Research
A collaborative research project on eutectic glazes across geological origins
Kaolin deposits worldwide form primarily through the breakdown of aluminum silicate minerals (such as feldspars and micas) in rocks like granite, rhyolite, and volcanic tuffs. Its geological origins are divided into two main categories: primary (in situ) and secondary (sedimentary/transported).
1. Primary (Residual) Kaolin
Primary kaolins form in situ, meaning the rock is altered into kaolin right where it originally solidified.
Hydrothermal Origin (Hypogene): Formed by hot, acidic fluids and gases rising from deep within the earth's crust, often associated with recent or ancient volcanic activity. These fluids alter the original rock through acid hydrolysis.
Key Locations: Emet Basin, Turkey; Mount Bigadiç area, Turkey; active geothermal zones in Japan; and some deposits in Mexico.
Weathering Origin (Supergene): Formed by the intense, long-term chemical weathering of feldspar-rich rocks near the earth's surface by circulating, slightly acidic groundwater and meteoric water. This process requires hot, humid climates over millions of years.
Key Locations: Cornwall, United Kingdom (developed from altered granites); Galicia, Spain; and extensive deposits in the Czech Republic and Germany.
2. Secondary (Sedimentary) Kaolin
Secondary kaolins are created when primary kaolin or kaolin-rich rocks are eroded, transported by rivers, and deposited in sedimentary basins, river deltas, or coastal environments.
Detrital/Fluvial Origin: Often termed "fluvial," "lacustrine," or "coastal plain" deposits, these clays settle in quiet waters. After deposition, groundwater and organic acids often naturally wash away iron and titanium impurities, leaving behind highly pure, white kaolin.
Key Locations: The Fall Line region of the Southeastern US (e.g., Sandersville, Georgia, known as the "Kaolin Capital of the World"); the Amazon region (Capim River) in Pará, Brazil; and Cretaceous-era deposits in Southwestern Sinai, Egypt.
3. Mixed / Multi-Stage Origins
Many of the largest and highest-quality economic deposits have a mixed origin. This typically occurs in geologically active or ancient environments where a pre-existing sedimentary or hydrothermal deposit undergoes a secondary phase of supergene weathering, diagenesis (post-depositional changes), or microbial alteration.
Chemical Composition and Crystal Structure
The Ideal Formula
The chemical formula of kaolinite, expressed in oxide notation as it is conventional in ceramics, is Al₂O₃·2SiO₂·2H₂O, with a theoretical composition of 39.5% Al₂O₃, 46.5% SiO₂, and 14.0% H₂O. In mineralogical notation, this same formula is written Al₂Si₂O₅(OH)₄. ChemicalBookWikipedia
Crystal Structure
In systematic mineralogy, kaolinite belongs to the phyllo silicates, tratified clay minerals built from networks of tetrahedral and octahedral layers. The kaolinite group consists of di-octahedral 1:1 minerals, each layer comprising one silica [SiO₄] tetrahedral sheet bonded to one aluminium [Al₂(OH)₄] octahedral sheet through shared oxygen atoms. ChemicalBook
The Al₂O₃ and SiO₂ layers bond firmly to each other, but the hydroxyl layer does not. This accounts for the characteristic flat particle morphology and the way kaolinite platelets stack and separate under physical force. It also explains the affinity for water: the attached water both binds particles together and lubricates particle-against-particle movement in the plastic matrix. DigitalFire
Kaolinite particles are comparatively the largest of all clay minerals, which makes them among the least plastic. This is directly observable when working with primary kaolins, the low plasticity that makes them difficult to throw on the wheel is a structural consequence of this large, flat platelet geometry. DigitalFire
Chemical Composition in Practice: Impurities
In reality, no kaolin is chemically pure. A representative oxide analysis from the Handbook of Mineralogy for kaolinite from Niigata, Japan gives: SiO₂ 45.80%, Al₂O₃ 39.55%, Fe₂O₃ 0.57%, FeO 0.18%, MgO 0.14%, CaO 0.41%, K₂O 0.03%, H₂O⁺ 13.92%, total 100.77%.
The Imerys Kaolin Polwhite B we use in Belgium has follow chemical analysis: SiO₂ 51,65%, Al₂O₃ 31.43%, Na2O 0,17%, K₂O 3,36%, Fe₂O₃ 1.01%, MgO 0,22%, CaO 0.07%, H₂O⁺ 11.85%, total 100%. (XRF-test at KU Leuven 2023).
Studies of kaolin from different geological origins show characteristic variation in impurity levels. Comparative oxide analyses across multiple kaolin samples consistently show the same suite of minor oxides: SiO₂, Al₂O₃, TiO₂, Fe₂O₃, CaO, MgO, K₂O, and loss on ignition (L.O.I.). Even chemically similar samples can differ significantly in technological behaviour due to differences in mineralogical composition and particle size distribution for example, a high crystallinity index in kaolinite determines good rheology and low drying shrinkage. University of South Florida
Beyond kaolinite itself, kaolin deposits typically contain impurities such as quartz, feldspar, tourmaline, and zircon. Iron oxides and titanium dioxide (TiO₂, present as anatase or rutile) are particularly significant for ceramicists: iron causes fired color to shift toward buff, cream, or red tones, while TiO₂ influences whiteness and can promote certain phase formations at high temperature. Taylor & Francis
Polymorphs
Kaolinite has three polymorphs, dickite, halloysite, and nacrite, all sharing the same Al₂Si₂O₅(OH)₄ composition but differing in layer stacking sequence. Halloysite is particularly relevant to ceramics: it has a tubular rather than platy morphology (due to an intercalated water layer), which gives it different plasticity and shrinkage behavior from kaolinite proper. Handbook of Mineralogy
Thermal Behavior: From Clay to Ceramic
Kaolinite is classified as a 1:1 dioctahedral phyllosilicate, and its thermal transformations are central to ceramic science. The firing sequence begins with dehydroxylation, which occurs through a three-dimensional diffusion process and is complete above approximately 650°C, yielding an amorphous intermediate called metakaolinite (Al₂Si₂O₅). ScienceDirect
Differential scanning calorimetry (DSC) reveals an endothermic event at 550–700°C from dehydroxylation, followed by an exothermic peak at approximately 980°C marking the onset of structural reorganization toward mullite.
At around 980–992°C, nanometer-scale needle-like primary mullite begins to form alongside a cubic Al-Si spinel phase and an amorphous silica-rich phase. From approximately 1136°C, mullite crystals grow further, and above 1200°C, high-temperature cristobalite (SiO₂) crystallizes from the silica-rich amorphous phase. ScienceDirect
Fired porcelain or stoneware is therefore not a single phase but a composite: mullite crystals embedded in a feldspathic glass matrix, with residual quartz or newly formed cristobalite, the proportions depending on firing temperature, atmosphere, and the specific impurity chemistry of the original kaolin. This is exactly why the geological origin of a kaolin and the minor oxide "fingerprint" it carries, shapes its behavior in the kiln and on the glaze surface.
In ceramics, "whiting" refers specifically to finely ground calcium carbonate used as the primary source of CaO in raw glazes. Calcite, aragonite, and vaterite are all pure calcium carbonate minerals; industrially important source rocks that are predominantly calcium carbonate include limestone, chalk, marble, and travertine. Wikipedia
Whiting (CaCO₃) contributes approximately 56.1% CaO to a glaze, the remaining 43.9% being CO₂ driven off as gas during firing. This loss on ignition is central to how it behaves in the kiln. Glazy
Geological Origins: Three Distinct Source Types
Unlike kaolin, which forms through active chemical weathering or hydrothermal alteration of alumino-silicate rocks, all whiting derives from a single process: the biological or chemical precipitation of calcium carbonate in aqueous almost always marine environments. The differences lie in when this happened, under what conditions, and what happened to the deposit afterward.
1. Chalk (Biogenic Fine-Grained Limestone)
Chalk is a soft, porous, sedimentary carbonate rock composed of the mineral calcite, originally formed under the sea by the accumulation and lithification of the hard parts of organisms mostly microscopic plankton that settled to the sea floor. It was deposited on extensive continental shelves at depths between 100 and 600 metres during a time of reduced terrestrial erosion, which explains its characteristically high purity. The coccolithophores, foraminifera, and other microscopic organisms from which chalk formed produce low-magnesium calcite skeletons, so the sediment was already in the form of highly stable low-magnesium calcite when deposited, in contrast to most other limestones, which originally formed from high-magnesium calcite or aragonite.
The most extensive chalk deposits date from the Cretaceous period (roughly 145–66 million years ago), from which the name "Cretaceous" itself derives (from Latin creta, chalk). Major formations occur across Western Europe south of Sweden, notably the chalk cliffs of Dover on the English Channel. Further deposits extend through the United States from South Dakota south through Texas and eastward to Alabama.
Other significant chalk outcrops include the Rügen cliffs on the Baltic coast of Germany, the Normandy coast of France, and the Møns Klint cliffs in Denmark, all part of the same Late Cretaceous seabed. The purest sources of ceramic-grade whiting are found in England, France, and Belgium.
2. Limestone (Biogenic and Chemical Precipitate)
Limestone forms through two main pathways: biological accumulation of shells, corals, algae, and microorganisms, or chemical precipitation of calcium carbonate from water. Depending on impurities and depositional conditions, limestone may appear white, grey, yellow, brown, or bluish, with textures ranging from fine-grained mudstone to coarse crystalline rock. The dominant mineral is calcite, but additional components may include aragonite, dolomite (CaMg(CO₃)₂), clay minerals, quartz, and organic matter. Geology Science
The presence of dolomite, the magnesium-rich carbonate, is the most practically significant variable. Whitings typically contain some dolomite as a contaminant, and limestone may contain considerable amounts of magnesium; some suppliers do not distinguish between limestone and pure whiting, so it is important to verify the source. Even small amounts of MgO in a "whiting" will shift glaze behavior: MgO from dolomite mattes the glaze surface, whereas CaO from pure calcium carbonate tends to produce a brilliant gloss (which requires slow cooling to develop). Digital Fire
3. Marble (Metamorphic Recrystallized Limestone)
Marble is limestone that has been subjected to heat and pressure deep in the earth's crust. During this metamorphic recrystallization, the original fine biogenic calcite grains recrystallize into larger interlocking crystite crystals, purging much of the organic material and some impurities in the process. The result is often a denser, harder material with very high CaCO₃ purity which is why ground marble is preferred for many industrial applications. China is the world's largest producer of calcium carbonate, followed by the United States and various European countries; India, Japan, and Russia are also significant producers. Other sources beyond chalk include marble, calcite ores, and sea shells.
Chemical Composition: Ideal and Real
The theoretical formula for pure calcite is CaCO₃, which on firing yields CaO (56.1%) + CO₂ (43.9%). In practice, a high-purity ceramic-grade calcite from Algeria showed an XRF composition of approximately 98% CaCO₃, with impurities including 0.15% SiO₂, 0.09% Al₂O₃, and 0.01% K₂O. ScienceDirect
The purest chalk varieties contain up to 99% calcium carbonate as calcite. Minor contributors to its composition include sponge spicules, diatom and radiolarian tests, detrital quartz grains, and chert nodules (flint), which introduce small amounts of silica. Encyclopedia Britannica
The critical impurity to watch for is MgO (from dolomite substitution in the crystal lattice or dolomite mineral inclusions), followed by SiO₂, Al₂O₃, Fe₂O₃, and occasionally SrO (strontium, which is geochemically close to calcium and commonly substitutes for it in marine shell material). Inexpensive non-ceramic grades of whiting tend to lack the quality and consistency needed for use in glazes, particularly for industrial use.
Compositional Differences Between Sources: Does It Matter for Ceramics?
The short answer is: less dramatically than with kaolin, but not negligibly. Unlike kaolin where differences in geological origin produce a broad spectrum of Al₂O₃/SiO₂ ratios, trace oxides, crystallinity, and plasticity whiting from different sources is chemically very similar at the macro level. The practical differences cluster around three variables:
Magnesium content. The main compositional fault line is between pure calcite sources and those containing dolomite. Even a few percent of MgO will subtly shift the melting behaviour and surface quality of a glaze. This is compounded by the fact that some suppliers market dolomitic limestone as "whiting" without clear labeling.
Silica and alumina contamination. Chalk and limestone grades sourced from deposits with significant clay or silica impurities introduce SiO₂ and Al₂O₃ that alter the effective UMF of a recipe particularly relevant in a system like K3 where the silica balance is already calibrated.
Particle size and reactivity. Particle size can make a significant difference in CaO's willingness to enter the glaze melt: a 325-mesh material may have a mean particle size of around 10 microns or even less, whereas a 200-mesh grade might be two or three times larger, yet both powders feel identical in the hand. In glazes sensitive to surface finish, this alone can shift the result from glossy to silky matte.
Thermal Behaviour
Calcite particles in a ceramic body decompose above 600°C during firing, releasing CO₂ gas; the decomposition is typically complete between 800 and 950°C. The resulting CaO reacts with silica and alumina from other materials to produce crystalline calcium silicate and aluminosilicate phases, including gehlenite, anorthite, and wollastonite, depending on the temperature and the available oxides. ScienceDirect
Whiting produces a very large volume of gas while decomposing, losing more than 40% of its weight. While these gases should be driven off well before 1100°C and therefore should not disturb the glaze melt in standard high-fire schedules, in low-fire or fast-fire contexts they can contribute to imperfections and surface faults. This is why wollastonite (CaSiO₃) has increasingly replaced whiting in industrial fast-fire applications: it delivers the same CaO without the CO₂ outgassing. Digital Fire
A Note for the K3 Research Context
For the K3 project specifically, this means that the whiting variable, while not as geologically complex as kaolin, is not entirely neutral either. If collaborators source their whiting from chalk-derived (low-Mg) European grades versus dolomitic limestone grades common in some Asian and North American markets, the effective MgO contribution to the melt will differ. This is worth documenting even if only as a note to specify CaCO₃ purity ≥ 98% and to report MgO content from the supplier's data sheet, it would be very helpful ;-)
The grain size of SiO2 is the most immediately tangible variable for ceramicists, but geological origin creates a second layer of difference that is subtler but real, primarily through the form of silica (quartz vs. flint vs. amorphous).
Silica is a major ingredient in refractories and whitewares, usually added as quartz sand, sandstone, or flint pebbles. In clay bodies, its primary role is that of a filler, imparting green (unfired) strength to the shaped object and maintaining form during firing, while also improving the properties of the fired result. In glazes, silica is the principal glass-former, controlling fusibility and viscosity depending on the amount present.
Silica is second only to clay in significance as a ceramic material and is the most abundant mineral in the earth's crust. The principal siliceous materials used in ceramics are: crystalline quartz, quartzite, sandstone, silica sand, and the organic and amorphous silicas including flint and diatomaceous earth.
Not all SiO₂ is the same
This is where silica becomes genuinely complex. Unlike whiting, which is essentially one mineral (calcite) from various sources, silica occurs naturally in a surprising number of structurally distinct forms all sharing the formula SiO₂ but behaving quite differently in the kiln.
The various forms of crystalline silica include α-quartz, β-quartz, α-tridymite, β-tridymite, α-cristobalite, β-cristobalite, keatite, coesite, stishovite, and moganite. The most abundant form is α-quartz, and it is thermodynamically stable under ambient conditions. The overwhelming majority of natural crystalline silica exists as α-quartz; all other forms exist in a metastable state. NCBI
The so-called cryptocrystalline forms of quartz (chalcedony, agate, flint, chert, novaculite) are the products of geological crystallization into fine-grained varieties of quartz. Biogenic silicas (such as diatomite) are readily converted into cristobalite under relatively mild temperature conditions of around 800°C, well below the thermodynamic stability range of cristobalite.
For ceramicists, the three practically relevant forms are:
Crystalline quartz: the standard material: large, well-ordered crystals, high purity, high refractory character. Found in igneous and metamorphic rocks (granites, pegmatites, quartzites).
Flint: the dark porous variety of chert that contains organic matter. True flint is very fine-grained (cryptocrystalline) hydrated silica, containing approximately 1% molecularly bound water, and is less dense than quartz sand due to its fine porous structure. The name derives from England and France, where flints were historically prepared by calcination and grinding. True flint is very high in silica at about 94%, but contains more inclusions of other minerals than quartz. The "flint" sold today by many pottery suppliers is in practice ground quartz rather than true biological flint. Sheffield PotteryCeramic School
Amorphous silica: including diatomaceous earth (diatomite), silica fume, and fused silica. Amorphous silica does not have long-chain crystalline order and is significantly more reactive than crystalline forms. Glass is an archetypal amorphous silica, created by heating and melting sand to approximately 1400–1600°C and cooling rapidly, which prevents the crystalline structure from reforming. ScienceDirect
Geological Origins of the Main Deposit Types
The geological origin of a silica deposit determines not just purity but the form of silica present, its crystal size, and the nature of its impurities.
1. Igneous and Pegmatitic Quartz: Quartz crystallizes directly from cooling magma and from hydrothermal fluids associated with granites. Pegmatites, very coarse-grained igneous bodies, produce the largest, most chemically pure quartz crystals, because slow cooling allows extensive crystal growth and self-purification. Granite pegmatites and hydrothermal quartz veins are the most promising repositories of high-purity quartz, because quartz from higher-temperature geological settings accommodates lower concentrations of impurities in its atomic structure. However, within pegmatites, the degree of magmatic fractionation matters: less fractionated pegmatites contain relatively high concentrations of Ti, Mg, Ca, and Cr, whereas more fractionated pegmatites show higher Fe, Li, and B. Academia.edu
The globally pre-eminent deposit for high-purity quartz is Spruce Pine, North Carolina, USA. The Spruce Pine deposit supplies more than 90% of the world's demand for high-purity quartz sand. The content of impurity elements is extremely low; after mechanical and chemical purification, the material is used in semiconductor crystals, precision optical glass, photovoltaics, and lighting. The BBC called it "the most strategic square acre on earth" in 2009. The quartz from the Spruce Pine leucogranite has a total trace element concentration of 18 ± 4 µg/g, which can be purified further to 8 µg/g. ScienceDirect
The second major global source is Norway. The Evje–Iveland pegmatite field in southern Norway is renowned for its large feldspar–quartz–mica pegmatites, containing significant quartz veins with relatively low levels of Fe and Ti oxides. Beneficiation combining hand sorting, magnetic separation, and acid leaching can produce quartz concentrates exceeding 99.9% SiO₂. The Tysfjord pegmatite quartz in northern Norway is of the second-best chemical quality produced today, with total trace element concentrations approximately 10–20 µg/g higher than Spruce Pine. ScienceDirect
2. Metamorphic Quartzite: When sandstone or chert is subjected to heat and pressure, it recrystallizes into quartzite, a dense, interlocking mosaic of quartz grains. Metamorphic quartzites across Norway and Central Europe can attain very high SiO₂ levels, but structural defects and accessory minerals, including Fe oxides, feldspar, clays, and locally kyanite-bearing fine grains, complicate upgrading and require more energy- and chemistry-intensive processing. MDPI
3. Sedimentary Silica Sand: Sand deposits result from the weathering, transport, and sorting of quartz-bearing rocks. Rivers and coastal currents naturally sort and concentrate quartz grains, removing lighter clay and feldspar. The result is typically high in SiO₂ (often 95–98%) but with characteristic impurities from the source rocks and transport environment. Although most industrial sand deposits contain a high percentage of quartz (≥95%), using raw materials with lower quartz content is becoming more common as demand outpaces production in certain markets. For ceramic-industry use, SiO₂ must exceed 97.5%, with Fe₂O₃ ≤ 0.2% and Al₂O₃ < 0.55%. For glass-making the threshold is even tighter: SiO₂ minimum 98.5–99%, Fe₂O₃ < 0.04%. ScienceDirect
Key producing regions for ceramic-grade silica sand include the United Kingdom (approximately 4 Mt/year), as well as Japan, Mexico, Canada, New Zealand, South Africa, and Iran (each producing 2–3 Mt/year).
4. Biogenic / Sedimentary Flint and Chert: Flint forms within chalk and limestone deposits through the dissolution and reprecipitation of silica derived from sponge spicules, radiolarians, and diatoms, the same microscopic marine organisms discussed in the whiting section. The silica dissolves in pore water and re-precipitates in nodular or bedded form as microcrystalline quartz (chert) or cryptocrystalline silica (flint). This is why the great chalk regions of England and France were historically the sources of ceramic flint. Flint is denser in biological origin history than in geological complexity, and its ceramic use persisted in English pottery traditions well into the 20th century before being largely replaced by ground quartz.
Chemical Composition: The Purity Spectrum
Unlike kaolin (with its rich suite of Al, Fe, Ti, K, Ca, Mg impurities) or whiting (where MgO from dolomite is the primary variable), silica is a nearly monomineralic material and its purity spectrum is dominated by trace element impurities at very low concentrations.
At the ceramic-grade level, SiO₂ content in quartz from various geological settings typically ranges from 98.46–99.75 wt%, with very low concentrations of all other elements.
Thermal Behaviour: The Inversion Problem
This is the aspect most directly relevant to glaze and body formulation. Silica undergoes several crystallographic phase transitions on heating that cause abrupt volume changes: the famous "inversions."
Quartz has a phase transition at 573°C (the quartz inversion). Upon firing, this inversion occurs rapidly and results in a volume expansion, which can lead to firing cracks in clay bodies high in free quartz. On cooling, the reverse contraction occurs with equal speed.
During firing, quartz particles in porcelain can convert to cristobalite. This has implications for the thermal expansion of the fired matrix: cristobalite undergoes its own inversion at approximately 220°C, with a volume change that affects cooling behaviour and glaze fit. Digital Fire
Crucially, the form of silica in the raw material determines how it behaves during firing. The formation of high-temperature silica polymorphs from quartz is influenced by the presence of alkali-metal impurities. Potassium in particular promotes the formation of cristobalite and tridymite from metastable silica glass during firing, and the balance between these phases depends on temperature, time, and the mineraliser/alkali combination present. ScienceDirect
The reactivity difference between quartz and flint is directly relevant here. Flint's finer, partially amorphous structure makes it more reactive at firing temperatures, it enters the melt more readily than coarse crystalline quartz and is less likely to persist as unreacted free silica in the fired body. Bodies containing quartz rather than flint have been shown to be more susceptible to dunting (thermal cracking), especially on re-fire, which may relate to the lower Young's modulus of quartz-based bodies. Wikipedia
A Note for the K3 Context
For this K3 research, silica is the most "standardized" of the three ingredients, ceramic-grade quartz from most suppliers is in the high-97% SiO₂ range, and the bulk chemistry is rarely the source of significant variation. However, two variables are worth noting in your protocol: grain size / mesh, which is critical, and the form of silica (ground quartz vs. flint vs. calcined flint), since studios in different countries will have different access. English and French studios may still use calcined flint; continental European and Asian studios almost universally use ground quartz. These are structurally different materials with different reactivity at temperature, and that difference could show up in how K3 behaves at the eutectic.
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