Sepiolite–palygorskite: textural study and genetic considerations

Abstract

The fibrous morphology, the small particle size, and the presence of tunnels and channels in their structure give sepiolite (Sep) and palygorskite (Pal) a large specific surface area (SSA). The surface properties vary greatly among different Sep or Pal deposits because they are strongly conditioned by their textural and microtextural features (size, morphology, and arrangement of fibres). In this work, a detailed study is presented based on the scanning electron microscopy (SEM), field emission scanning electron microscopy (FEG) and transmission electron microscopy (TEM) observations of the microtextural features of a wide range of deposits of extremely pure Sep and Pal fromall over theworld. It has been confirmed that although all Sep and Pal have “the same characteristic” fibrous morphology, the samples fromdifferent deposits have their own characteristic signatures that vary greatly fromone locality to another, explainingwhy each deposits has different physical and chemical properties. Sep and Pal can consist of fibres with different sizes, curls, or types of aggregation. Several morphological fibre classifications have been made according to length, the width–length ratio (W/L), or curliness. In addition, local heterogeneities have been found as a consequence of particular genetic conditions. Aside from local heterogeneities, each Sep or Pal deposit has its own characteristic signature. As the minor width of fibres observed depends on the resolution of the study technique,we use three terms: lath (the smallest units that can be observed, the true unit crystal), rod (several laths in a crystallographical arrangement), and bundle (several rods parallel to the c-axis). Laths are approximately 10–30 nm wide and are the primary stable particles of Sep and Pal. Small crystals will not be stable andwill have dissolved. After lath formation (nucleation), they should grow by oriented aggregation, forming rods and bundles after which they grow by adding ions to the ends of the fibres.