (having various levels of purity from amber to crystal clear). The interesting thing about these is where they come from: A very dirty-looking terra cotta clay. Something else interesting: At 50, 80 and 100 mesh the iPhone, even with its computational photography, was also able to capture the particles - and they looked exactly the same. Their presence explained a number of physical properties of the clay that otherwise seemed odd.

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