Thermal behaviour of tobermorite from N'Chwaning II mine (Kalahari Manganese Field, Republic of South Africa) II. Crystallographic and spectroscopic study of tobermorite 10 A

摘要

X-ray diffraction studies of "tobermorite 10 A", obtained through heating of the "anomalous" tobermorite 11 A from N'Chwaning II mine (Kalahari Manganese Field, RSA), show the widespread structural disorder that arises during the transformation from tobermorite 11 A to the 10 A phase. The diffraction pattern of the final phase points to an orthorhombic (or pseudoorthorhombic) "family cell" with a 11.2, b' 3.67, c 20.1 A. The b' unit-cell parameter is typical for the average structure of the members of the tobermorite group. Completely streaked reflections double the b' periodicity, indicating that the true b parameter is equal to 7.3 A, which is characteristic for wollastonite-like chains. The a unit-cell parameter of the "family cell" of tobermorite 10 A is doubled with respect to that of the other tobermorite-group minerals (11.2 instead of 5.6 A). Through the application of the OD theory, a structural model is proposed and the atomic positions of one of the two possible MDO polytypes have been refined, using a set of 399 reflections, up to a R1 = 15.7 %. Notwithstanding the quite high reliability index, the main structural features of tobermorite 10 A could be observed. Its structure consists of layers of six- and seven-fold coordinated Ca cations, linked together by double silicate chains, with a conformation similar to that present in xonotlite. Additional information has been obtained through 29Si nuclear magnetic resonance (NMR) and micro-Raman spectroscopies. 29Si Single Pulse Excitation Magic Angle Spinning (SPEMAS) spectra show the occurrence of two main sets of peaks, related to Q2 and Q3 sites, respectively, confirming the presence of double silicate chains. An additional weak peak at -106 ppm, typical of Q4 sites, may be interpreted as due to minor amorphous silica. In addition, the 29Si Cross-Polarization Magic Angle Spinning (CPMAS) spectra indicate that H20 is present in tobermorite 10 A and that Si-OH bonds are present prevalently in the bridging Q3 tetrahedra. The results of the micro-Raman spectroscopy are in agreement with those of the NMR study, indicating the occurrence of double chains. The observed conformation of the double chains can be easily reached through the shrinking of clinotobermorite-like chains. An intermediate clinotobermorite-like phase was actually observed during the topotactic transformation from tobermorite 11 A to tobermorite 10 A. The comparison between structural, X-ray powder diffraction, and micro-Raman spectroscopic data on tobermorite 10 A and oyelite allows to exclude that oyelite may be the natural counterpart of tobermorite 10 A. On the contrary, similarities between the tobermorite 10 A obtained in this study and its analogue obtained in hydrothermal synthesis experiments are clear.