You might try alkali fusion following van den Boorn et al. (2006):
van den Boorn, S. H. J. M., Vroon, P. Z., van Belle, C. C., van der Wagt, B., Schwieters, J., and van Bergen, M. J. (2006). Determination of silicon isotope ratios in silicate materials by high-resolution MC-ICP-MS using a sodium hydroxide sample digestion method. J. Anal. At. Spectrom. 21, 734–742. doi: 10.1039/b600933f
Or the "cold" version as described in the methods part in our last publication: