Supersaturation evaluation is an essential requirement to describe, confront and explain crystal growth experiences. However, in the particular case of crystal growth in gels, experiences are often described by attending to the initial concentration of reagents. This fact is wnnected with deficiencies in the theoretical quantification of mass-transfer, and therefore in both time and location prediction for the first precibitate. In this paper laboratory experiences have been specifically designed to test supersaturation evolution through an actual (finite) diffusion system. The problem is carried out by keeping into account several complexity factors: free ions as well as complexes and silica gel Na+ and C1-"unloading" are considered to evaluate the supersaturation.

Abstract--The kaolinite crystallization by homogeneous precipitation with previous hydrolysis of the feldspars added has been followed by pH and potassium concentration measurements. The synthesized products were studied by X-ray powder diffraction and electron microscopy and consist of kaolinite and mica, accompanied occasionally by traces of smectite. The relative quantities of the synthesized minerals depend fundamentally on the supply rate of the alkaline ions.

A B ST R ACT: A method of precipitation in homogenous solution has been used for the synthesis of kaolin minerals at room temperature. The method consists essentially in producing conditions for which a sufficiently slow ratc of precipitation is maintained. In the particular case described in this paper, the conditions for homogenous precipitation are obtained by addition of an anion exchange resin in the OH form to an acidic solution (pH ~ 4) of sodium silicate and aluminium chloride or acetate.The mineral species synthesized in each particular case depend on the kind of anions present in the original solution (chloride or acetate) and also on the nature of the exchange resin used.

A detailed study has been made of the dehydration of sepiolite. This work, together with other studies of dehydration in vacuo and over phosphoric anhydride, rehydration of samples heated to various temperatures, specific surface, water uptake, and x-ray examination of natural and heated samples (to be published later), lead us to certain conclusions regarding types of water in sepiolite. A comparison is made with attapulgite.

It is suggested that the minerals of the palygorskite-sepiolite group occupy the region of discontinuity between dioctahedral and trioctahedral minerals. There is thus a series of minerals, the two extremes having a planar lattice, and the intermediate members a fibrous lattice. The lamellar-fibrous change in structure takes place as the number of vacancies increases progressively.Among the minerals of fibrous structure, the structural change from sepiolite to attapulgite occurs in such a way as to allow no more than one vacancy per structural fiber, and per half cell, as if this vacancy were not distributed at random, but symmetrically among the octahedral positions. The greater the nmnber of vacant octahedral positions, the smaller the extension of the octahedral layer. The margin of variation in the number of vacant octahedral positions per structural fiber is greater in sepiolite than in attapulgite, probably because of the greater width of the fiber in the former mineral.