The structure and dynamics of alkylammonium-intercalated smectites were simulated using molecular dynamics employing the clayff-CVFF force field, the reliability of which was firstly validated for these systems. The layering behaviors of alkyl chains confirm the scenarios of the monolayer, transition and bilayer configurations for short, medium-length and long carbon tails, respectively. In the systems without water, the alkylammonium groups are all anchored firmly above the surface six-member rings through H bonds between ammonium hydrogen and surface oxygen, and the alkyl tails are a little more mobile. With water involved, some ammoniums are dragged out of the potential barriers of the six-member rings by water molecules through the strong H bonds between water oxygen and ammonium hydrogen. The intercalated water scarcely affects the basal spacing, alkyl chain layering or alkylammonium dynamics. It is also found that the systems with alkyl chains of 11 to 14 exhibit the greatest density, resulting in the extremely limited mobility of the intercalated species.

In the 12 km2 catchment area of Syv creek, Denmark, moderate to high concentrations of nitrate (NO3−) occurred in the upper part of the oxidized zone (oxic-I), but dropped within the lower suboxic part (oxic-II), to below the detection limit in the unoxidized zone. Structural Fe2+ in the clay minerals made up 10 to 12% of the Fe in the oxidized zone and increased to approximately 50% in the unoxidized zone. Concurrent with changes in the distribution of structural Fe2+ the clay mineral constituents changed. Vermiculite was typically found in the oxidized zone whereas chlorite was found in the unoxidized zone only. A conversion of illite and chlorite into vermiculite seems to take place. A significant correlation between NO3− and the amount of reduced Fe2+ in the suboxic (oxic-II) zone, indicates that primary structural Fe2+ in the clay minerals is the reductant in a NO3− reduction process.

57Fe Mössbauer spectra of two synthetic samples of ferrihydrite, recorded at 4.2 K in applied fields of up to 9 T, have been analysed by a mean-field model. The samples exhibit two and six X-ray diffraction peaks. It is shown that only one ferric ion site is present in the mineral, and that in this site the ions are octahedrally coordinated. The spectra show the presence of different magnetic states: ferrimagnetism in two-line ferrihydrite, and antiferromagnetism in six-line ferrihydrite. The ferrimagnetism in two-line ferrihydrite is analysed in terms of random fluctuations arising from the small numbers of ferric ions per particle, and it is shown that the different magnetic states may arise purely as a result of these fluctuations.

The influence of surfactants on the flow behavior of sodium montmorillonite dispersions (2% w/w) was studied for a cationic (cetylpyridinium chloride, CPCl) and an anionic surfactant (sodium dodecylsulfate, SDS). When the dispersion pHs were >3.5 and <7, CPCl concentrations >10−4 M increased the shear stress but the Bingham yield value remained virtually unchanged (τo ≈ 100 mPa). At pH ≈ 7, the shear stress and yield point decreased with increasing CPCl concentration (τo from 430 to 100 mPa). The flow properties of sodium calcium bentonite dispersions were independent of pH and CPCl concentrations ≤ 10−4 M; they increased modestly at higher concentrations. At pH < 4, SDS addition to the sodium montmorillonite dispersions increased the shear stress and yield value to a maximum value (τo = 2100 mPa) at 10−3 M SDS; higher SDS concentrations reduced the shear stress and yield value. At pH > 4, the flow values decreased to a minimum value at 10−2 M SDS (τo from 430 to 50 mPa). The flow of the sodium calcium bentonite dispersions at pH > 4 was independent of SDS concentrations ≤ 10−3 mole/liter; at higher SDS concentrations, the flow values increased more strongly in sodium calcium bentonite than in sodium montmorillonite dispersions.

Surfactants influence the flow behavior of sodium montmorillonite dispersions by their action on the card-house networks in strongly acidic medium and, at higher pH, by the electroviscous effect. At the highest surfactant concentrations without flocculation, the shear stress and yield value are increased by interacting chains of opposed particles.

Addition of the surfactants increases the salt (NaCl) stability of the dispersions because the adsorbed surface active agents influence the counterion distribution between the Stern and the diffuse ionic layer.

Results from earlier laboratory and field experiments were interrogated for the possibilities of sequestration, or long-term accumulation, of carbon from excess greenhouse gases in the atmosphere. In the laboratory study, samples of three (top) soils dominated by kaolinite and illite (together), smectite, and allophane were examined for the adsorption and desorption of dissolved organic carbon (DOC). Adsorption and desorption of DOC were carried out on clay fractions extracted physically and after first native organic matter and then iron oxides were removed chemically. Labeled organic material was added to the soils to assess the priming effect of organic carbon (OC). In the field, changes in OC were measured in sandy soils that had been amended by additions of clay for between 3 and 17 years, both through incorporation of exogenous clay and delving of in situ clay. The laboratory experiments demonstrated that a portion of DOC was held strongly in all soils. The amount of DOC adsorbed depended on clay mineral types, including Fe oxides. Much adsorbed DOC was lost by desorption in water and a substantial amount of native OC was lost on priming with new OC. Addition of clay to soils led to increased OC. Therefore, addition of clay to soil may enhance net sequestration of C. Organic carbon close to mineral surfaces or within microaggregates is held most strongly. Carbon sequestration may occur in subsoils with unsaturated mineral surfaces. However, incorporation of carbon into macroaggregates from enhanced plant growth might be most effective in removing excess carbon from the atmosphere, albeit over the short-term.

Several dioctahedral clay minerals are related through reaction series. These series can be produced hydro-thermally from beidellite gel and montmorillonite by making simple changes in interlayer and solution chemistry. The series are:

• gel-K-beidellite-random illite/smectite-K-rectorite-illite

• K-montmorillonite-K-rectorite-illite

• Na-montmorillonite or beidellite-Na-rectorite-paragonite

• Li-montmorillonite-Li-tosudite-Li-rectorite-cookeite(?)

• Mg-montmorillonite-Mg-rectorite-tosudite-sudoite(?)

• Ca-montmorillonite-Ca-rectorite-margarite(?)

• Al-Ca-montmorillonite-kaolinite/smectite-kaolinite (150°C)

• Al-Ca-montmorillonite-pyrophyllite/smectite-pyrophyllite (320°C).

Assuming stability for the mixed-layer phases, paragenesis is a function of P, T, and X conditions. If the phases are considered to be metastable, paragenesis is a function of the speed and path of reaction.

After a temperature-dependent period when little dissolution occurs, the dissolution of rhyolitic glass can be described by dC/dt = k(Cs − C), where Cs is the concentration of dissolved silica at saturation, C is the instantaneous silica concentration, and k is a rate constant equal to 1.6 × 10−5, 3.0 × 10−5, and 4.5 × 10−5 sec−1 at 115°, 130°, and 140°C respectively, in 2 M Na-K carbonate solution at 1 kbar pressure. At 130°C, a Cs value of 0.177 M SiO2 is reached in 30 hr, and phillipsite, clinoptilolite, and mordenite begin forming at 34, 64, and 76 hr, respectively, in 2 M CO3, 1:1 Na/K. During glass dissolution and zeolite formation, the concentration of Al as Al(OH)4− is buffered at 3.7 × 10−4 M by an unidentified phase. The ratio of SiO2 to Al(OH)4− at the onset of zeolite formation is 475. In 2 M CO3 solution, phillipsite crystallization begins at 144 hr at 115°C, at 34 hr at 130°C, and at 20 hr at 140°C. Phillipsite crystallization begins at 48 hr in 1.5 M CO3, at 168 hr in 1.0 M CO3, and in excess of 550 hr in 0.2 M CO3 at 140°C. In addition to OH− catalysis, CO32- appears also to catalyze the glass-dissolution and zeolite-formation processes.

Thermodynamically, phillipsite is unstable relative to clinoptilolite and mordenite in silica-rich alkaline hydrothermal solutions. Phillipsite forms first, followed by clinoptilolite, and then mordenite. Phillipsite formation is favored by runs of one-week duration, temperatures less than 150°C, and K-rich fluids. Clinoptilolite formation is favored in runs of more than one week, temperatures less than 150°C and K-rich fluids. Mordenite formation is favored by runs of more than one week, temperatures greater than 140°C, and Na-rich fluids. In 8-day runs at 140°C, clinoptilolite formation was favored by liquid: solid reactant (volume: mass) ratios less than 1.0, mordenite by ratios from 0.85 to 1.5, and phillipsite by ratios greater than 1.5. The mechanism of formation of the different zeolites, particularly phillipsite, may involve silica-cyclic tetramers which are abundant in concentrated solutions under alkaline hydrothermal conditions but which are almost absent in dilute low-temperature solutions. Thus, the results of hydrothermal experiments may not be directly applicable to zeolite formation at low temperatures.

Model cation-exchange curves are presented for an idealized kaolinite surface where the charge on the surface (1) has its origin in cation substitution in the structure, and hence, is pH independent; and (2) is produced by protonation/deprotonation reactions of oxide-like sites, and hence, depends on the acid and base strengths of the surface sites, as well as the ionic strength. Two pH-independent situations are considered: one where the exchanging ions have no selectivity for the surface and are all in the diffuse layer; and one where selectivity exists for one ion and where that ion is partly in a Langmuir-Stern layer and partly in the diffuse layer. If one of the exchanging ions is a proton, the shape of the curves and their position on the pH scale depend on ionic strength and ionic selectivity.

The model curves are compared with data for actual kaolinites. Under most conditions exchangeable Al is released from the structure, and the shape of the charging curves becomes similar to that of an oxidelike surface. However, if titration is carried out rapidly, or account is taken of the presence of Al, the proton binding curves are similar in shape to those expected for sites resulting from cation substitution in the structure of kaolinite, either near the surface or at the edge of the crystal.

Surface diffusion, or migration within the electrical double layer next to mineral surfaces, is often invoked as a significant contributor to the overall diffusion coefficient in compacted clays, particularly where model predictions underestimate measured diffusion coefficients. The potential for surface diffusion of Sr2+, Ca2+ and Na+ on three clays compacted to dry bulk densities of 1.25 and 1.60 Mg/m3 was examined. The clays were a bentonite, an illite/smectite, and a glacial lake clay (composed mainly of smectite, illite, kaolinite and quartz). The clays were saturated with a Na-Ca-Cl-dominated synthetic groundwater solution with an effective ionic strength of 220 mol/m3. Total intrinsic diffusion coefficients for the cations were determined from their steady-state flux through compacted clays, and apparent diffusion coefficients were obtained from the time-lag technique. Models of diffusive transport in compacted clays, based only on diffusion in the pore solution, adequately described the diffusion data for all clays and diffusants, and there was no need to invoke other transport mechanisms, like surface diffusion. The data indicate that surface diffusion is not a significant transport mechanism in compacted clays at least to a clay density of 1.60 Mg/m3.

Samples of smectites and vermiculites were evaluated to (1) determine the effects of source and amount of charge, (2) determine if a continuum of properties exists, and (3) improve the basis for differentiating criteria for identification.

The montmorillonites expanded to the equivalent two-layer complex upon solvation by condensation of ethylene glycol or glycerol vapor. Beidellite exhibited the one-layer complex with glycerol vapor and the two-layer complex with ethylene glycol. The vermiculite samples did not yield regular two-layer complexes regardless of saturating cation, prior moisture or solvating agent. A number of different complexes of vermiculite were obtained depending on conditions of solvation. These corresponded to spacings of approx 13·6, 14·0, 14·3, 15·0, and 15·3 Å. Two or more of the complexes may be present in the same sample.

Potassium-saturated vermiculites and smectites both exhibited collapsed layers in a dry atmosphere. Hydration of smectites occurred at humidities above 20 per cent and resulted in poorly ordered diffraction maxima of 11–12 Å. The K-saturated vermiculites tended to retain the collapsed lattice and exhibited diffraction maxima of 10–10·6 Å with integral higher orders. Some hydration occurred as evidenced by peak asymmetry or the appearance of a small 14·3 Å line at higher humidities, although the collapsed spacing predominated. The tendency of vermiculite to hydrate decreased as the exchange capacity increased.

Differences in properties can be related to both source and amount of charge. Nevertheless, a continuum in the properties of expansion and collapse between the smectites and vermiculites was not observed. The data indicate two discrete populations. These properties may be used as differentiating criteria for identification.

Cretaceous bentonites were collected in outcrop from the Sweetgrass Arch and the Disturbed Belt in Montana. The mixed-layer illite-smectite (I/S) components of the bentonites from the Sweetgrass Arch have from 0 to 25% illite layers and no detectable structurai Fe2+, whereas the samples from the Disturbed Belt have from about 25 to 90% illite layers, and all contain Fe2+. A positive correlation (r = 0.89) exists between the percentage of structural iron that is Fe2+ and the amount of fixed interlayer K in the I/S.

The higher percentage of illite layers in the samples from the Disturbed Belt is attributed to reactions related to elevated temperatures caused by burial beneath thrust sheets. The increase in Fe2+/Fe3+ with increasing percentages of illite layers is tentatively attributed to a redox reaction involving the oxidation of organic matter. Although there is no statistical evidence for an increase in octahedral charge with an increase in illite layers when all the samples are considered together, iron reduction may have contributed as much as 10 to 30% of the increase in total structural charge that occurred in any given sample during metamorphism. The remaining structural charge increase can be attributed to the substitution of Al3+ for Si4+ in the tetrahedral sites.

Because layered Fe(II)Fe(III)-hydroxides (Green rusts, GRs) are anion exchangers, they represent potential orthophosphate sorbents in anoxic soils and sediments. To evaluate this possibility, two types of experiments with synthetic sulphate-interlayered GRs (GRSO4 = Fe2+4Fe3+2(OH)12SO4 ×H2O) were studied. First, sorption of phosphate in GRSO4 was followed by reacting suspensions of pure GRSO4 synthesized by oxidation of Fe(II) with an excess of Na2HPO4 (pH 9.3). Second the possible incorporation of phosphate in GR during formation by Fe(II)-induced reductive dissolution of phosphate-containing ferrihydrites was examined in systems containing an excess of Fe(II) (pH 7). With excess phosphate in solution, GRSO4 initially sorbed phosphate in the interlayer producing a basal layer spacing of 1.04 nm, but only ~50% of the interlayer sulphate was exchanged with phosphate. This GR slowly transformed to vivianite within months. In the Fe(II)-rich systems, reaction with synthetic ferrihydrites produced GRSO4 similar to that produced by air oxidation. Reaction of Fe(II) with phosphate-containing ferrihydrites initially produced amorphous greenish phosphate containing precipitates which, after 3–4 h, crystallized to GRSO4 and vivianite. In these solutions, stable phosphate-free GRSO4 can form since precipitation of vivianite produced low phosphate activity. Consequently, in both systems GR or amorphous greenish precipitates act as reactive intermediates, but vivianite is the stable end-product limiting phosphate concentration in solution. It is also inferred that Fe(OH)2 is an unlikely phosphate sorbent in mixed Fe(II)-Fe(III) systems because GR phases are more stable (less soluble) than Fe(OH)2.

The formation of mixed-metal-Al layered double hydroxide (LDH) phases similar to hydrotalcite has been identified as a significant mechanism for immobilization of trace metals in some environmental systems. These precipitate phases become increasingly stable as they age, and their formation may therefore be an important pathway for sequestration of toxic metals in contaminated soils. However, the lack of thermodynamic data for LDH phases makes it difficult to model their behavior in natural systems. In this work, enthalpies of formation for Ni LDH phases with nitrate and sulfate interlayers were determined and compared to recently published data on carbonate interlayer LDHs. Differences in the identity of the anion interlayer resulted in substantial changes in the enthalpies of formation of the LDH phases, in the order of increasing enthalpy carbonate<sulfate<nitrate. Substitution of silica for carbonate resulted in an even more exothermic enthalpy of formation, confirming that silica substitution increases the stability of LDH precipitates. Both mechanical mixture and solid-solution models could be used to predict the thermodynamic properties of the LDH phases. Modeling results based on these thermodynamic data indicated that the formation of LDH phases on soil mineral substrates decreased Ni solubility compared to Ni(OH)2 over pH 5–9 when soluble Al is present in the soil substrate. Over time, both of these precipitate phases will transform to more stable Ni phyllosilicates.

Scan electron micrographs are shown of (1) kaolinite and dickite which crystallized from solution within cavities, (2) kaolinite weathered from clastic primary silicate material, (3) residual kaolinite from primary silicate rock, (4) kaolinite above and below a basal unconformity on granitic rock, and (5) hydrothermally altered kaolinite. The texture of kaolin reflects the environment in which the clay was formed.

Euhedral crystals of kaolin minerals characterize cavity fillings. The weathering environment produces large crystal flakes of kaolinite, expanded books, mats of elongates, high porosity, and low bulk density (less than 2.0). The hydrothermal environment produces smaller crystals, singles, sheaves and packets of crystals, low porosity and high bulk density (2.0 and above).

The interrelationships of genetic environment of kaolin minerals and accompanying texture are discussed in light of available geochemical information.

Hornblende of the Carrol Knob mafic complex (southern Blue Ridge Mountains, North Carolina) has weathered under humid, temperate conditions. Hornblende weathering appears to have been a dissolution-reprecipitation reaction, in which hornblende dissolved stoichiometrically, and the ferruginous and aluminous weathering products (goethite, gibbsite, and kaolinite) precipitated from solution (neoformation). During the earliest stage of alteration, ferruginous weathering products formed as linings of fractures within and around crystals and cleavage fragments of hornblende. Side-by-side coalescence of lenticular etch pits during more advanced weathering produced characteristic “denticulated” terminations on hornblende remnants in dissolution cavities bounded by ferruginous boxworks. Dissolution cavities are devoid of weathering products. Small “pendants” of ferruginous material project from the boxwork into void spaces. Because these products are separated from the hornblende remnants by void space, they must have been produced by dissolution-reprecipitation reactions. Complete removal of the parent hornblende left a ferruginous microboxwork or “negative pseudomorph.” Only Al and Fe were conserved over microscopic distances; alkali and alkaline-earth elements were stoichiometrically removed from the weathering microenvironment during the weathering process.

Previous investigations of goethite revealed a substantial variation of color and diffuse reflectance spectra (DRS) in the extended visible range (350–2200 nm). To better understand the causes of this variability and to assess the potential of DRS as a mineralogical tool, we investigated the DRS of pure and Al-substituted goethite, α-Fe1−xAlxOOH with x from 0 to 0.33, and mean crystal lengths (MCL) from 170 to 1800 nm. The strongly overlapping ligand field bands were extracted by fitting the single-electron transitions 6A1 → 4T1, 6A1 → 4T2, 6A1 → (4E; 4A1), and 6A1 → 4E(4D) as functions of the ligand field splitting energy, 10 Dq, and the interelectronic repulsion parameters, Racah-B and -C. With x increasing from 0 to 0.33, 6A1 → 4T1 decreased from 10,590 to 10,150 cm−1 (944 to 958 nm), and 6A1 → 4T2 decreased from 15,310 to 14,880 cm−1 (653 to 672 nm), while 10 Dq increased from 15,770 to 16,220 cm−1. From the change of 10 Dq we calculated a decrease of the Fe-(O,OH) distances from 202.0 to 200.9 pm (−0.5%). This decrease is smaller than the average decrease of all (Al,Fe)-(O,OH) distances (−1.8%) calculated from the change of the unit-cell lengths (UCL). That is, there remains a substantial difference in size between the larger Fe- and the smaller Al-occupied octahedra in the solid solution which may indicate the existence of diaspore clusters within the goethite structure. The increasing strain in the crystal structure due to the size mismatch and limited contractibility of the oxygen cage around Fe may be the primary reason for Al substitution being restricted to x < 0.33. The bands 6A1 → (4E; 4A1) and 6A1 → 4E(4D) did not shift, indicating a constant covalency of the Fe-(O,OH) bonds with B = 628 cm−1 and C = 5.5B. Whereas variation of band energies could be explained in terms of the Fe-(O,OH) ligand field, the variation of color and band intensities was mainly determined by crystal size. Although our study confirmed the potential of DRS for mineralogical investigations, there is still a gap between the fundamental theory and the explanation of some spectral features.

Clayey fragments colored deep bluish-green are widely found in glassy rhyolitic tuffs at Oya, Tochigi Prefecture. In room-air the color changes to black or gray within one hour and finally to brown in a few weeks. The fragments are composed of an intimate mixture of two kinds of smectite: a ferrous iron-rich smectite (IR) with b0 = 9·300 Å; and an iron-poor smectite(IP) with b0 = 9·030 Å. Microscopic examination shows a vesicular texture and that IR occurs at the core and IP at the marginal parts of each vesicle. Analysis by EPMA gave the following structural formulas: IR, (Na0·60K0·04Ca0·44) (Mg2·04Fe3·982+Al0·02) (Si6·36Al1·64)O20(OH)4; IP, (Na0·52K0·08Ca0·26) (Mg0·90Fe0·952+Al2·54) (Si7·66Al0·34)O20(OH)4. IR has a much larger amount of iron in trioctahedral sites than that found in any earlier data. Acid-dissolution data, infrared absorption spectra, Eh-values, and DTA and TG curves are also given. Ferrous iron in the structure is easily oxidized in room air with loss of protons from the clay hydroxyls and with contraction of the lattice. We call the IR before and after oxidation the ferrous and ferric forms, respectively, of iron-rich saponite. They strongly suggest the existence of the iron-analogue of saponite. On exposed weathered surfaces in the field, brown fragments tend to be differentiated into two parts: one light yellow montmorillonite-beidellite; the other a brown incrustation due to hisingerite.

The acid dissolution of 8 chlorites in 2N Si-saturated HCl showed that there was no preferential dissolution of Al in octahedral positions and that the octahedral and tetrahedral sheets in the chlorite structure were equally attacked. An analysis of chemical data suggested a simple two-dimensional diffusion model in which the acid attack on the chlorite particles proceeded from the edges inward leaving a reacted layer which formed a diffusion barrier. Microscopic examination showed, however, that the acid attack was not simply an edge attack but started at any point where cracks, structural defects and weaknesses apparently predisposed sites to acid attack. It was also clear from the formation of etch figures and solution channels that the product layer at least partially dissolved or disintegrated during the progress of acid attack. The residue which remained after acid attack appeared to be an opaline amorphous hydrated silica as was indicated by index of refraction measurements. The orderly arrangement of etch figures on flakes of some chlorites reflected certain crystal symmetry elements. The results indicate that the acid dissolution technique to determine the ratio of Al in octahedral and tetrahedral positions can not be used as an aid in the calculation of structural formulae for chlorites.

Nine hematites and 22 goethites were synthesized by a variety of methods to obtain monomineralic samples having a range of Al substitutions and particle sizes. The second derivative of absorbance and Munsell color designations were calculated from visible reflectance spectra obtained from the dry powders. Unit-cell dimensions, Al substitution, infrared band positions, mean crystallite dimensions (MCD) from X-ray powder diffraction, and particle size from fiber-optic Doppler anemometry (FODA) were determined. Previously reported correlations between Al substitution, goethite unit-cell dimensions, and OH-stretching and -bending band positions were confirmed. For hematite, the position of the second derivative peak at ∼600 nm was negatively correlated with Al substitution (r = −.86). Munsell value and chroma were positively correlated with Al substitution (r =.94 for both), but hue was not related to Al substitution. Hue appeared to become redder, however, as particle size measured either by FODA or MCD increased. For goethite, the position of the second derivative minimum at ∼485 nm was negatively correlated with Al substitution (r = −.99). Munsell hue appeared to be related to both Al substitution and MCD perpendicular to (110), MCD110, with hues becoming redder with increasing Al substitution and yellower with increasing MCD110. Correlations between Munsell value and chroma and parameters such as Al substitution, particle size, and OH-stretching and -bending band positions were poor, but goethites synthesized by oxidation of Fe2+ solutions at room temperature had higher chromas than goethites synthesized hydrothermally from an Fe3+ system. Visually determined colors agreed well with calculated ones. Second-derivative spectra and color designations calculated from visible spectra appear to be potentially useful for quickly estimating other properties of goethite and hematite, such as Al substitution and particle size.

In cases where the provenance of raw materials used in the manufacture of local archeological ceramics is of interest, a detailed study of thermal transformations of minerals may be useful. The purpose of this study was to measure mineralogical transformations of different types of clays obtained during experimental firing runs, carried out at different temperatures, with the main goal of establishing Algarve reference groups based on the composition of raw material and high-temperature mineralogy, which may be compared with ceramics in studies of provenance. Eleven samples of clay-rich raw materials from the Algarve Basin (southern Portugal) were fired to temperatures ranging from 300 to 1100°C in increments of 100°C under oxidizing conditions. These were chosen to have variable chemical and mineralogical compositions, representing the main compositional range of the clay deposits from the region. Mineralogical and geochemical characterizations of the original clays were carried out by X-ray diffraction (XRD) and X-ray fluorescence (XRF), respectively. Mineral transformations on the fired products were also studied by XRD.

Three groups of clays were distinguished according to the type of neoformed high-temperature minerals: (1) non-calcareous clays; (2) clays containing calcite as the only carbonate; and (3) clays with dolomite or dolomite + calcite. Firing of non-calcareous clays produced mullite at 1100–1200°C. Gehlenite and wollastonite formed by firing calcite-rich clays above 900°C, accompanied by anorthite or larnite in samples with small or large calcite contents, respectively. Firing of dolomite-rich clays at temperatures >900°C yielded a member of the gehlenite-åkermanite group and diopside. Anorthite, enstatite, periclase, forsterite, and monticellite may also form in the firing products.