Mullite Crystals Research Articles

Effect of ZrSiO4 as additive on the mechanical properties of DLP printing porous mullite ceramics

In order to satisfy the demand for mullite porous ceramics with complex geometries in the engineering field, the digital light processing (DLP) technology is employed to prepare mullite porous ceramics in this study. However, the poor mechanical properties of the mullite porous ceramics prepared using the DLP technology limits its further application in engineering field. In this work, the microstructure of mullite crystals was regulated by adding the sintering aid ZrSiO4 to enhance the mechanical properties of the mullite ceramics, and the effects of sintering aid ZrSiO4 content and sintering temperature on the internal microstructure evolutions of mullite porous ceramics were investigated. Results showed that the decomposition of ZrSiO4 promoted the generation of the silica-rich liquid phase at high temperatures, which effectively promoted the nucleation and growth of needle-like mullite crystals. When the sintering temperature was 1600 °C and the ZrSiO4 content was 10 wt%, the well-developed elongated crystals formed an interlocking nonwoven structure. The corresponding mullite porous ceramics exhibited a compressive strength of 2.52 MPa and a porosity of 74.51 %. The strategy of this study can be a feasible route for preparing lightweight and high-strength mullite porous ceramics via digital light processing.

In-situ grown columnar mullite reinforced SiC porous ceramic membrane support with excellent mechanical performance and corrosion resistance: Using TiO2 as the key to form columnar mullite

In this study, in-situ growth of columnar mullite reinforced silicon carbide porous membrane supports were prepared, and the addition of TiO2 helped to reduce the sintering temperature for the anisotropic growth of necked columnar mullite crystals. The synthesis of in situ columnar mullite makes reasonable use of the oxidized silica of silicon carbide, and in turn reduces the loss of the silicon carbide porous membrane support under strong corrosive conditions. The material showed a flexural strength of up to 72.95 MPa, and a water flux of 31612.83 L·m−2·h−1·bar−1 when sintered for 2 h in an air environment at 1250 °C. The flexural strength after corrosion in 20 vol% H2SO4 and 1 wt% NaOH solution (90 °C) exceeded 93 %. This shows that this porous silicon carbide ceramic membrane scaffold has potential applications in the field of water treatment under harsh environments.

Microwave‐sintered mullite structural ceramics based on low‐grade bauxite applied for fracturing proppants

AbstractThis study aimed to assess the feasibility of manufacturing fracturing proppants by microwave sintering and using low‐grade bauxite as raw material. The effects of microwave hotspot SiC and sintering additive MnO2 content on the performance of the mullite‐based structural materials were studied, respectively. The optimum sintering condition was determined by single‐factor experiments. The sintering process and mechanism were explored based on the analysis of physicochemical properties, phase transitions, and microstructure. The results showed that (1) mullite ceramic composites could be successfully prepared only with SiC added and with poor interparticle bonding microstructure. (2) With the addition of MnO2 and CaO, the granular‐shaped mullite crystals transformed into rod‐like mullite crystals, forming a net structure. (3) As the input power increased, the overfast sintering rate would reduce proppants' mechanical properties, and it was also necessary to select a reasonable sintering time to avoid overburning. (4) When the mass ratio of MnO2:CaO:SiC:bauxite was 2:1.5:12:84.5 and under the sintering condition of 1000 W, 2 h, the performance (breakage ratio of 8.5% under 28 MPa closed pressure, apparent density of 2.58 g/cm3, turbidity of 52 FTU, and acid solubility of 6.77%) could meet the requirements of the Chinese Petroleum and Gas Industry Standard (SY/T 5108–2014). This study provides a powerful way for reducing the fracturing cost, which not only improves the low‐grade bauxite utilization scale within the ceramic industry, but also expands the application of microwave sintering technology in mullite structural materials for the petroleum and gas industry.

Preparation and Properties of High Sound-Absorbing Porous Ceramics Reinforced by In Situ Mullite Whisker from Construction Waste.

Porous sound absorption ceramic is one of the most promising materials for effectively eliminating noise pollution. However, its high production cost and low mechanical strength limit its practical applications. In this work, low-cost and in situ mullite whisker-reinforced porous sound-absorbing ceramics were prepared using recyclable construction waste and Al2O3 powder as the main raw materials, and AlF3 and CeO2 as the additives, respectively. The effects of CeO2 content, AlF3 content, and sintering temperature on the microstructure and properties of the porous ceramics were systematically investigated. The results showed that a small amount of CeO2 significantly promoted the growth of elongated mullite crystals in the resultant porous ceramics, decreased the growth temperature of the mullite whiskers, and significantly increased the biaxial flexural strength. When 2 wt.% CeO2 and 12 wt.% AlF3 were added to the system, mullite whiskers were successfully obtained at a sintering temperature of 1300 °C for 1 h, which exhibited excellent properties, including an open porosity of 56.4 ± 0.6%, an average pore size of 1.32-2.54 μm, a biaxial flexural strength of 23.7 ± 0.9 MPa, and a sound absorption coefficient of >0.8 at 800-4000 Hz.

Preparation of β-SiAlON/SiC composite ceramic foam filters and their oxidation resistance

In this study, ceramic foam filters with high porosity, strength, and oxidation resistance are produced at low temperatures by preparing porous β-SiAlON/SiC composite ceramic foams using the foam impregnation method with Si, Si, α-Al2O3, AlN, and SiC as the raw materials. The presence of SiC inhibits the growth of β-SiAlON grains, which reduces the amount of β-SiAlON produced. As the proportion of β-SiAlON increases, the porosity of the β-SiAlON/SiC composite ceramic foam decreases, whereas the bulk density and compressive strength increase. Non-isothermal oxidation experiments show that β-SiAlON/SiC composite ceramic foams exhibit high oxidation resistance and their macroscopic morphology does not change significantly after oxidation. As the proportion of β-SiAlON increases, the smooth SiO2 oxide layer on the surface of the ceramic foam gradually changes to acicular and columnar mullite crystals, and the oxidation resistance increases. Therefore, β-SiAlON acts as a high-temperature binder when it is added to SiC. Hence, the prepared β-SiAlON/SiC composite ceramic foams can be used as ceramic filters in the field of molten metal filtration.

Binary anorthite‐cordierite ceramic composite from kaolin and dolomite rock with improved characteristics

AbstractBinary CaO‐MgO‐Al2O3‐SiO2 (CMAS) ceramics composites of anorthite‐cordierite composition were synthesized from natural raw materials (kaolin and dolomite rock). The change of SiO2/Al2O3 ratio in kaolin/dolomite mixtures (12, 24, and 36 wt.%) and synthesized composites along with analysis of their phase composition transformations (X‐ray diffraction) during heat treatment (thermogravimetric and differential thermal analysis) give the controlled design of aluminosilicate matrices. The obtained ceramic composites are represented by anorthite, a cordierite‐like phase whose ratio varies from 1.3:1 to 2.8:1. Mullite crystals, also included in the composition, reinforce the anorthite‐cordierite matrix. The morphostructural features of the samples were studied using optical and scanning electron microscopy. The porosity ranges from 7.8% to 24.2% depending on dolomite content. The impurities of iron and titanium interfering with obtaining a qualitative product are leveled by the presented technique including the heat treatment scheme justified by thermogravimetric analysis. Obtained CMAS‐ceramics of anorthite‐cordierite composition correspond to industrial international standards by their technical characteristics and exceed the requirements for heat‐insulating and chemically resistant materials by compressive strength.

Preparation and properties of mullite ceramic-based porous aggregates with high closed porosity utilizing low-voltage electroceramics waste

Mullite ceramic-based porous aggregates, known for their high strength and closed porosity, are widely utilized in engineering. This study successfully prepared mullite-ceramic-based porous aggregates from low-voltage electroceramics waste, investigating the impact of pore-forming agents, bauxite chamotte, and sintering temperature on the physical properties and cold compressive strength. Finally, a mechanism of closed and open pores formation was proposed. The results show that when 10 wt% activated carbon was added, enhanced particle dispersion and stability in apparent porosity, bulk density, and diameter shrinkage ratio were observed, with an optimal range of activated carbon addition identified at around 10.0 wt%. Furthermore, increasing bauxite chamotte addition from 5.0 wt% to 15.0 wt% at 1000°C promoted mullite nucleation, facilitating mullite crystal formation and reducing the relative content and viscosity of the liquid phase. This led to increased apparent porosity, reduced closed porosity, and a decrease in average pore size. However, stress concentration around surface pores resulted in reduced cold compressive properties. The sintering temperature was then increased from 900°C to 1200°C using 10.0 wt% activated carbon and 15 wt% bauxite chamotte, leading to changes in crystallinity and porosity. At 1100°C, isolated islands of the liquid phase between grain boundaries resulted in the highest cold compressive strength and bulk density, while further heating to 1200°C led to a reduction in these properties due to excess liquid phase filling grain boundaries. The formation of closed and open pores is primarily influenced by changes in sintering temperature, which affect the quantity, viscosity, and flowability of the aluminum-silicon liquid phase.

Significantly improved mechanical properties of mullite ceramics by adding AlOOH with different sizes and morphologies

AbstractMullite ceramics with high purity and toughness were prepared by hot‐press sintering of pyrophyllite at 1300°C using AlOOH nanomaterials with different sizes and morphologies (nanoparticles, nanorods, nanoflakes, and micro‐sized sea urchin–like) as additives. Among the four types of AOOH additives, the incorporation of nanoflakes and sea urchins resulted in the formation of a relatively uniformly distributed and tightly packed microstructure within the ceramics, which significantly improved the density and mechanical properties of the ceramic materials. Compared to nano‐sized AlOOH, the addition of micron‐sized sea urchin–like AlOOH could produce mullite ceramics with best purity and flexural strength. The flexural strength and fracture toughness of ceramics prepared from micro‐sized sea urchin–like AlOOH and pyrophyllite reach 427.34 ± 1.99 MPa and 4.68 ± .31 MPa m1/2, respectively. During the ball milling process, the originally micron‐sized sea urchin–like AlOOH particles were broken down into micro‐ and nano‐sized AlOOH particles. The resulted micron and nanoscale AlOOH particles exhibited synergistic and multi‐scale effects with pyrophyllite, which contributed to the formation of uniformly sized and densely arranged mullite crystals within the ceramics. Additionally, the bridging between the mullite crystals further improved the mechanical properties of the mullite ceramic material.

Study on comprehensive properties of mullite porous ceramics prepared from gangue based on sintering temperature

Based on coal gangue and clay with median particle diameters of 0.22 and 0.21 nm as the main raw materials, starch is used as the pore-forming agent, the mullite porous ceramics are prepared by the hydraulic forming method and the solid sintering method, and the sintering temperature and the amount of clay added are studied. The mechanism influence on the performance of mullite porous ceramics. Through thermogravimetric analysis (TG-DSC), flexural properties, X-ray diffractometer (XRD), scanning electron microscopy (SEM), mercury intrusion method and other technical methods, the thermal stability, mechanical strength, crystal phase composition, Performance indicators such as microscopic morphology and pore size are characterized. The results show that the mullite porous ceramics prepared under the conditions of sintering temperature of 1300 °C and clay content of 40 wt.% have the best performance, with a porosity of 34%, a bulk density of 1.79 g/cm3, and a volume shrinkage rate. 11.88%, flexural strength of 33.28 MPa, uniform pore distribution and more mature mullite crystals. The research results open a new way to explore the high-value utilization of coal-based wastes.

SINTERING PROCESSES, PHASE AND STRUCTURE FORMATION AND PROPERTIES OF ELECTRICAL PORCELAIN WITH THE USE OF MARSHALLIT AND WOLLASTONITE ADDITIVES

The effect of marshallite upon its complete replacement of quartz and wollastonite additions (2.0 wt.%) on the sintering and phase formation processes, as well as the structure and properties of insulation porcelain was studied. The object of study is insulation porcelain of typical component composition (kaolin, soft clay, feldspar, quartz sand). It has been established that the processes of melting and sintering at firing of the investigated porcelain compositions are not equilibrial. The formation of ternary eutectics fusions of the R2O-Al2O3-SiO2 systems at temperatures of 985 and 1050 °C in amounts of 45–60% does not occur. The formation of a fusion during firing of ceramics occurs due to partial binary eutectics “white feldspar-earth silicon” and “orthoclase feldspar-earth silicon” during the interaction of feldspar with silica-containing components (990–1060 °C). This is due to the fact that primary mullite is formed at temperatures of its stable state above 1200 °C. The actual processes of fusion formation during porcelain firing are not equilibrium. Recrystallization of mullite occurs due to the dissolution of primary mullite in the fusion, followed by crystallization of acicular mullite. Replacing of quartz sand with marshallite leads to a decrease in the temperatures of earth silicon with feldspar interaction and the formation of a primary fusion, which results in a decrease in the ceramics firing temperature by 70° from 1340 to 1270 °C. The addition of wollastonite – CaSiO3 (2 %wt.) intensifies the process of primary mullite dissolution and crystallization of acicular mullite. The length of the mullite needles increases from 2–3 μm up to 7–10 μm, the mullite needles form aggregates, which provides an increase in the bending strength of porcelain by 36% up to 80 MPa. For citation: Vereshchagin V.I., Kurbanbayev M.Ye., Yessimov B.O., Root L.O., Mogilevskaya N.V. Sintering processes, phase and structure formation and properties of electrical porcelain with the use of marshallit and wollastonite additives. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 5. P. 87-98. DOI: 10.6060/ivkkt.20246705.6960.

Evaluation of Ceramic Properties of Bauxitic Materials from SE of Iberian Range

The use of aluminum-rich clays and bauxites as refractory materials is common. Upon firing, these materials form mullite crystals in the shape of needles embedded in a siliceous and vitreous matrix, with mullite being responsible for the refractory properties. In this study, bauxite samples for use in refractory applications have been characterized. Chemical analysis revealed that the alumina content varied between 34 and 40%, with silica values generally being high (around 40%), except for one sample (26%). Two samples were found to be the most suitable for use as “refractory clay” refractories. However, high silica or Fe oxide contents can affect mineralogical transformations at high temperatures. Mineralogical analysis confirmed the presence of several minerals in the bauxite materials, including kaolinite, halloysite, anatase, rutile, gibbsite and boehmite. Differential thermal analysis (DTA) showed the decomposition of gibbsite and its partial transformation to boehmite and alumina, and the dehydroxylation of kaolinite, with primary mullite crystallization observed at a high temperature. These findings provide valuable information for the selection and optimization of bauxite materials for refractory applications, considering their chemical composition and mineralogical characteristics.

ОПРЕДЕЛЕНИЕ ВЗАИМОСВЯЗИ СВОЙСТВ ВЫСОКОПРОЧНЫХ КЕРАМИЧЕСКИХ ИЗДЕЛИЙ С МИКРОСТРУКТУРОЙ И ФАЗОВЫМ СОСТАВОМ

The article investigates interconnection of high physical and mechanical characteristics of highstrength building ceramics and phase composition of ceramic shard. The authors identify phase composition and microstructure of clinker brick and porcelain stoneware samples by powder X-ray diffraction and using a scanning electron microscope. The shard structure affects many properties of products, including strength, water absorption, frost resistance, etc. The article considers components of building ceramics microstructure, which give high physical and mechanical characteristics to product. High physical and mechanical properties of clinker bricks are achieved due to the content of needle-shaped mullite. The microstructure of clinker bricks is represented by fused quartz grains and mullite crystals with glass interlayers between them. In porcelain stoneware, fine-crystalline mullite is distributed in the X-ray amorphous phase. Porcelain stoneware has a welldeveloped X-ray amorphous phase due to intensive liquid-phase sintering. In samples of high-strength building ceramics the study establishes leucite crystals formation, having reinforcing effect on structure of a ceramic shard.

The kinetic characteristics of mullite crystallization in slag with high SiO2 and Al2O3 content at gasification condition of solid fuel

Mullite crystallization leading to poor slag discharge, is the bottleneck of solid fuel gasification. In order to quantify the crystallization behavior and prevent slag blocking, the crystallization thermodynamics and kinetics of slags in the main crystalline phase region of mullite were studied using FactSage, XRD, SEM-EDS and DSC. Mullite crystallization temperatures ranged from 1189 °C to 1325 °C, and an increase in the cooling rate reduced the mullite onset crystallization temperature as detected by DSC. Kinetic analysis showed that mullite possesses strong nucleation capacity and crystallize tendency, and the minimum activation energy of crystallization of slags was 304.4 kJ/mol. The Si-O-Al structure was regarded as the key short-range ordered structure for mullite crystallization ability, and its concentration was reduced by increasing SiO2/Al2O3. Then the crystallization tendency could be weakened by adjustment of SiO2/Al2O3. Meanwhile, slags with SiO2/Al2O3 = 2.0 or Fe2O3 ≤ 15 wt% showed weak crystallization tendency or glassy slag, which was favorable for slagging operation. The results of this study can provide theoretical guidance for the smooth slagging of ash slag in the main crystalline phase of mullite.

Crosslinking and pyrolysis of a methyl-silsesquioxane: Effect of heating rate on fabrication of polymer derived mullite ceramics using thermoplastic shaping

Poly-silsesquioxanes have been numerously used to produce SiO2 or SixOyCz ceramics through pyrolysis under different atmospheres. In this study, crosslinking and pyrolysis behavior of a commercial methyl-silsesquioxane, was examined using thermogravimetry analysis and a lower SiO2 yield was observed after pyrolysis at low heating rates of 0.3 and 0.6 K/min under air atmosphere, namely 69.1 and 75.0 wt%, respectively. Using ceramic thermoplastic shaping techniques, a low heating rate, below 2 K/min, is usually required to remove processing additive (e.g., binders) gradually and avoid failures like blisters, pores, and cracks. A mixture of SILRES® MK and γ-Al2O3 was prepared to produce a mullite ceramic with Al2O3/ SiO2 ratio of 60/40 wt%. As expected, the stoichiometric ratio of alumina to silica was altered using a heating rate of 0.6 K/min and mullite ceramic with 61.70/37.70 wt% of Al2O3/ SiO2 ratio was obtained. By mixing above the crosslinking temperature of SILRES® MK, a constant ratio of alumina to silica could be achieved successfully, even at low heating rates. In addition, microstructure indicated the growth of needle-like mullite crystals with some amount of SiO2-rich ternary amorphous phase (MgO–Al2O3–SiO2) between the needle grains. Elemental analysis revealed that MgO sintering additive was mainly in the amorphous SiO2-rich phase.

Analysis of Clay Mineral Transformation in Plambik Village, Central Lombok Using X-Ray Diffraction and Scanning Electron Microscope Methods

Analysis of minerals and microfabric changes during the burning process of clay in Plambik village, Central Lombok has been carried out using 2 methods: X-Ray Diffraction and Scanning Electron Microscope. A brick of dimension (10 x 20 x 5) mm was prepared which was formed through a gypsum mould. After drying it is burned using an electric furnace with temperatures: 800oC, 1000oC and 1100oC. The results show that using the X-ray diffraction method when burned at 800oC there are quartz minerals with several illite peaks, at 1000oC there is only quartz, the illite peaks disappear while at 1100oC the quartz mineral peaks remain accompanied by the appearance of mullit peaks. Then the effect of the firing temperature on the clay microfabric was analyzed by observing the sample with a Scanning Electron Microscope. This test aims to obtain information about the changes in structure and texture that develop during the combustion process and more specifically on the morphology of the particles and their interaction with the development of the vitrification process and the distribution of the shape and size of the pores. The results showed that before burning, the kaolinite and illite particles were randomly distributed with porous aggregates and the grain size was between 1 – 5 microns. During firing at 800oC some parts have undergone melting and welding phases with the development of secondary porosity. Meanwhile, when burning with a temperature of 1100oC, it appears that the welding and smelting processes that occur are wider and the emergence of mullite crystals from the glass matrix.

Microstructure and thermal shock resistance of ZrO2–Al2O3–A3S2 composites for casting filter materials

ZrO2–Al2O3-A3S2(3Al2O3·2SiO2) composites are ideal Casting filter materials with excellent mechanical properties and thermal shock resistance. In this paper, 3Y–ZrO2 (3 mol% Y2O3-stabilized ZrO2) was added to kaolin and α-Al2O3 to prepare ZrO2–Al2O3-A3S2 composites by pressureless sintering. The results showed that when the addition amount of 3Y–ZrO2 is 15 wt%, the bending strength and fracture toughness of the composites sintered at 1600 °C are 152.51 MPa and 3.52 MPa m1/2, respectively. The bending strength increased to 161.83 MPa after 30 thermal shocks (1100 °C to room temperature), with an increase rate of 6.11 %. The ZrO2 crystals are evenly distributed between corundum and mullite crystals, effectively preventing abnormal growth of corundum and mullite crystals, and a small number of pores exist in the samples, which could act as a “buffer zone” to prevent the expansion of cracks in the thermal shock, resulting in high mechanical strength and excellent thermal shock resistance for composites.

Mullite crystallization in zirconia incorporated aluminosilicate ceramics prepared from novel monophasic liquid precursor

This paper deals with the synthesis of a series of zirconoaluminosiloxane (ZAS) as monophasic liquid precursors in toluene for zirconia/mullite ceramics. Precursors are synthesized by the zirconium modification of aluminosiloxane. The effect of zirconium incorporation in the aluminosilicate network is studied by FTIR, NMR, and TG/DTA analysis. 29Si and 27Al clearly indicates that zirconium is chemically bonded to the aluminosilicate network and the network connectivity increases with increase in zirconium content. 27Al NMR also reveals the progressive distortion of octahedral symmetry of AlO6 due to zirconium incorporation, validating the insertion of Zr through Al–O–Zr bonds where Al is four coordinated. These observations attest the monophasic nature and the chemical homogeneity of the synthesized precursors, which is the main factor contributing to the low temperature crystallization of ceramic phases. Ceramic conversion studies of the precursors confirmed the formation of crystalline zirconia/mullite ceramic phases at a low temperature of 900 °C. The XRD and microstructure analysis by SEM/EDX indicated that the mullite crystallization can be controlled by zirconia incorporation. This effect is discussed in terms of the occupancy of zirconia either as intra-granular or inter-granular particles. The zirconia concentration for maximum crystallization of mullite is optimized.

Viability of Bauxite Deposits from Catalonia (Spain) for Ceramic Applications

This study provides a characterization of materials from wastes and outcrops of two inactive bauxite mines located close to Sant Joan de Mediona and Peramola, Spain. Mineralogy was determined via powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Thermal properties were measured via differential thermal analysis–thermogravimetry (DTA-TG) and gresification tests. The crystalline phases are medium-high crystalline kaolinite and variable amounts of illite, quartz, calcite, boehmite, hematite and rutile/anatase. DTA show two endothermic peaks produced by the dehydroxylation of minerals: the first peak, at 530–538 °C, belongs to boehmite; the second peak, at 535–568 °C, corresponds to kaolinite. An exothermic peak at 950–978 °C is associated with mullite crystallization. The optimal sintering temperatures obtained from the gresification curves (firing shrinkage and water absorption) were 970 °C for carbonate-poor, illite-rich clays; 1100 °C for illite- and carbonate-poor samples; and near 1190 °C for carbonate-rich materials. The carbonate-poor samples fired at 1300 °C contain sillimanite and mullite, and the carbonate-rich materials are rich in gehlenite, anorthite, and hedenbergite. The mineralogy of these materials is sufficient to obtain ceramic materials with suitable properties, but not their low plasticity. They could be used in the formulation of bricks or as part of mixtures to produce refractory ceramics.

A kinetic approach for valorisation of hydrodesulphurisation catalyst residue into mullite

Mullite (3Al2O3·2SiO2) is a silicoaluminate widely used for the production of ceramics, and it is usually synthesised from different raw materials with solid-state reactions. In this work, petrochemical industry catalyst waste (Ni–Mo waste) composed principally of alumina and silica has been envisaged as a suitable precursor to prepare mullite via thermal treatment. The kinetics of mullite crystallisation from the catalyst waste were studied with differential thermal analysis (DTA). The starting Ni–Mo waste was an amorphous material, and the crystallisation process takes place at temperatures between 1200 and 1300 °C. The activation energies calculated with isothermal and nonisothermal methods were 1059 and 743 kJ mol-1, respectively. The Avrami exponent (n) obtained with the Ligero method and the parameter m calculated with the Matusita method indicated two-dimensional crystal growth, probably through a diffusion-controlled bulk crystallisation mechanism from a constant number of nuclei. The frequency factor calculated by the isothermal treatment was 8 x 1033 s-1.

Cristobalite Clinker and Paralavas of Ferroan and Melilite–Nepheline Types in the Khamaryn-Khural–Khiid Combustion Methamorphic Complex, East Mongolia: Formation Conditions and Processes

Abstract—Rock samples from the Khamaryn-Khural–Khiid combustion metamorphic (CM) complex, including cristobalite clinker, ferroan tridymite–sekaninaite and cristobalite–fayalite paralavas, which are rock types new to the complex, as well as clinker xenoliths in melilite–nepheline paralava, have been studied in terms of chemistry and mineralogy. The obtained data on rock-forming, minor, accessory, and rare phases (silica polymorphs, cordierite-group minerals, fayalite, Fe and Ti oxides, ferrosilite, etc.) have implications for the formation conditions and processes of the CM rocks. The Raman spectra of sekaninaite, indialite, ferroindialite, mullite, and anhydrous Fe–Ca–Mn phosphate, presumably from the graftonite group, have several specific features. The diversity of mineral assemblages in the CM rocks is due to heterogeneous lithology of the sedimentary protolith and to local effects in the multistage history of the Khamaryn-Khural–Khiid complex. According to geochemical data, all CM rocks of the complex are derived from the Early Cretaceous Dzunbain Formation, their protolith molten to different degrees. The cristobalite clinker and tridymite–sekaninaite and cristobalite–fayalite paralavas were produced by partial melting of pelitic rocks containing different amounts of iron in a wide temperature range. The formation of mullite developed from dehydration–dehydroxylation and incongruent partial melting of amorphous pelitic matter. Large-scale crystallization of mullite in clinker, occurred from the high-silica potassic aluminosilicate melt at >850 °C. Combustion of subsurface coal seams heated the overburden to >1050 °C or locally to >1300–1400 °C (melting point of detrital quartz) or even, possibly, to >1470 °C corresponding to the stability field of β-cristobalite. Melilite–nepheline paralava was formed by incongruent melting of silicate (pelitic) and carbonate (calcite) components of marly limestone under elevated CO2 partial pressure. Oxygen fugacity (fO2) during combustion metamorphism changed from strongly reducing conditions favorable for crystallization of Fe phosphides (barringerite, schreibersite) and metallic iron from silica-undersaturated melts parental to melilite–nepheline paralava to high fO2 values that can maintain the formation of hematite in Fe-rich CM rocks.