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 so- called '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 palygors- kite. 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,

Properties and applications of palygorskite-sepiolite clays

An analogy can be made between the mechanisms of water transport in partially frozen soils and those in unsaturated soils. By use of this analogy a Darcian approach is applied to the analysis of coupled heat-fluid transport in porous media with freezing and thawing. With the aid of a numerical model, freezing-affected soil-water redistribution and infiltration to frozen soil are examined from a phenomenological point of view, and the effects of soil type and initial conditions on the response of a hypothetical soil column are studied. In general, the model shows that the rate of upward redistribution of soil water to a freezing zone at the soil surface decreases from coarse-textured soils to fine-textured soils and decreases with increase in depth to the water table. Subsequent redistribution during melting of the frost wedge is shown to occur at a rate less than that associated with freezing. Infiltration to partially frozen soil is also shown to have a significant influence on soil-water redistribution and the response of the groundwater table.

Analysis of coupled heat-fluid transport in partially frozen soil

Abstraet-A knowledge of the distribution of pore sizes in clay and soil bodies is a useful element in the microstructural characterization of such materials. Pore-size distributions and total porosity of a number of reference clays, naturally-occurring subsoils, and commercial clay samples prepared in various ways were determined by mercury porosimetry. The range of equivalent pore diameter explored covered almost five orders of magnitude, from several hundred microns down to approximately 150A. The method and its assumptions are critically evaluated, and measurements of the contact angle of mercury on clays yield values of 139 ~ for montmorillonite and 147 ~ for kaolinite and illite clays. The extent of shrinkage on oven-drying prior to mercury intrusion is assessed in each case and found to vary from insignificant to as much as 30 per cent of the pore space, depending on microstructural state and degree of initial saturation. The development of techniques for water removal which do not involve change in pore structure is explored. Some preliminary results for structurally weak saturated clays suggest that critical-region drying and perhaps freeze-drying procedures may be practical.

Pore Size Distributions in Clays

Fibrous clays based bionanocomposites

Direct measurement of the relation between interlayer force and interlayer distance in the swelling of montmorillonite

During quick-freezing with liquid nitrogen all but two molecular layers of water moved from between the crystals of saturated bentonite gels. The water molecules appear to have migrated to ice crystals forming near the clay crystals. Lyophilizing removed the ice crystals and further reduced interlayer water. Both quick-freezing and lyophilizing altered the original gel structure.

https://science.sciencemag.org/content/sci/136/3522/1116.2.full.pdf

Freezing and Lyophilizing Alters the Structure of Bentonite Gels

In vitro studies of the destruction (lysis) of bovine red blood cells (erythrocytes) by some silicate minerals showed the reaction to be complete in less than 1 hr and very destructive to the cell membrane. The activity as lysing agents was found to be in the order smectites > silica > palygorskite -~ sepiolite > chrysotile > kaolinite. Different compositions (Fe, A1, Mg, Li, vacancy) of the octahedral sheet of the smectite and fibrous clay minerals did not appreciably alter their hemolytic activity. The most active particle size range for kaolinite and montmoriUonite was 0.2-2 ~m. Structural folding of palygorskite reduced lysis suggesting that edge surfaces and silanol groups are important in this process. Aluminum oxides and hydroxides caused no lysis, and coatings of positively charged aluminum-hydroxy polymers on montmorillonite, silica, palygorskite, and kaolinite significantly reduced lysis.

James L. Ahlrichs

Papers

Freezing and Lyophilizing Alters the Structure of Bentonite Gels

Lysis of erythrocytes by silicate minerals

Effect of Poly-2-vinylpyridine-N-oxide and Sucrose on Silicate-Induced Hemolysis of Erythrocytes