Cordierite (Mg2[Al4O3(SiO3)5])

Hydrolysis

Description of key information

The hydrolysis of Cordierite was determined via an examination of the composition of the substance and the molecular structure of its constituents and similar aluminosilicates as well as experimental solubility data available for the substance (OECD 105 (water solubility, shake flask method) report reference: Intertek 1335142-2). Cordierite was found to show no hydrolysis potential.

Additional information

Hydrolysis of cordierite will most likely involve breaking the Si-O-Si or Si-O-Al bonds in the crystal lattice. To evaluate the hydrolysis of cordierite at extremes of pH, the hydrolysis of these bonds will be evaluated. Many studies have been performed to determine the release of silicon and aluminium from silica and alumina at different pH’s. in the case of silica and alumina the dissolution and hydrolysis products are assumed to be one and the same. The dissolution of silica below pH 8.5 is negligible (less than 170 ppm) but as the pH of the solution increases above 8.5 silica becomes slightly soluble as the SiO2 macrostructure is converted to Si(OH).

Silica is reported in the literature as being sparingly soluble over a broad range of pH’s (1.1-8.9) with the maximum concentration dissolved reported to be 2.7mmol L-1 . 6 Alumina is amphoteric and has the potential to react at both high and low pH. Alumina exhibits higher solubility at the extremes of the pH range with virtually no solubility at pH 6.3,7 no detectable concentrations of aluminium were observed during experiments at pH 5.3 and 8.3 over 60 days, therefore no hydrolysis of alumina occurs over this range.8 Cordierite also contains some Mg ions at low concentration (reported as MgO at 9.6%w/w of the total composition). The ions are present in the crystal structure surrounded by the SiO4 and AlO4 tetrahedra and are held in place by ionic interactions with oxygen. The magnesium is bound within the crystal structure of cordierite and therefore not available for interaction with the surrounding solutes without degradation of the crystal lattice which would require forcing conditions. One use of aluminosilicate materials is as filtration aids where high mechanical strength and hydrolytic stability are essential qualities, for example sodium aluminosilicates have been shown to be resistant to concentrated mineral acid solutions (pH 1) over 72 h, with less than 0.1%w/w Al and Si detected in the solute after exposure. Both silica and alumina is very low over a range of pH’s. In the case of cordierite the combination of the alumina and silica into a complex crystal lattice strengthens the structure, increasing the energy barrier to dissolution making cordierite more resistant to hydrolysis than its component parts. This is highlighted by the hydrolytic stability reported for other aluminosilicate rocks and clays. It is for this reason that cordierite can be said to be hydrolytically stable.