Abstract
The conditions of formation and the structural formula of a smectite fluoride (tetrasilicic, trioctahedral fluorosilicate of type 2/1 which undergoes inter-layer hydratation) have been specified. The fluorine, in the synthetic smectite, replaces the hydroxyl group in the corresponding natural mineral. The smectite was synthesized as the major phase using several starting compositions : a single phase product has not been obtained by synthesis. There are two conditions necessary for the synthesis of a phase with swelling properties : a neutral or reducing environment for the melt and rapid cooling. X-ray diffraction, chemical studies and infra-red spectroscopy permit the characterisation of the phase. The formula is Na⁺₁,₈₀ (Mg²⁺₅,₁ Si⁴⁺₈ O²⁻₂₀ F⁻₄) and measurements of the exchange capacity give the structural formula : Na⁺₁,₁₀[xii] [(Mg⁺₅,₁ Na⁺₀,₇₀ ⃞₀,₂₀)[vi] (Si⁴⁺₈)[iv] O²⁻₂₀ F⁻₄]. This formula corresponds both to a « fluor-hectorite » and to a « fluorostevensite » by analogy with the natural minerals.
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