Diffuse reflectance optical spectroscopy measurements were conducted using an Ocean Optics tungsten-halogen light source (Mikropack HL-2000) and an Ocean Insight flame miniature spectrometer (Flame-S-VIS-NIR) equipped with an integrating sphere. A fiberoptic cable was used to illuminate the flat base of a borosilicate glass vial filled with loose powder of the reaction products. A vial containing BaSO 4 was used as a white reflectance standard. The vials containing the reaction products were sealed with electrical tape to maintain an inert atmosphere (argon-filled glovebox) before being removed for measurement. The Kubelka-Munk transform was calculated using the formula (k/s) =