Formation d'argile par altération d'albite à l'autoclave à 200°C

Abstract

The alteration of albite monocrystals in an autoclave at 200°C (390°F) with a monomerous silica saturated solution yielded kaolinite. In contact with albite, this kaolinite is three-dimensional and its crystallization is good, whereas in solution it yields, in the initial phase, a type Te-Oc mineral characterized by disordered crystallization and an aluminium deficiency. This mineral then develops into a properly crystallized kaolinite. The formation of kaolinite and the differences between the two development processes -in contact with feldspath, and in solution -were shown to result from the presence of a greater or lesser quantity of boehmite fibres and to local variations in the ration • The importance of A 1 2 O g this ratio was confirmed by albite alteration experiments conducted with a solution having an excess colloïdal silica content of 700 ppm. In the latter case, the mineral produced seems to belong to the Te-Oc -Te group.

Illinois Basin coal fly ashes. 2. Equilibria relationships and qualitative modeling of ash-water reactions

“…The acidic fly ash extract (12) was also in equilibrium with either gibbsite Al(OH)3 or amorphous Al(OH)3; the alkaline extract, 18, appeared to be in equilibrium with boehmite AIO(OH), a metastable phase of gibbsite (Figure 3) during the latter part of the extraction period. The results of solubility modeling could be interpreted as indicating that aluminum hydroxide species controlled by solubility of aqueous Al3+.…”

“…The alkaline extract (18) was supersaturated with respect to calcite during the entire extraction interval. The acidic extract (12) remained undersaturated with respect to calcite because of the absence of carbonate at the low pH. Calcium concentrations in highly alkaline solutions in contact with atmospheric C02 (such as 18) should be controlled by calcite solubility.…”

“…Although no aluminum hydroxide compounds were detected by XRD in the solid samples, such compounds could have been present in amorphous forms or could have formed as precipitates during the extraction interval. Boehmite has been found to form as an amorphous precipitate on silicate surfaces during the dissolution of feldspar (12). The source of the Al3+ that forms the aluminum hydroxide precipitates in the extracts could have been the aluminosilicates such as mullite detected in 18 but not in 12 by XRD.…”

“…Two of the LTE solutions discussed in Roy et al (4) were chosen for presentation since it was believed that these two samples best represented the diversity of the extracts from Illinois Basin fly ashes. One sample (12) is an acidic fly ash, an acid C-Modic silt loam by the nomenclature of Roy and Griffin (7); the other sample (18) is an alkaline Modic silt.…”

“…Table I presents the possible sequence of reactions that were operative in the initially acidic fly ash extract (12).…”