THE LONG-TERM CORROSION OF GLASS BY GROUNDWATER

Specimens of corroded soda and potash glasses, which had been exposed to groundwater for periods of up to about 1 650 years, have been physically and chemically analysed. The morphology and compositional profiles of their finely laminated weathering crusts were determined by scanning electron microscopy and electron-probe microanalysis. It is shown that the surface layers on all specimens are depleted, to varying extents, of their principal constituents, with the exception of Si, Al and Fe. X-ray and electron diffraction Studies have revealed the (tentative) identities of complex, poorly crystalline silicates and aluminosilicates within the largely amorphous crusts on potash glasses. The mean thickness of the crusts was found to correlate well with the free energy of hydration DELTAG-degrees, of the pristine glasses. The rate of layer formation on the most durable specimens (DELTAG almost-equal-to -15 kJ mol-1) was about 4 x 10(-3) mum year-1. Deposits of calcite, calcium phosphate and manganese rich minerals occurred within the crusts; they were largely of external origin. Thus, in addition to the composition of the glass, the geochemistry of the local groundwater plays a decisive role in determining the identity of the compounds present within the weathering crusts.