Carbonate-cancrinite: In-situ real-time thermal processes studied by means of energy-dispersive X-ray powder-diffractometry P. Ballirano a) and A. Maras Dipartimento di Scienze della Terra, Universita di Roma La Sapienza, P.le Aldo Mow 5, 1-00185, Roma, Italy R. Caminiti and C. Sadun Dipartimento di Chimica, Universita di Roma La Sapienza, P.le Aldo Mow 5, 1-00185, Roma, Italy (Received 30 June 1994; accepted for publication 26 November 1994) New powder X-ray data for cancrinite [ideally Na 8 Si 6 Al 6 0 24 (CO 3 ) 2 -2 H 2 O] are reported along with in-situ real-time thermal processes recorded using energy dispersive X-ray diffractometry (EDXD). A completely anhydrous phase is obtained after heating the sample up to 600 °C and quickly cooling it to room temperature, as shown by means of both Rietveld analysis and IR spectroscopy. The anhydrous phase does not show any tendency to re-acquire molecular water. During the heating process, at around 450 °C, a peak splitting is observed, possibly due to a reversible phase transition. I. INTRODUCTION Cancrinite is the parent of the homonymous group of minerals. Its structure is relatively well known (Jarchow, 1965; Smolin et al., 1981; Emiraliev and Yamzin, 1982; Grundy and Hassan, 1982) showing an ordered Si-Al distri- bution inside the tetrahedra constituting a framework. The AB-type stacking sequence gives rise to two cancrinite cages and a large channel for every unit cell. The cancrinite cages contain a [NaH 2 O] cluster, whereas the large channel houses the remaining cations (Na,Ca) and anions (CO 3 ,SO 4 ,F,C1): the empirical formula can be written out as [NaH 2 O] 2 [Na 6 (CO3) 2 ][Si6Al6O 24 ] highlighting the relevant structural units. Cancrinite is commonly nonstoichiometric and the ordering of vacancies inside the channel is consid- ered to be the reason for the occurrence of a wide variety of superstructures (Grundy and Hassan, 1982; Hassan and Buseck, 1992). A large number of isostructural phases are also commonly synthesized (Belokoneva et al., 1986) and studied by means of powder techniques (Norby et al., 1991). The powder diffraction data (PDF 25-0776) refer to Foit etal. (1973). II. EXPERIMENTAL The sample studied is from Satom Quarry (Cameroon). Microprobe analysis was conducted on a Cameca SX50, op- erating in full WDS mode, with the following experimental setup: accelerating voltage 15 kV, beam current 15 nA. In order to minimize the alcalies' loss, a beam diameter of 10 fim and a counting time of 10 s were used. The raw data were corrected on-line for drift, dead time, and background using a standard ZAF procedure. IR spectroscopy was done using the KBr pellet technique on a Perkin Elmer FT 1760. The IR spectra were obtained averaging 128 scans and using a 4 cm"' nominal resolution. XRD data were collected in summation step-scan mode in the 6-100° 20 range on a Seifert PAD IV 2002 equipped with two Soller slits, a 1° divergence slit, a 0.2° receiving slit, and a diffracted-beam pyrolitic graphite mono- a) Present address: Dipartimento di Scienze della Terra, Universita di Pisa, V. S. Maria 53,1-56126, Pisa, Italy. chromator. External standards, NBS675 (fluorophlogopite) and NBS640 (silicon), were used to minimize the effects of severe overlapping of standard and sample peaks. Cell pa- rameters were calculated using both DBWS program (Wiles and Young, 1981) and the Appleman and Evans (1973) least- squares program, modified for PC computers by Benoit (1987). A prototype of an energy dispersive (ED) diffractometer (Caminiti, 1992) in did geometry, equipped with a vacuum chamber fitted with a heating system was used to study the effect of thermal processes on the cancrinite sample. The powdered specimen was located inside the sample holder forming an angle of 3° with the direct beam, corresponding to the 5-35° 20 angular range for CuKa radiation. In EDXD a continuous polychromatic X-ray beam is required and the diffracted beam is energy resolved by a solid-state detector set at a suitable scattering angle. Using the properties of EDXD technique a significant portion of the diffracted spec- trum can be collected simultaneously allowing the evolution of the structure to be studied in real time. As polychromatic radiation the bremsstrahlung of a tungsten tube was used with the power supply kept at 55 kV and 35 mA. An intrinsic Ge solid state detector with energy resolution of about 200 eV at 6.5 keV was used. X-ray data were collected by a multichannel analyzer with 1024 channels and were stored in a PC. Escape peaks from Ge did not perturb the spectra significantly. III. RESULTS AND DISCUSSION Microprobe analysis shows that our sample is non- stoichiometric with a 0.8 anion and cation deficiency (Table I): according to the analysis the empirical formula is Na 6 . 3 Ca 0 . 9 Si 6 Al 6 O 24 (CO 3 ) ,F 0 . 2 • 2(H 2 O). Carbonate and water content were calculated according to the procedure fully described in Maras and Ballirano (1994). The presence and the position of molecular water was studied using Fourier /ransform mfrared (FT-IR) spec- troscopy (Figure 1): The occurrence of two well-developed absorption bands located at 3607 and 3539 cm" 1 points out to 0 Framework ...O H o distances of around 3.1 and 3.0 A (Fig- ure 2), according to the Nakamoto plot (Nakamoto etal., 173 Powder Diffraction 10 (3), September 1995 0885-7156/95/10(3)/173/5/$6.00 ' 1995 JCPDS-ICDD 173