1. apatite-like calcium phosphate. This leads siliceous with less aluminium than in
2. to the precipitation on or the formation albite. This result strongly suggests that
3. into the Si-rich surface layer of a sparingly there is a gradient of concentration of
4. soluble calcium phosphate which effec- aluminium and sodium from the interface
5. tively reduces the measured weight loss. to the bulk of the residual product.