Sepiolite

Sepiolite, formerly known as Meerschaum (sea froth), is a non-swelling, lightweight, porous clay with a large specific surface area. Unlike other clays, the individual particles of sepiolite have a needle-like morphology. The high surface area and porosity, as well as the unusual particle shape of this clay account for its outstanding sorption capacity and colloidal properties that make it a valuable material for a wide range of applications. Sepiolite is a very uncommon clay because of both its peculiar characteristics and scarce occurrence. There are very few commercial deposits in the world. Most of the world production of this clay comes from deposits of sedimentary origin located near Madrid, Spain. Chemically, sepiolite is a hydrated magnesium silicate with the ideal formula Si12Mg8O30(OH)4(OH2)4.8H2O. Sepiolite, unlike other clays, is not a layered phyllosilicate. Its structure can be described as a quincunx (an arrangement of five objects, so placed that four occupy the corners and the fifth the centre of a square or rectangle) of talc-type sheets separated by parallel channels. This chain-like structure produces needle- like particles instead of plate-like particles like other clays. Sepiolite has the highest surface area (BET, N2) of all the clay minerals, about 300 m2/g, with a high density of silanol groups (-SiOH) which explains the marked hydrophilicity of this clay. The silicate lattice has not a significant negative charge and therefore the cation exchange capacity of this clay is very low. The tiny elongated particles of sepiolite have an average length of 1μm to 2 μm, a width of 0.01 μm; and contain open channels with dimensions of 3.6 Å x 10.6 Å running along the axis of the particle. These particles are arranged forming loosely packed and porous aggregates with an extensive capillary network which explains the high porosity of sepiolite and its light weight because of the large void space. The high surface area and porosity of sepiolite account for the remarkable adsorptive and absorptive properties of this clay. It adsorbs vapours and odours and can absorb approximately its own weight of water and other liquids. Sepiolite is a non-swelling clay and its granules do not desintegrate even when saturated with liquids. Colloidal grades of sepiolite must be dispersed into water or other liquid systems using high-shear mixers. Once dispersed in the liquid, it forms a structure of randomly intermeshed elongated particles, which is maintained by physical interference and hydrogen bonding, and entraps the liquid, increasing the viscosity of the suspension. This structure is stable even in systems with high salt concentrations, conditions that produce the flocculation of other clay’s suspensions, as bentonite. The random network of sepiolite particles holds coarser particles in the liquid preventing their settling by gravity, acting as a suspending agent. Sepiolite provides to its suspensions a pseudoplastic and thixotropic behaviour which make it a valuable material in multiple applications to improve processability, application or handling of the final product.