223 macla nº 9. septiembre ‘08 revista de la sociedad española de mineralogía A TEM and 2D-XRD Study of the Thermal Decomposition of Calcite /ENCARNACIÓN RUIZ-AGUDO (*), CARLOS RODRÍGUEZ-NAVARRO, ANA LUQUE, ALEJANDRO RODRÍGUEZ-NAVARRO, MIGUEL ORTEGA HUERTAS Departamento de Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, Fuentenueva s/n 18002 Granada (España) INTRODUCTION. The thermal decomposition of calcium carbonate results in the formation of lime (CaO) and the release of CO2 (Boyn- ton 1980). Lime, which has been tradi- tionally used for building purposes (Elert et al. 2001), is nowadays also employed in agriculture, food processing, disinfec- tion and disease control, water treat- ment, flue-gas desulfuration, steelmak- ing, plastics and glass fabrication, and sugar refining (Boynton, 1980). The thermal decomposition of calcite also plays a role in a number of geologic processes such as high grade metamor- phism, pyrometamorphism and meteor- ite impacting (Best, 1982; O’Keefe and Ahrens, 1989; Grapes, 2006). Despite the numerous efforts dedicated to the understanding of this apparently simple decomposition reaction, the actual atomic scale reaction mechanism re- sponsible for the observed textural rela- tionships between reactant and product phases is not fully understood (Beruto et al., 2004). Here we have studied the thermal decomposition of Iceland Spar single crystals in air, at T ranging from 600 up to 1150°C. It is the aim of this work to elucidate by means of transmis- sion electron microscopy (TEM) and selected area electron diffraction (SAED) analysis, as well as texture X-ray diffrac- tion (2D-XRD) analysis, if the transfor- mation mechanism is a solid state topo- tactic reaction as has been suggested, but never convincingly proved (Beruto et al. 2004). MATERIALS AND METHODS. Calcite crystals (Iceland spar, from Mex- ico) were calcined in an air-ventilated electric furnace (Select-Horn, Selecta), cooled at room T and kept in dry N2 atmosphere vials. An X-ray diffraction (XRD) analysis of the phase evolution with T, was performed on a Philips PW- 1710 diffractometer with an automatic slit, Cu Kα radiation (λ = 1.5405 Å), 20 to 80 °2θ explored area, with steps of 0.03 °2θ and 0.03 °2θ s -1 goniometer speed (static mode, 1 s counting time). The evolution of CaO crystallite size with T was calculated from peak broadening analysis using the XPowder software package (Martin-Ramos, 2004). Pole figures describing the 3-D orientation relationships between calcite pseudo- morphs and product lime crystals were determined using a X-ray single crystal diffractometer equipped with an area detector (D8 SMART APEX, Bruker). The working conditions were: Mo Kα (λ = 0.7093 Å), 50 kV and 30 mA, a pin-hole collimator of 0.5 mm in diameter, and an exposure time of 20 s per frame. Iceland spar pseudomorphs were meas- ured by reflection. A set of frames (2D diffraction patterns) was registered while rotating the sample around ϕ angle. Pole densities for the strongest lime reflections were calculated and displayed in stereographic projection using the XRD2DScan software (Rodri- guez-Navarro, 2007). Ex situ TEM obser- vations and SAED analyses of calcite pseudomorphs were performed on a Philips CM20 operated at 200 kV. In situ decomposition of calcite crystals (about 5 µm in size) due to electron beam damage was also observed in the TEM (i.e., high vacuum conditions). RESULTS AND DISCUSSION. XRD Analysis of Phase Evolution with T The products of the thermal decomposi- tion of calcite single crystals retained the external shape of the {10 1 4} rhom- bohedron, as it has been already indi- cated by several researchers (Beruto et al., 2004). Partially decomposed crystals are formed by a core of unaltered car- bonate, surrounded by a lime brownish shell. The reaction occurs through the formation and advance of a reactant- product interface through which the CO2 diffuses outside (Beruto et al., 2004). The former observation suggests that there is a crystallographic control in the interface advancement towards the nucleus of the crystal. CaO peaks were first detected in XRD patterns of powdered samples at 750°C. Calcite Bragg peaks disap- peared at T > 800°C. At this point, the main CaO peaks were clearly observed. Upon further increase the temperature, CaO crystallite size increased from 27 nm (800°C) up to 76 nm (1150°C) while peak breadth decreased, which suggests that a sintering process oc- curred. The lime 220 reflection was the most intense in the XRD pattern of non grinded pseudomorphs (instead of 200, as in powdered samples). This suggests a preferred orientation of CaO crystals, which was further confirmed by pole figures of (111), (200) and (220) planes of CaO, obtained from 2D-XRD single- crystal analysis (Fig. 1). In particular, {110}lime planes were parallel to {10 1 4}calcite (Fig. 1a). Curiously, pole figure of 200 lime planes showed 4 maxima instead of the 2 expected for a single crystal. This suggests that two set of CaO crystals with {110}lime //{10 1 4}calcite (oriented at an angle of palabras clave: calcita, descomposición térmica, nanocristales de CaO, TEM-SAED, 2-D XRD, agregación orientada, topotáctica. key words: calcite, thermal decomposition, CaO nanocrystals, TEM- SAED, 2-D XRD, oriented aggregation, topotactic. resumen SEM/SEA 2008 * corresponding author: encaruiz@ugr.es fig 1. Pole figures of calcite pseudomorphs calcined at 850°C. Note the well defined preferred orientation of: a) (110); b) (111); and c) (100) planes. Note also the existence of two sets of crystals with (100) planes rotated ca. 80 ° with respect to each other resulting in 4 maxima in the pole figure shown in (c). Due to experimental constrains, only pole figures corresponding to planes with ρ < 85° are represented.