Mechanisms of Palygorskite and Sepiolite Alteration as Deduced from Solid-State ²⁷Al and ²⁹Si Nuclear Magnetic Resonance Spectroscopy

Abstract

Abstract--The mechanisms of palygorskite and sepiolite alteration to smectite under mild hydrothermal conditions were investigated by solid-state 27A1 and 29Si magic-angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy, X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Palygorskite altered to smectite in the presence of NaOH at 150"C. 27A1 MAS-NMR spectroscopy showed that the Al coordination changed from chiefly octahedral in palygorskite to chiefly tetrahedral in the smectite product. 29Si MAS-NMR spectroscopy showed that the nearest neighbor environment of Si also changed when palygorskite altered to smectite. The XRD data showed that the synthetic smectite is trioctahedral in nature with tetrahedral charge. The TEM results revealed that the needle-like morphology of palygorskite was preserved in the product smectite. The MAS-NMR results in conjunction with the above XRD and TEM studies suggest that the mechanism of palygorskite alteration was a dissolution and recrystaUization process rather than a solid-state reorganization to form 2:1 layer silicate units from the preexisting chain structure. Sepiolite altered to smectite in the presence of 2 N salt solutions at 300*(2. The trioctahedral nature of the product smectite as detected by XRD and the foil-like morphology of product smectite as shown by TEM suggest that the mechanism of sepiolite transformation to smectite was also a dissolution and recrystaUization process. The tetrahedral A! coordination detected by 27Al MAS-NMR in the smectite altered from sepiolite corroborated the XRD and TEM results.

Synthesis of Smectite Clay Minerals: A Critical Review

“…The addition of MgCO3 strongly increased the amount of Mg 2 § in solution and, consequently, favored the formation of saponite. Based on the XRD and NMR results, both Komarneni (1989) and Golden and Dixon (1990) concluded that a dissolution and reprecipitation mechanism was responsible for the saponite formation. Images obtained by TEM revealed that the newly formed saponite had a lath-like morphology similar to that of palygorskite.…”

“…Images obtained by TEM revealed that the newly formed saponite had a lath-like morphology similar to that of palygorskite. Komarneni (1989) suggested a nucleation and epitaxial-growth mechanism for saponite on the surface of the palygorskite remnants. Hydrothermal experiments with sepiolite at 300~ and 300 bars in the presence of CsC1, MgC12, and CaC12 (Komarneni, 1989) revealed a similar dissolution and reprecipitation mechanism, but the nucleation and epitaxial-growth mechanism was absent; no explanation was given for this absence.…”

“…Orlemann (1972) used talc as an inexpensive starting material for the formation of hectorite. The disadvantage of this method, however, is that talc must first be calcined in the presence of Li2CO 3 at high temperatures before the remaining procedure is performed at 185~ Komarneni (1989) and Golden and Dixon (1990) studied the formarion of saponite from palygorskite and sepiolite and showed that the addition of Mg is essential for smectite formation. This was also shown by Levinson and Vian (1966) and Levinson and Day (1968), who studied the formation of montmorillonite from mixtures of several carbonates, kaolinite, and quartz.…”

Application of27Al NMR techniques to structure determination in solids

“…Pyrophyllite dehydroxylate has been shown to have a well defined Al(5) environment from XRD and from the 14.1 T NMR speetrum, whieh was fitted to a quadrupolar lineshape [180] but field dependent simulations have pointed out some discrepancies [58]. Other systems studied include pyrophyllite [181], montrnorillonite [182], muscovite [183], imogolite [184,185], polygarskite/sepiofite [186] and laponite [187] have been reported. The transformation in the pillaring of clays has also been followed [188][189][190].…”

Hydrothermal alteration of a saponitic bentonite: mineral reactivity and evolution of surface properties

“…The undesirable cementing processes that would be produced seem to be inhibited by the transformation (reconstitution or dissolution-precipitation) under hydrothermal conditions of sepiolite into stevensite or saponite. This reaction has been documented previously by Güven & Carney (1979) and Komarneni et al (1989) using a solid:water (s:w) ratio of <1:1 at 3008C. This s:w ratio is far from the 1:20 excess water used in the preliminary study on the hydrothermal reactivity of the Madrid Basin saponitic clays.…”