Surface Properties of Sepiolite from Amboseli, Tanzania, and Its Catalytic Activity for Ethanol Decomposition

Magnesite pebbles in Miocene lacustrine conglomerates in northwest Turkey have been partially to totally replaced by sepiolite. Only 5% of the magnesite pebbles have been converted to essentially pure sepiolite; the rest represent mixtures of magnesite and sepiolite. This process of sepiolitization is documented by X-ray diffraction (XRD), chemical, scanning electron microscopy (SEM) and BrunauerEmmett-Teller (BET) techniques. These techniques show that the sepiolitization proceeds from the rim towards the core of the pebbles. The conglomerates, with pebbles of magnesite and ultramafic rocks, were deposited in a near-shore environment on the margin of a large Miocene lake with an ophiolitic substratum. The diagenetic transformation of magnesite to sepiolite is believed to have been caused by the interaction of mixed meteoric and lacustrine waters, which were undersaturated with respect to magnesite. The sepiolitization occurred during the highstands of the lake, when the near-shore conglomerates were flooded by the silica-rich lake waters. The pH of the water during the sepiolitization was probably on the order of 10.5-11.5. The sedimentary magnesite beds in the center of the Miocene basin show no sepiolitization, which is explained by the presence of pore water saturated with respect to magnesite.The magnesite-sepiolite replacement process is chemically modeled as a 4-stage process from dimerization to polymerization.

Section: Surface Area (Bet) Studiesmentioning

confidence: 99%

Diagenetic Transformation of Magnesite Pebbles and Cobbles to Sepiolite (Meerschaum) in the Miocene Eskişehir Lacustrine Basin, Turkey

Ece

1998

“…The cause for these differences was assumed to be a result of differences in the particle size distribution, crystal imperfections and impurities ( 'Nishimura et al 1972, Dandy Copyright 9 1995, The Clay Minerals Society and Nadiye- Tabbiruka 1982, Inagaki et al 1990. This assumption however was not derived from empirical data.…”

Section: Introductionmentioning

confidence: 99%

Model Calculation of Sepiolite Surface Areas

Hibino¹,

Tsunashima²,

Yamazaki

et al. 1995

Abstract--In general, the N2-BET surface areas ofsepiolite samples range from 95 to 400 m2/g depending on deposits.The surface areas of five sepiolites, all varying in crystallite size, were measured on heating, and were compared with a model calculation. A sharp decrease in the surface area, due to crystal folding, was observed between 200* and 400.C. Both before and after the folding, each sepiolite sample had peculiar values. Our model sufficiently explains this difference in surface areas among the samples. In the model, which is based on the Brauner-Preisinger structural model, surface area is a function of the crystaUite size and the ratios of the coverage for nitrogen adsorption on both the internal and external surfaces. These ratios of the coverage can be inversely estimated from the model. The ratios of the coverage on the internal surface are less than 0.19, and that on the external surface between 0.7 and 1.0.

“…More recently, acid hydrolysis reactions of many different clay minerals have been investigated as routes to high surface area silicates with properties well suited as adsorbents, catalysts, and composite filling materials (Gonzalez et al 1982(Gonzalez et al , 1984Dandy and Nadiye-Tabbiruka 1982;Corma et al 1984;Corma and Perez-Pariente 1987;Acosta et al 1984). Octahedral cations such as A13+, Fe 3+, Fe z+, and Mg 2+ can be depleted by treating the clay minerals with acids at elevated temperatures (Johnson et al 1964; with the rates of depletion generally following the order Mg 2+ > Fe 2 § Fe 3+ > AP + Luce et al 1972;Rice and Strong 1974).…”

Section: Introductionmentioning

confidence: 99%

Acid Hydrolysis of Octahedral Mg²⁺ Sites in 2:1 Layered Silicates: An Assessment of Edge Attack and Gallery Access Mechanisms

Kaviratna

Pinnavaia

1994

101

Abstract--The acid hydrolysis products of trioctahedral fluorohectorite and phlogopite have been investigated by XRD, MAS NMR spectroscopy and nitrogen BET surface area analysis in an effort to assess the relative importance of edge attack and gallery access mechanisms. A dramatic loss of X-ray crystallinity and the formation of Q3 and Q" SiO4 sites accompanied the depletion of Mg 2+ from the octahedral sheet of both 2:1 layered structures. Depending on the extent of hydrolysis, the products derived from fluorohectorite exhibited surface areas up to 208 mVg, whereas phlogopite hydrolysis products gave values <20 mY/g. The dramatic differences in surface areas were not related to differences in hydrolysis mechanisms. 19F MAS NMR studies indicated that the hydrolysis of fluorohectorite occurred primarily by an edge attack mechanism equivalent to the hydrolysis pathway for phlogopite. A gallery access mechanism contributed to the hydrolysis of fluorohectorite only at the later stages of octahedral Mg 2+ depletion. Solvation effects appeared to be important in determining the surface areas of the reaction products derived from the swelling (fluorohectorite) and non-swelling (phlogopite) precursors.