Integration of iron in natural and synthetic Al-pyrophyllites: an infrared spectroscopic study

Sébastien Lantenois 1,2 * Jean-Michel Bény1 Fabrice Muller1 Rémi Champallier1

1 Institut des Sciences de la Terre d’Orléans (ISTO), CNRS – Université d’Orléans, 1A rue de la Férollerie, 45071 Orléans Cedex 2, France.

2 Laboratoire des Agrégats Moléculaires et Matériaux Inorganiques (LAMMI), CNRS – Université Montpellier 2, Bât 15 – Case courrier 015, Place Eugène Bataillon, 34095 Montpellier CEDEX 5 – France.

• Author to whom correspondence should be addressed. e-mail: sebastien.lantenois@univ-orleans.fr

2 ABSTRACT Numerous studies focus on the relationships between chemical composition and OHband positions in the infrared (IR) spectra of micaceous minerals. These studies are based on the coexistence, in dioctahedral micas or smectites, of several cationic pairs around the hydroxyl group which each produce a characteristic band in the IR spectrum. The aim of this work is to obtain the wavenumber values of the IR OH vibration bands of the (Al-Fe3+)-OH and (Fe3+- Fe3+)-OH local cationic environments of ‘pyrophyllite type’ in order to prove, disprove or modify a model of dioctahedral phyllosilicate OH-stretching band decomposition. Natural samples are characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopies and electron microprobe; the hydrothermal synthesis products are also analysed by powder XRD and FTIR after inductively coupled plasma measurements to obtain the chemical compositions of nascent gel phases. Natural samples contain some impurities which were eliminated after acid treatment; nevertheless, a small Fe content is found in the pyrophyllite structure. The amount of Fe which is incorporated within the pyrophyllite structure is much more important for the synthetic samples than for the natural ones. The IR OH bands were clearly observed in both natural and synthetic pyrophyllites and assigned to hydroxides bonded to (Al-Al), (Al-Fe) and (Fe-Fe) cationic pairs. During this study, three samples were analysed by DTG to check the cis- or trans- vacant character of the layers and to determine the influence of this structural character on the OH-stretching band position in IR spectroscopy.

KEYWORDS: Pyrophyllite, hydrothermal syntheses, FTIR spectroscopy, iron defaults, cis- trans-vacant.

3 INTRODUCTION

Dioctahedral phyllosilicates have a wide spectrum of chemical composition [1] and their cations distribute with different degrees of order – disorder in the tetrahedra and octahedra of the sheet structure. Infrared spectroscopy is an efficient tool for the determination of local cationic environments as well as fine structural features. The main relationships between chemical composition and OH bands positions in infrared spectra of various dioctahedral materials have been established for celadonites and glauconites [2-5], micas [6-8] or smectites [9-14]. With the computer science development it is really possible to decompose the broad infrared band of O-H vibrations of phyllosilicates. The first model of (cation-cation)-OH vibrations corresponding to different wavenumber bands in the OH stretching range was proposed by Besson et al. [6-7] for dioctahedral trans-vacant mica and more recently adapted to dioctahedral smectites by Zwiagina et al. [13]. These models were based on the coexistence in dioctahedral micas or smectites of (cation-cation)-OH bands linked to an interlayer cation (mica-type bands) and three bands not linked to an interlayer cation (pyrophyllite type bands) [6]; Al-Al-OH, Al-Fe3+-OH and Fe3+-Fe3+-OH bands. The Al-Al-OH stretching band position in pyrophyllite (Si4Al2O10OH2) is clearly identified [2, 15- 17] in comparison with the Fe-pyrophyllite OH bands ones. The scarce occurrence of natural ferripyrophyllite (iron rich equivalent of pyrophyllite, described by Choukhrov et al. [18]) and the very low content of iron substitution in natural Al-pyrophyllite structure explain the lack of Fe-pyrophyllite infrared studies. The identification of these OH pyrophyllite vibrations are necessary to approve disapprove or modify the model of dioctahedral phyllosilicate OH stretching decomposition. The aim of this study is to identify the infrared OH vibration bands in Fe-pyrophyllite structure. Two complementary approaches were carry-out: the first one consisting to the study

4 of natural Fe-pyrophyllites and the second one using hydrothermal synthetic samples. These approaches have been realized with fast Fourier transform infrared (FTIR) spectroscopy coupled with chemical analyses, transmission electron microscopy (TEM), powders X-ray diffraction (XRD) and Raman spectroscopy.

MATERIALS AND METHODS

Materials

A variety of natural and synthetic samples were used in this study. The natural samples are described in Table 1. Synthetic pyrophyllites we

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