The crystal structure of fl-FeSi2 has been determined by three-dimensional methods, using the Patterson function, Fourier synthesis and least-squares refinement. The final reliability index R is 0.043 with 569 reflexions. The structure, which belongs to the distorted fluorite structure type CI is compared with those of dilicides, digermanides and distannides of the group VIII metals. The structure is held by Fe-Si and Fe-Fe bonds. Each iron is bound to eight Si and two Fe atoms.

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Three clay samples (a kaolinite, a halloysite and an inseparable mixture of kaolinite-palygorskite-illite-montmorillonite) were, after a mild pretreatment, successfully tested as catalysts in the polymerization of styrene. The catalytic activity of the clay (measured by the conversion rate), the MW and the colour of the polystyrene obtained depended not only on the chemical composition and the structure of the clay catalyst but also on its concentration in the medium. The polystyrenes obtained on the clay catalyst were composed of two groups of macromolecules, one with low MW and the other with high MW.

: The study of 13C NMR spectra of polystyrene thermally obtained on clays shows that thermal polymerization of styrene on these minerals proceeds by two parallel reaction mechanisms: the radical mechanism due to the thermal effect, and the cationic mechanism in which the initiatory cation H + is provided by the clay. The latter mechanism, accelerated by increase in temperature, becomes more important as the clay content increases in the reaction medium. Otherwise, the thermal polymerization on clays is accompanied by the hydrogenation of some g carbons of the polystyrene molecule and the oxidation of certain aromatic carbons.In a previous study, it was shown that mildly acid-treated clays catalyse the thermal polymerization of styrene. The gel permeation chromatograms of the polystyrenes formed revealed the intervention of two polymerization mechanisms (Njopwouo et al., 1987). Heat acts on polystyrene by producing free radicals to start polymerization, and in this case, the radical mechanism is evident. A supplementary polymerization test was carried out at ambient temperature to demonstrate the intervention of the other mechanism; the control tests (without clay), and those with the three clays used in the preceding study showed that at the same clay catalyst concentration P (P = 0.8%), polymerization of styrene started after 3 days on Bo4 (KPIM* mixture), 4 days on Bal (halloysite) and 5 days on Bol (kaolinite) with conversion rates z of 1-9%, 1-5% and 1.0%, respectively. In the control test, polymerization began after 7 days. These results showed that clays could initiate the polymerization of styrene by a non-radical mechanism. A cationic mechanism can be envisaged, and according to Fripiat et al. (1965Fripiat et al. ( , 1971, silico-alumina surfaces possess Lewis and BrSnsted acid sites, the latter being predominant in hydrated silico-alumina (as in clays). In other respects Conley & Althoff (1971) showed that acid-treated kaolinite possesses BrSnsted acid sites (H3 O+) on its surface at low temperature due to silanol groups, and Lewis acid sites (A13+) at high temperature, from the octahedral gibbsitic sheet. In the trials reported, the acid sites, especially Brfnsted style, could be responsible for the cationic polymerization mechanism occurring in parallel with the radical mechanism. The present study is aimed at confirming the presence of these two mechanisms from observations on ~3C nuclear magnetic resonance (NMR).* KPIM --kaolinite-palygorskite-illite-montmorillonite.9 1988 The Mineralogical Society

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