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Clay Minerals; September 2008; v. 43; no. 3; p. 393-403; DOI: 10.1180/claymin.2008.043.3.05
© 2008 Mineralogical Society of Great Britain and Ireland
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Elucidating the crystal-chemistry of Jbel Rhassoul stevensite (Morocco) by advanced analytical techniques

B. RHOUTA1,*, H. KADDAMI2, J. ELBARQY1,2, M. AMJOUD1, L. DAOUDI3, F. MAURY4, F. SENOCQ4, A. MAAZOUZ5 and J.-F. GERARD5

1 Laboratoire de Matière Condensée et Nanostructures (LMCN), Faculté des Sciences et Techniques Guéliz, BP 549, Marrakech, Morocco, 2 Laboratoire de Chimie Bioorganique et Macromoléculaire, Faculté des Sciences et Techniques Guéliz, BP 549, Marrakech, Morocco, 3 Laboratoire de Géosciences et Géoenvironnement, Faculté des Sciences et Techniques Guéliz, BP 549, Marrakech, Morocco, 4 CIRIMAT; UMR 5085 CNRS-UPS-INP, ENSIACET, 118 Route de Narbonne, 31077 Toulouse, France, and 5 LMM-INSA-Lyon, 20 Av A Albert Einstein, 69621 Villeurbanne, France

* E-mail: brhouta{at}gmail.com or rhoutab{at}yahoo.fr

(Received 21 December 2006; revised 17 March 2008)

The composition of Rhassoul clay is controversial regarding the nature of the pure-mineral clay fraction which is claimed to be stevensite rather than saponite. In this study, the raw and mineral fractions were characterized using various techniques including Fourier transform infrared spectroscopy and magic angle spinning nuclear magnetic resonance (MAS NMR). The isolated fine clay mineral fraction contained a larger amount of Al (>1 wt.%) than that reported for other stevensite occurrences. The 27Al MAS NMR technique confirmed that the mineral is stevensite in which the Al is equally split between the tetrahedral and octahedral coordination sites. The 29Si NMR spectrum showed a single unresolved resonance indicating little or no short-range ordering of silicon. The chemical composition of the stevensite from Jbel Rhassoul was determined to be ((Na0.25K0.20)(Mg5.04Al0.37Fe0.20{square}0.21)5.61(Si7.76Al0.24)8O20(OH)4). This formula differs from previous compositions described from this locality and shows it to be an Al-bearing lacustrine clay mineral.

KEYWORDS: Rhassoul, stevensite, structural characterization, structural composition, vacancies, short-range ordering, FTIR, MAS NMR






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