Aqueous-Chemical Control of the Tetrahedral-Aluminum Content Of Quartz, Halloysite, and other Low-Temperature Silicates

Abstract

Abstract--Aqueous AI passes from octahedral to tetrahedral coordination over a narrow pH interval, or threshold. This interval is 5.5-6.5 at 25~ and shifts to lower pH as temperature increases. The concentration of aqueous tetrahedrally coordinated A1 is a quasi-step function of the solution pH, and, by the mass-action law, so should be the amount of tetrahedral A1 incorporated by a silicate that crystallizes from the aqueous solution. Qualitative support for this prediction (which applies to quartz, opal-CT, kaolin-group minerals, pyrophyllite, micas, chlorites, and other low-temperature silicates) comes from the very topology of equilibrium activity diagrams and from several pairs of associated waters and authigenic silicates from weathering, hydrothermal, and diagenetic environments. The uptake of tetrahedral A1 also depends on the aqueous concentrations of monovalent cations and silica, and on the mineral's structural constraints.Solid solution of tetrahedral A1 in halloysite in turn produces the characteristic bent or tubular crystals of this mineral. This genetic link between aqueous chemistry (mainly pH), tetrahedral-A1 uptake by a low-temperature silicate, and the mineral's crystal morphology may operate also in other silicates.

Structural studies of Silica Springs allophane

“…This suggests that the A1 vi sites are on the inside of the curved surfaces of Silica Springs allophane in contrast to the proto-imogolite allophanes where they are on the outside. This suggestion is in accord with the accepted explanation for the curvature of halloysite (Drever, 1982) and with the suggestion that the greater the degree of Al-for-Si substitution, the more bent become the layers of halloysite (Merino et al, 1989).…”

“…Al(H20)63+). Just below pH6, four-coordinated species (especially AI(OH)4-) become dominant (Farmer, 1987;Merino et al, 1989). Thus A1 iv in Silica Springs allophane corresponds to the solution form of A1 from which the allophane precipitates.…”

Dioctahedral Tosudite in Hydrothermally Altered Pliocene Rhyolitic Tuff, Neutla, Mexico

“…150~ that favor tetrahedral coordinated A1 (Inoue, 1983;Howard and Roy, 1985;Merino, 1989). High K and K/(Ca + Mg) ratio conform with the observations reported from mixed-layers in bentonites, which required K to balance the layer charge (Velde and Brusewitz, 1982;Brusewitz, 1982).…”

Tubular microfossils from ∼2.8 to 2.7Ga-old lacustrine deposits of South Africa: A sign for early origin of eukaryotes?

“…The in vivo or very early post mortem precipitation of the aluminosilicate coatings on the Omdraaivlei algae was most probably the critical factor responsible for their unique 3-D preservation by making the cell wall resistant to lytic processes and the thalli stiff to compaction. Both chamosite and muscovite are mineral phases preferring lower oxygen fugacity and circum-neutral pH in the ambience, during formation (Harder, 1978;Merino et al, 1989). Their association with the Omdraaivlei microalgae-like organisms may therefore be an indirect evidence for their existence in 26 dysoxic or at least not highly oxygenated environment.…”