ABSTRACT

The paragenesis and the sequence of formation of silica polymorphs have been studied in three examples of silicification in the regolith of the Paris Basin and the Central Massif of France. The initial silica deposits are most frequently opal with varying degrees of structural organization. Subsequently, recrystallization produced varieties of progressively better-organized quartz; namely, fibrous quartz (chalcedony and lutecite), which developed from deposits of microlaminated opal, and microcrystalline quartz, which formed from nodular opal. However, even during the formation of microcrystalline quartz facies, dissolution and recrystallization processes continued, resulting in progressively bigger quartz crystals. In the final stages of silcrete development, hyaline quartz crystals rew in voids.

These mineral sequences are determined by the solubility of each of the component parts; solubility is controlled by the "purity" and the degree of order of the crystalline structure. The most disordered minerals are the most soluble. The degree of order is controlled, in part, by foreign ions incorporated into the lattice, but also by the degree of supersaturation of silica in solution in the system which, in turn, controls the rate of formation of nuclei.

Dissolution and recrystallization processes are dependent on the percolation of dilute solutions through the silcretes. The concentrations of silica and other cations in these solutions is a function of the solubility of the silica phases in the host material and of the rate of percolation. Thus, the porosity of the host material is important. Rapid percolation favors dissolution, but in zones with slow percolation, equilibrium is attained and no transformation of existing silica phases occurs. It is in the zones of intermediate porosity that progressive recrystallization of quartz occurs. The effect of rate of percolation on dissolution and recrystallization explains the vertical distribution of different silica polymorphs observed in numerous silcrete duricrusts.