The effect of magnesia and lime on the durability of synthetic basaltic glasses

Abstract

The effect of varying $\frac{\left[\text{MgO}\right]}{\left[\text{MgO}\right]+\left[\text{CaO}\right]}$ ratios on the chemical durability of a series of synthetic basaltic glasses is examined via monolith (MCC-1) and powder (PCT-B) tests for times up to 1800 days (MCC-1) or 224 days (PCT-B). It is demonstrated that the magnesium-rich compositions generally have a lower durability than the calcium rich ones. For the shorter MCC-1 tests the normalised losses of all elements increase with time but at longer times both Mg and Al are removed from solution. Higher Mg content basaltic glasses developed thicker alteration layers with reduced Al and Mg contents along with secondary precipitates (potentially aluminous hectorite clays). The laboratory assessed alteration rates, based on alteration layer thicknesses from the accelerated MCC-1 tests at 90 °C, are demonstrated to be at least 2 orders of magnitude greater than the alteration rates reported for natural basalts in a range of continental and oceanic settings.