Abstract: Volcanic activity was important in Spain during the second half of the Cenozoic Era. It was closely related to the late kinematic and subsequent periods of the Alpidic orogenic cycle. Manifestations of the volcanic activity are seen in several parts of the Iberian Peninsula, but the main ones were confined to a few rather individualized zones. Cabo de Gata, Cartagena, and Mazarrón show a strong similarity, as does the Guelaya volcanic province, Morocco. The rocks (rhyolites, dacites and andesites) are clearly calcalkaline and, in many places, strong contamination processes have been demonstrated. The eruptions were, at least in part, fissural and fill important fractures, mainly northeast-southwest. They occurred just after the more important folding in the Betic-Riffien orogen. In these regions bentonite has formed by alteration of volcanic material.
The most important deposits are in Sierra of Nijar and Gata (Almería, Spain) and in the Monte Tidinit (Morocco). The Cabo de Gata bentonites are composed mainly of calcic and calcosodic montmorillonites. Palygorskite has been found in some places.
The bentonite of the Monte Tidinit is mainly calcium montmorillonite but some samples contain cristobalite. In Monte Maazza there is also a sedimentary deposit, consisting of a complex mixture of endellite, halloysite, gibbsite, montmorillonite and alunite— probably of hydrothermal origin. Halloysito is the main component: it is accompanied by gibbsite in the bottom layers and by montmorillonite and alunite in the top ones.
The ground clay, after processing by cation exchange or acid activation, is used for several industrial products, such as soaps and detergents, cosmetics and pharmacy; but the main use is for drilling muds, foundry molds, and clarification of mineral and vegetable oils.
All the bentonites are amenable to acid activation but the natural mixture of halloysite and montmorillonite from Monte Maazza gave the best results in tests on lubricating oils at a laboratory scale.
Research still in progress on the rheology of suspensions of the Almeréa bentonite contaminated with electrolytes gave interesting results using several thinner and filter oss control agents.