Carbonate and silicate phase reactions during ceramic firing

Mineralogical, textural and chemical analyses of clay-rich materials following firing, evidence that initial mineralogical differences between two raw materials (one with carbonates and the other without) influence the textural and mineralogical evolution of the ceramics as T increases from 700 to 1100°C. Mineralogical and textural changes are interpreted considering local marked disequilibria in a system that resembles a small-scale high-T metamorphic process (e.g., contact aureoles in pyrometamorphism). In such conditions, rapid heating induces significant overstepping in mineral reaction, preventing stable phase formation and favoring metastable ones. High-T transformations in non-carbonate materials include microcline structure collapse and/or partial transformation into sanidine; and mullite plus sanidine formation at the expenses of muscovite and/or illite at T 3 800°C. Mullite forms by muscovite-out topotactic replacement, following the orientation of mica crystals: i.e., former (001)muscovite are ^ to (001)mullite. This reaction is favored by minimization of free energy during phase transition. Partial melting followed by fingered structure development at the carbonate-silicate reaction interface enhanced high-T Ca (and Mg) silicates formation in carbonate-rich materials. Gehlenite, wollastonite, diopside, and anorthite form at carbonate-silicate interfaces by combined mass transport (viscous flow) and reaction-diffusion processes. These results may add to a better understanding of the complex high-T transformations of silicate phases in both natural (e.g., pyrometamorphism) and artificial (e.g., ceramic processing) systems. This information is important to elucidate technological achievements and raw material sources of ancient civilizations and, it can also be used to select appropriate clay composition and firing temperatures for new bricks used in cultural heritage conservation interventions. Key-words: Ceramics, carbonates, clay, high-T reactions, gehlenite, mullite, wollastonite, reaction-diffusion fingers, muscovite-out reaction, architectural conservation. 0935-1221/01/0013-0621 $ 3.50 ã 2001 E. Schweizerbart’sche Verlagsbuchhandlung. D-70176 Stuttgart DOI: 10.1127/0935-1221/2001/0013-0621 *E-mail: [email protected] G. Cultrone, C. Rodriguez-Navarro, E. Sebastian, O. Cazalla, M.J. De la Torre ence of and the interactions between various phases that coexist, disappear, or form, are not well established. Little is known on the transformations undergone by silicate and carbonate phases at the reaction interfaces, or the phyllosilicate-out reaction. In particular, there is no clear understanding of the transport mechanisms of reactants (i.e., diffusion vs. mass transport/viscous flow) and the processes involved in mineral transformation at high-T (i.e., solid-state reactions vs. crystallization from a melt). These issues are not solely relevant for understanding ceramics, as there is a close similarity between ceramic formation and the development of reaction textures resulting from marked disequilibrium during pyrometamorphism (i.e., in contact aureoles or in xenoliths where heating rates were very rapid; Brearley, 1986, Brearley & Rubie, 1990; Preston et al., 1999). They also may have important implications in understanding ancient ceramic technologies, or elucidating raw material sources, as well as in the designing of new ceramic materials in general, or appropriate (i.e., compatible) conservation materials (i.e., bricks) for architectural heritage conservation interventions. It is the aim of this work to study the mineralogical, chemical and textural changes of both residual minerals and newly formed phases taking place upon firing, and, in particular, the mechanisms of mineral transformation of raw clay-rich materials with and without carbonates. 2. Materials and methods Two Pliocene clay-rich materials were selected, one from Guadix (G) and the other form Viznar (V), two villages located in the vicinity of Granada, Spain. Raw material was collected, milled and sieved, discarding the fraction with grain size > 1.5 mm. Bricks paste was prepared adding 400 cc of water to 1000 g of raw material. G and V bricks were fashioned using a wooden mould 24.5 ́ 11.5 ́ 4 cm in size, and fired in an air-ventilated electric oven (Hoerotec, CR-35) at the following temperatures: 700, 800, 900, 1000, and 1100°C. T was raised at a heating rate of 3°C per minute, first up to 100°C with one hour soaking time, and later up to the peak T with a soaking time of three hours. Fired brick samples were stored at constant T and relative humidity (RH) conditions of 21°C and 55 %, respectively. Separation of the fractions < 2 μm, 2 to 20, and > 20 μm was performed using a KUBOTA 2000 centrifuge. Grain size distribution in the < 20 μm fraction was determined using a laser-beam particle size analyzer (GALAIh CIS-1). Grain size distribution of > 20 μm fraction was determined using standard ASTM sieves (50 μm up to 1 mm mesh f). The mineralogy of the raw material as well as the mineralogical changes taking place upon firing were studied by powder X-ray diffraction (XRD) using a Philips PW-1710 diffractometer with automatic slit, CuKa radiation (l = 1.5405  ), 3 to 60°2q explored area, and 0.01°2q s-1 goniometer speed. At least three samples (~ 1 g each) of each brick type/firing T were analyzed. They were milled in agate mortar to < 40 μm particle size. XRD analysis of the clay fraction (i.e., fraction with grain size < 2 μm) was performed using oriented aggregates (air-dried, ethylene-glycol and dimethyl sulfoxide solvated, and 1 h heated at 550°C). The bricks texture and microstructure, as well as the progress of mineral transformation and reactions upon firing, were studied by means of optical microscopy (OM) and scanning electron microscopy (SEM; Zeiss DMS 950) coupled with EDX microanalysis. Two thin sections per sample type and firing T were prepared. A polished thin section was prepared using one of them. Both, SEM secondary electron (SE) and backscattered electron (BSE) images were acquired using either small brick pieces (5 ́ 5 ́ 10 mm in size; gold coated), or polished thin sections (carbon coated). The same thin sections were used to analyze small-scale compositional changes of selected minerals following firing by means of an electron microprobe (EMPA; Cameca SX 50). The EMPA working conditions were 20 keV beam energy, 0.7 mA filament current, and 2 μm spot-size diameter. 14 analyses of muscovite crystals (4/700°C; 6/800°C, and 4/1100°C) and 25 of carbonates (6/700°C; 9/800°C, and 10/1100°C) were performed. Albite, orthoclase, periclase, wollastonite and oxides (Al2O3, Fe2O3, and MnTiO2) were used as standards (Govindaraju, 1989). Detection limit for major elements after ZAF correction (Scott & Love, 1983) was 0.01 wt %. Bulk chemical analyses of ceramic materials before and after firing at each target T were performed by means of X-ray fluorescence (XRF; Philips PW-1480). 1 g per raw material or fired brick sample was finely ground and well mixed in agate mortar before being pressed into Al holder for disk preparation. ZAF correction was performed systematically (Scott & Love, 1983). International standards (Govindaraju, 1989) were used thoroughly. The estimated detection limit for major elements was 0.01 wt %. 622