The role of magnesium in the formation of smectite minerals

Abstract

The synthesis of clay minerals at elevated temperatures has been studied in many papers but only very few investigations have been made of the synthesis of clay minerals at surface conditions. It has been shown that amorphous hydroxides of Al, Fe, Mg, etc. are capable of co-precipitating silica even from very dilute surface solutions. By aging of these X-ray amorphous hydroxide—silica precipitates under certain conditions clay minerals can be synthesized at low temperatures. The presence of magnesium particularly favours the formation of smectite minerals. From Mg-hydroxide silica precipitates montmorillonite and talc could be synthesized under basic conditions at low temperatures. From pure Al-hydroxide—silica precipitates framework silicate (quartz and feldspar) and bayerite was formed, while the synthesis of pure Al-clay minerals at low temperatures seems to be more difficult. From Mg-containing Al-hydroxide silica precipitates tri- and di-octahedral smectite could be synthesized. The following conditions favour the formation of montmorillonite at low temperatures. 1. (1) Low silica concentration in solution. Formation of clay minerals and other minerals (quartz, feldspar, etc.) takes place at low temperatures (20° and 3° C) only in those precipitates that are in contact with solutions undersaturated with SiO 2 (in respect to amorphous silica). If the SiO 2 concentration in the solution has higher values the hydroxide—silica precipitates stay amorphous. It seems that polymerisation of the silicic acid inhibits the formation of clay minerals. 2. (2) Composition of precipitates. The concentration of silica and the other elements in these hydroxide silica precipitates should be similar to the composition of naturally occurring smectite minerals. 3. (3) Magnesium content in solution and precipitates. The hydrogen ion-concentration effects the precipitation and co-precipitation of magnesium. Under basic conditions (pH 10) only 10 p.p.m. Mg are necessary in the solution for the formation of smectite minerals. Under neutral conditions it is possible to co-precipitate Mg with Al-hydroxide, the Mg-content of such Al-hydroxide—silica precipitates depends on the Mg concentration in the solution and the pH during precipitation. The formation of smectite minerals is only possible when the precipitates contain at least 6% of MgO. The precipitates stay amorphous if the magnesium content is lower. The magnesium seems to favor the formation of all three-layer silicates.