364 CLAY MINERALS chlorite-vermiculite is found in a wide range of soil types on chlorite-bearing rock Kaolinite-smectite (or more probably the kaolin mineral is halloysite, though this has only recently been recognized) is particularly abundant in soils formed on basic igneous rock Globally sepiolite and palygorskite are fairly rare though locally abundant Recent and Quaternary deposits occur in North Africa and the Middle East, while there are well-known older Tertiary deposits in southern Spain and France Neoformed sepiolite and palygorskite are commonly found in desert soils, ephemeral lakes, and playas, and occasionally in hydrothermal veins in basic and ultrabasic rocks Formation of palygorskite requires confined conditions, either in shallow, enclosed water bodies at times of low lake level or in palaeosols and slow deposition It occurs in mature, slow-formed calcrete, and in association with smectite and dolomite Palygorskite does occur in deep water sediments For example, the origin of palygorskite in deepwater sediments of the North Atlantic has been much debated However, using SEM and TEM analysis, this clay has been demonstrated to be wind blown, reworked material from shallow water saline basins in Morocco glauconite) forms just below the sediment water interface, again, largely through replacement of faecal pellets Glauconite forms principally on the outer shelf, in a water depth of 100–300 m, at a temperature of 7–15C and between 50 N and 50 S It is also favoured by a low sedimentation rate because it is slow to form and has an He close to the redox boundary as it contains both ferrous and ferric iron Hence glauconite is associated with transgressive sediments, and can be a useful environmental indicator, though there are many instances of reworked glauconite in turbidites, shallow marine, and nonmarine sediments Deep sea clays cover by far the greatest area, but are deposited very slowly Much of the clay in these sediments is a combination of detrital and neoformed Ferich smectite, often associated with palygorskite (which may or may not be authigenic) and authigenic clinoptilolite In the central Pacific, where there is perhaps the least detrital input, smectite makes up >70% of the clay fraction Nontronite has been reported from deepsea areas of volcanic activity Apart from neoformation of smectite, the deep ocean clays largely reflect the pedogenic processes occurring on adjacent landmasses Marine Clays Nonmarine Clays The clay mineral suite of marine environments is controlled by provenance (closely linked to climate), salinity, particle size, and neoformation Detrital clay minerals distributions in estuaries and deltas reflect the different settling rates of the clays present and their tendency to flocculate Where flocculation does not occur, the finer clay particles (which roughly equates with smectite) remain in suspension longest and are transported further offshore However, no such depth profile will occur if mixing of freshwater suspended clay with saline water results in flocculation, or where marine currents are strong The effective grain-size of clay particles is also dramatically increased through the formation of mucous-bound faecal pellets by marine invertebrates Due to the incorporation of reactive organic matter into faecal pellets, they are prone to allogenic and early diagenetic mineralisation An example is odinite, a ferric iron-rich clay that forms exclusively within the tropics, offshore from major river systems (which introduce large amounts of iron into the marine environment) Odinite replaces faecal pellets and also forms ooidal coatings on grains In ancient sediments, odinite is believed to transform into ferrous iron-rich 7A˚ (berthierine) and 14A˚ (chamosite) clays Further offshore in marine shelf sediments, the better known iron-rich clay, glauconite (and intermediate clays which range from glauconitic smectite to smectitic Most continental, freshwater, and saline lacustrine clays are detrital A few recent saline lakes contain unequivocal neoformed Fe and Mg-rich smectite However, studies of lake clays not always include sufficient investigation of the drainage basin to be certain that just because a lake clay is dominated by one clay type that it is neoformed Some Tertiary lake sediments are almost monomineralic ferric rich illite, but the same provision regarding the drainage basin applies to these There appear to be two distinct processes whereby this unusual clay forms, firstly in volcanic lakes with a high dissolved cation content, and secondly in ephemeral lakes through repeated wetting and drying of smectite Laboratory experiments have demonstrated that such a process is possible Clay Stratigraphy As a consequence of diagenetic replacement by illite, smectite is virtually unknown from pre-Mesozoic sediments, while throughout the Mesozoic and Early Tertiary, smectite is the principal clay mineral found in sediments of most types Glauconite is most abundant at the end of the Cambrian and the end of the Cretaceous, while berthierine/chamosite are mostly found from the Ordovician to the end of the Devonian and from the earliest Jurassic until the end