Properties and Applications of Palygorskite-sepiolite Clays

The palygorskite-sepiolite group of clay minerals has a wide range of industrial applications derived mainly from its sorptive, rheological and catalytic properties which are based on the fabric, surface area, porosity, crystal morphology, structure and composition of these minerals. For assessing potential industrial uses, the mineralogical and chemical composition of the clay and its basic physical and physico-chemical parameters must be determined. Then some particular properties of commercial interest can be modified and improved by appropriate thermal, mechanical and acid treatments, surface active agents, organo-mineral derivatives formation, etc. In this paper, a revision of the principal characteristics of commercial palygorskite-sepiolite clays is presented, and potential uses are suggested according to these data. New products and applications are being investigated and those concerning environmental protection in particular, are noted. Finally, possible health effects of these elongate clay minerals are discussed. While commercial deposits of palygorskite and sepiolite are rare, these two clay minerals occur relatively frequently in sediments and soils. Both palygorskite and sepiolite clays are socalled ‘special clays’. The annual production in the western world for the former is estimated to be around two million tonnes; production comes mainly from the southeastern USA (Miocene deposits in Florida and Georgia) , about 1.8 million tonnes in 1989 (Ampian, 1991), and from Senegal, Spain, Australia, India, Turkey, South Africa and France to a much smaller extent. Sepiolite is less abundant than palygorskite. The former is a typically Spanish mineral; in fact, over 90% of sepiolitic clays are produced in Spain (also from Miocene deposits). Other small operations are in Nevada (USA), Turkey and China. The annual production of sepiolite in Spain is close to 800,000 tonnes (O’Driscoll, 1992). The interest in these minerals lies in their special sorptive, colloidal-rheological, and catalytic properties which are the basis for many technological applications. STRUCTURAL FEATURES AND PHYS ICO -CHEMICAL BEHAVIOUR Palygorskite and sepiolite are phyllosilicates inasmuch as they contain a continuous two-dimensional tetrahedral sheet; however, they differ from other layer silicates in that they lack continuous octahedral sheets. Their structure can be considered to contain ribbons of a 2:1 phyllosilicate structure, each ribbon being linked to the next by inversion of SiO4 tetrahedra along a set of Si!O!Si bonds. Ribbons extend parallel to the X-axis and have an average width along Y of three linked pyroxene-like single chains in sepiolite and two linked chains in palygorskite (Fig. 1); in this framework, rectangular channels run parallel to the X-axis between opposing 2:1 ribbons. As the octahedral sheet is discontinuous at each inversion of tetrahedra, oxygen atoms in the octahedra at the edge of the ribbons are coordinated to cations on the ribbon side only, and coordination and charge balance are completed along the channel by protons, coordinated water and a small number of exchangeable cations. Also, a variable amount of zeolitic water is contained in channels. ClayMinerals (1996) 31, 443–453 # 1996 The Mineralogical Society This structure of chain phyllosilicate results in a fibrous habit (Fig. 2) with channels running parallel to the fibre length. Channels are 3.7 6 10.6 Å in sepiolite and 3.7 6 6.4 Å in palygorskite. Fibre sizes vary widely but generally range from c. 100 Å to 4!5 mm in length, c. 100 Å to c. 300 Å in width, and c. 50 Å to c. 100 Å in thickness. Sepiolite is a Mg silicate close to trioctahedral phyllosilicates, while palygorskite appears to lie midway between dioctahedral and trioctahedral phyllosilicates (Fig. 3), with 1.1!2.5 octahedral positions occupied by Al + Fe and 1.1!2.8 by Mg at the five octahedral sites (Paquet et al., 1987; Galán & Carretero, unpublished results). Four H2O molecules per half unit-cell are present in the channels (zeolitic water) and four others are bound to octahedral cations. The cation exchange capacity of both minerals is quite low; it ranges from 4 to 40 mEq/100 g, but the higher values are probably related to impurities. b=17.9Å