Genetic constraints on crystallinity, thermal behaviour and surface area of sepiolite from the Cerro de los Batallones deposit (Madrid Basin, Spain)

• Sepiolite can form throughout distinct genetic pathways in the same deposit.
• The sepiolite origin may constrain its physical properties and industrial uses.
• Contrasted crystallinity values allow differentiation of sepiolite types.
• Thermal behaviour and BET are clearly influenced by the crystallinity of sepiolite.

The characterisation of sepiolite-bearing samples from five lutite associations occurring in the Cerro de los Batallones deposit has been carried out. Based on both the composition and texture of the sepiolite, two main formation patterns for this clay mineral, transformation from saponite by dissolution–precipitation and direct precipitation from solution, is proposed. From a compositional point of view, sepiolite oscillates between typical Mg-rich sepiolite and near Al-rich sepiolite, all samples showing variable swelling with ethylene glycol (up to 0.25 Å).There is a relationship between mechanism of formation and crystallinity of the sepiolite mineral. Both DRX and FTIR analyses led to differentiate between low crystallinity sepiolite (LCS) and high crystallinity sepiolite (HCS). The presence of the triplet 2.61 Å, 2.58 Å and 2.56 Å is especially useful for HCS identification. The two types of sepiolite show slightly a different thermal behaviour. In particular, coordinated water loss at 350 °C in LCS is higher than in HCS. BET surface area is high in all studied samples, reaching up to 360 m2/g and 328 m2/g in LCS and HCS, respectively. Differences in the microporosity and mesoporosity contribution have also been recognised. The reported dissimilarities account for differences in the technological behaviour of sepiolite clays and therefore these differences must be taken into account as they constrain industrial uses.