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Thermal behaviour of tobermorite from N'Chwaning II mine (Kalahari manganese field, Republic of South Africa); II, Crystallographic and spectroscopic study of tobermorite 10 Aa

Cristian Biagioni, Elena Bonaccorsi, Stefano Merlino, Danilo Bersani and Claudia Forte
Thermal behaviour of tobermorite from N'Chwaning II mine (Kalahari manganese field, Republic of South Africa); II, Crystallographic and spectroscopic study of tobermorite 10 Aa
European Journal of Mineralogy (October 2012) 24 (6): 991-1004

Abstract

X-ray diffraction studies of "tobermorite 10 Aa", obtained through heating of the "anomalous" tobermorite 11 Aa from N'Chwaning II mine (Kalahari Manganese Field, RSA), show the widespread structural disorder that arises during the transformation from tobermorite 11 Aa to the 10 Aa phase. The diffraction pattern of the final phase points to an orthorhombic (or pseudo-orthorhombic) "family cell" with a 11.2, b' 3.67, c 20.1 Aa. 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 Aa, which is characteristic for wollastonite-like chains. The a unit-cell parameter of the "family cell" of tobermorite 10 Aa is doubled with respect to that of the other tobermorite-group minerals (11.2 instead of 5.6 Aa ). 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 R (sub 1) = 15.7 %. Notwithstanding the quite high reliability index, the main structural features of tobermorite 10 Aa 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 (super 29) Si nuclear magnetic resonance (NMR) and micro-Raman spectroscopies. (super 29) Si Single Pulse Excitation Magic Angle Spinning (SPEMAS) spectra show the occurrence of two main sets of peaks, related to Q (super 2) and Q (super 3) sites, respectively, confirming the presence of double silicate chains. An additional weak peak at -106 ppm, typical of Q (super 4) sites, may be interpreted as due to minor amorphous silica. In addition, the (super 29) Si Cross-Polarization Magic Angle Spinning (CPMAS) spectra indicate that H (sub 2) O is present in tobermorite 10 Aa and that Si-OH bonds are present prevalently in the bridging Q (super 3) 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 Aa to tobermorite 10 Aa. The comparison between structural, X-ray powder diffraction, and micro-Raman spectroscopic data on tobermorite 10 Aa and oyelite allows to exclude that oyelite may be the natural counterpart of tobermorite 10 Aa. On the contrary, similarities between the tobermorite 10 Aa obtained in this study and its analogue obtained in hydrothermal synthesis experiments are clear.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 24
Serial Issue: 6
Title: Thermal behaviour of tobermorite from N'Chwaning II mine (Kalahari manganese field, Republic of South Africa); II, Crystallographic and spectroscopic study of tobermorite 10 Aa
Affiliation: Universita di Pisa, Dipartimento di Scienze della Terra, Pisa, Italy
Pages: 991-1004
Published: 201210
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 47
Accession Number: 2013-002485
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
S27°27'35" - S27°27'35", E23°24'45" - E35°23'45"
Secondary Affiliation: Universita di Parma, ITA, ItalyIstituto di Chimica dei Composti Organo Metallici, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201302
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