Croissance naturelle de paracristaux de quartz dans une saumure sulfatée calcique à basse température

Abstract

In brines with dissolved Ca-sulphates, the growth of quartz is attained by a close packed assemblage of nuclei. These are depicted as sharp spots of 250 Å in size by SEM mode imaging. These are either spherical nuclei or electron optical diffusion features which are induced by smaller particles. Nuclei give quartz fibers by a coalescence process. The average diameter of fibers is 1 000 Å. Their length is 5 to 10 nm. By successive coalescences, fibers give ribbons, bidimensional sheets and tridimensional quartz ultimately. In the quartz paracrystal resulting from repeated attachment of nuclei, the tridimensional continuity is attained by ageing of the first precipitate. At intermediate stages of this evolution, the quartz paracrystal is crossed by very numerous inframicroscopic channels, leaving a high internal porosity in each paracrystal. The primary structure, characterized by a close packed assemblage of nuclei, is very similar to the organization of spherical amorphous silica particles in precious opal. The ordered precipitation of both quartz nuclei in a paracristalline structure and amorphous silica particles in a precious opal are the products of the colloidal ordered precipitation due to the second potential well of the interaction energy between two particles. Quartz nucleation appears by the salting out of dissolved monomere silica. All this results from mixing of a brine and of a silica saturated solution. Quartz nucleation may be improved by impurities (Al⁺⁺⁺, hydrocarbures) in the growth medium.