Singer / Galan Developments in Palygorskite-Sepiolite Research

A New Outlook on these Nanomaterials
1. Auflage 2011
ISBN: 978-0-444-53608-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, Band Volume 3, 520 Seiten

Reihe: Developments in Clay Science

The aim of this comprehenvise book is to present the most important results achieved in the research of the clay minerals palygorskite and sepiolite. Palygorskite and sepiolite have found to be useful in a huge variety of industrial and medical applications. As a result, research on these clays has been intensified during the last two decades, and important advances in their characterization have been made. The book contains contributions from distinguished scientists in the field. - Comprehensive treatment of palygorskite and sepiolite - Cutting-edge developments in industrial minerals and applications - Written by distinguished scientists in the field

Autoren/Hrsg.

Singer, Arieh

Galan, Emilio

Leseproben

Developments in Palygorskite-Sepiolite Research, Vol. 3, No. Suppl C, 2011 ISSN: 1572-4352 doi: 10.1016/B978-0-444-53607-5.00002-5 Chapter 2 Advances in the Crystal Chemistry of Sepiolite and Palygorskite Mercedes Suárez* and Emilia García-Romero†‡ * Área de Cristalografía y Mineralogía, Departamento de Geología, Universidad de Salamanca, 37008 Salamanca, Spain † Departamento de Cristalografía y Mineralogía, Universidad Complutense de Madrid, Facultad de Geología, Ciudad Universitaria, Madrid, Spain Instituto de Geociencias (UCM-CSIC), Ciudad Universitara, 28003 Madrid, Spain ‡ Instituto de Geociencias (UCM-CSIC), Ciudad Universitara, 28003 Madrid, Spain Abstract The structure and chemical composition of sepiolite and palygorskite are known from the first half of the twentieth century. Ideal palygorskite Si8O20(Al2Mg2)(OH)2(OH2)4·H2O)4 has dioctahedral character, and sepiolite Si12O30Mg8(OH)4(OH2)4(H2O)8 is a pure trioctahedral mineral. In the two minerals, isomorphic substitutions in both tetrahedral and octahedral sheets are frequent. In addition, Mg-rich palygorskites, Fe-rich palygorskites and Al-rich sepiolites have been reported and it seems that the compositional limits accepted until now could be too narrow. Therefore, sepiolite and palygorskite can be classified into different types depending on the composition. In this chapter, the variations in the chemical composition of natural sepiolites and palygorskites as well as the limit of chemical composition of both minerals, if it exists, using the data from the literature available are established. Sepiolite can be classified into two types: sepiolite and Al-sepiolite. A limit for these two types can be established from the octahedral occupancy, and Al-sepiolites are those that have more than 10% of octahedral positions vacant and more than 0.5% VIAl atoms. On the other hand, palygorskite is classified into Ideal palygorskite with an octahedral composition near to the ideal palygorskite. Common palygorskite: where VIAl content is less than in the ideal formula and as a consequence that the Mg content is higher, but the number of octahedral cations is close to 4. Magnesic palygorskite is the most trioctahedral extreme, and Aluminic-palygorskite, which is defined by a total number of octahedral cations (p.h.u.c.) < 4, with Mg < 2 and consequently (Al + Fe3) > Mg. Magnesic palygorskite and aluminic sepiolite can have very similar chemical composition. There is no a compositional gap between the two minerals. Palygorskite can be so rich in Mg and sepiolite so rich in Al that is possible to affirm that a continuous composition series exist and all the intermediate compositions between the two extremes corresponding to the two pure minerals can be found. The intermediate compositions can be explained by intergrowths of sepiolite and palygorskite ribbons or polysomes. Keywords • Palygorskite–sepiolite chemical composition variations • Compositional limits • Intermediate minerals 1. INTRODUCTION
The structure and chemical composition of sepiolite and palygorskite are known from the first half of the twentieth century based on work by Caillère (1936, 1951), Bradley (1940), Nagy and Bradley (1955), Brauner and Pressinger (1956), Martin Vivaldi and Cano Ruíz (1953, 1955, 1956a,b), Drits and Aleksandrova (1966), and others. Later, studies on the structure and chemical composition of both minerals have found many different aspects, and logically, as more cases are studied more differences are found. Recently Mg-rich palygorskites, Fe-rich palygorskites and Al-rich sepiolites have been reported and it seems that the compositional limits accepted until now could be too narrow. The aim of this chapter is define the limit of chemical composition of both minerals, if it exists, using the data from the literature available today. Sepiolite and palygorskite are modulated phyllosilicates. The modulated components are the octahedral sheets (Guggenheim et al., 2006). Both minerals can be described as 2:1 type ribbons running parallel to the c-axis. Ribbons are connected by oxygen atoms. There are continuous oxygen planes, but the periodical inversion of the apical oxygen (each two tetrahedral chains in palygorskite and each three in sepiolite) limits the lateral dimensions of the octahedral chain (Figure 1). Ideal palygorskite has a dioctahedral character (80% of octahedral position occupied) and sepiolite is a pure trioctahedral mineral. Figure 1 Structural schemes of palygorskite and sepiolite. (A and B) Tetrahedral sheet of palygorskite and sepiolite, respectively, projected on (001), black and grey mean tetrahedrons with apical oxygens pointing in opposite directions. (C and D) Tetrahedral sheet of palygorskite and sepiolite, respectively, projected on (100), grey shadow corresponds to the octahedral sheet. (E and F) Octahedral sheet of palygorskite and sepiolite, respectively, projected on (001). (G and H) Schematic view (from Sánchez del Rio et al., 2005) of a 1 × 1 × 2 supercell of palygorskite and sepiolite, respectively. A great number of sepiolite and palygorskite references (159 sepiolites and 216 palygorskite analyses) were compared and studied to obtain this general review of their composition. The number of articles offering chemical data on the fibrous clay minerals is more numerous than those used in this report, since the number of studies made on the genesis, properties and application is very abundant and is increasing yearly. However, we only consider analyses coming from pure or almost pure samples in this chapter.1 In general, sepiolite and palygorskite, as do most clay minerals, appear in natural occurrences as mixtures with other clay minerals and with impurities like carbonates, feldspar and quartz. The presence of small amounts of other clay minerals on the final data of composition could change significantly the results of analyses. As an example, palygorskite frequently has illite and montmorillonite impurities containing ~ 30 wt% of Al2O3, that is, approximately twice of the fibrous mineral and, consequently, lower amounts of silica. If the whole sample analysis is considered, and the structural formula is fitted from it, the formula of palygorskite obtained could have more tetrahedral substitutions than it really does. The chemical composition of the two pure dehydrated minerals calculated from the theoretical formulae is very simple: 69.1 wt% SiO2 and 30.9 wt% MgO for sepiolite; 72.47 wt% SiO2, 15.37 wt% Al2O3 and 12.5 wt% MgO for palygorskite. The deviations from these values are due to three different possibilities: (1) isomorphic tetrahedral and octahedral substitutions; (2)contamination by impurities of other minerals in the sample analysed mainly other clay minerals like illite, smectites and quartz or zeolite; and (3) instrumental errors. In the past decades, the use of transmission electron microscopy to obtain analyses (AEM) of very small particles makes it possible to obtain more accurate data from the individual particles and to avoid the influence of impurities. Bound or structural water is used in many different ways in the literature; so in this chapter, most of the reported chemical data have been recalculated on an anhydrous basis to compare them. The objective of this chapter is to establish the variations in the chemical composition of natural sepiolites and palygorskites from the ideal composition which correspond to pure and “perfect” minerals of sepiolite and palygorskite without any type of isomorphic substitutions. The range and type of isomorphic compositions in the two fibrous minerals can be obtained by analysing already published data of pure or almost pure sepiolites and palygorskites. Therefore, it is possible to look for the compositional limits between the two minerals, taking into account the existence of recently reported Al-sepiolite and Mg-palygorskite. 2. CHEMICAL COMPOSITION OF SEPIOLITE
Ideal sepiolite, Si12O30Mg8(OH)4(OH2)4(H2O)8, is a pure trioctahedral mineral, and the four possible octahedral positions (Figure 1) are occupied by Mg. The earliest experimental references about chemical composition of sepiolite or palygorskite are from the early decades of the twentieth century (Fersmann, 1913; Kauffman, 1943; Lapparent De, 1935; Shannon, 1929). However, the most outstanding works are those of Caillère (1936, 1951), Bradley (1940), Nagy and Bradley (1955) and Brauner and Pressinger (1956) or the review of chemical analyses for both sepiolite and palygorskite done by Caillère and Henin (1961). We must not forget, in this review, a special mention to the pioneer works of Martín Vivaldi and collaborators. In 1953, Martín Vivaldi and Cano Ruíz reported a chemical characterization of five different Spanish sepiolites. They compared the analyses with others from the bibliography and defined the SiO2/MgO ratio for an ideal, wholly magnesian sepiolite. In 1956 (Martín...