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

Abstract

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.

On the Chemical Composition of Sepiolite and Palygorskite

“…The MER, TPO, and SAN sepiolites have a structural formula close to the theoretical, both for the tetrahedral and the octahedral content. The MER sample is the most 'perfect' sepiolite from these data, in good agreement with those published by Ece (1998) for sepiolite from the same locality. Twelve samples contain variable amounts of tetrahedral AI, with >7 atoms of Mg p.h.u.c.…”

Characterization of two forms of sepiolite and related Mg-rich clay minerals from Yenidoğ an (Sivrihisar, Turkey)

“…It was a very poorly crystallized sepiolite with fairly broad and poorly resolved reflections and with the 110 reflection appearing at 12.3 Å . Sepiolites with fairly similar XRD patterns with higher d values of the 110 reflection were reported in later studies (Yeniyol & Ö ztunalı, 1985;Yeniyol, 1986Yeniyol, , 1993aEce & Ç oban, 1994;Yalçın & Bozkaya, 1995;Chahi et al, 1997;Ece, 1998). These works describe poorly-ordered sepiolite having broad 110 reflections with high d values, showing a convoluted single reflection in the 26À30º2y region and remaining reflections which are not well resolved, rather similar to the work of Brindley (1959).…”

ORIGIN OF SEPIOLITE AND LOUGHLINITE IN A NEOGENE VOLCANO-SEDIMENTARY LACUSTRINE ENVIRONMENT, MIHALlCClK ESKISEHIR, TURKEY

“…If there were any transitional phase between these minerals, there would be a gradational contact between sepiolite-and loughlinite-dominated claystone. Moreover, SEM images also indicate that loughlinite developed as a network of fibers on the inner surfaces of dissolution voids in domains of volcanic glass, in contrast to sepiolite, which occurs as bridging fibers between crystallites of dolomite, indicating that these minerals are formed authigenically under different physicochemical environmental conditions rather than resulting from a transformation process from one to the other (Galán & Castillo 1984, Jones 1986, Jones & Galán 1988, Ece & Çoban 1994, Ece 1998, Singer et al 1998. On the other hand, Wollast et al (1968) observed the precipitation of sepiolite under laboratory condi-tions, but they did not explain the precipitation of loughlinite.…”