Sepiolite-Palygorskite Equilibria in Saline Lakes and Basins

Comparison of these assemblages with the phase diagram deduced for 

The use of the closed system to describe the assemblages in these closed basins seems justified in that frequently, most always, in fact, the number of clay minerals present in the sediments discussed above is two or more. The omnipresence of amorphous silica or chert raises the total number of phases to three. In an essentially three-component system, Mg-Si-Al or possibly four if H+ is considered, this indicates that the chemical components of the minerals are present in relatively fixed quantities in the chemical system which produces the mineral assemblages. None of the first three components is mobile , i.e., its activity is independent of its relative mass in the solids or crystals present. However, there are sediments which present a monophase assemblage where only one variable need be fixed. Under these conditions sepiolite can be precipitated from solution and pre-existing solid phases need not be involved. 

The relationship between aluminous sepiolite and the magnesian form is not at all clear as yet. The question of whether or not aluminous sepiolite can be precipitated directly from solution should be posed since alumina concentration in alkaline solutions (pH 7-9) is assumed to be quite low. For the present, palygorskite should probably be considered as a phase produced through solid-solution equilibria due to its high alumina content. Possibly most sepiolites are produced in this way. Even the inonomineralic deposits in saline lakes or deep sea cores contain sepiolites with high alumina contents (Parry and Reeves, 1968 McLean, et al., 1972). 

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