Metakaolin is a valuable pozzolanic material. It is obtained by heating kaolin, nominally Al-4[Si4O10](OH)(8), at 450-800degreesC. However kaolin from natural sources may be notably impure, even after beneficiation. During heating, essential to convert unreactive kaolin to reactive metakaolin, impurities present in the precursor kaolin may become activated with respect to dissolution in alkaline cement pore fluid. The most important impurities in this context are muscovite and potassium-rich felspar. potassium felspar and muscovite, heated and unheated, were slurried with Ca(OH)(2) and water at similar to22degreesC for up to 28 days. Significant alkali releases were obtained even from unheated minerals. However the extent of release increased with increasing calcination temperatures in the range 500-700degreesC. The results indicate that supposedly 'inert' aggregate minerals may become reactive towards cement. In cement-metakaolin mixes, much of the alkali liberated from mineral impurities is probably incorporated into cement solids but the overall effectiveness of metakaolin in decreasing the alkali contents Of cement pore fluid is reduced. However alkali release may also be beneficial: metakaolin may release sufficient alkali to enhance slag hydration in slag-Ca(OH)(2)-metakaolin blends.