A common form of neomorphism in ancient carbonate rocks is the recrystallization of micrite to microspar. This occurs when a "cage" of magnesium ions is flushed from 2 mu m micrite grains and permits them to recrystallize to microspar grains 5 to 10 mu m in diameter. Some clays play an important role in this type of neomorphism. In the Bromide Formation (Middle Ordovician, Oklahoma) shale beds of chlorite and/or montmorillonite attracted magnesium ions from adjacent micrite beds and greatly facilitated the formation of microspar. In the absence of clays, porosity controlled the formation of microspar. Some neomorphism of micrite to microspar occurred during outcrop weathering, but most neomorphism seems to have occurred after burial to moderate depths and after cementation of associated limestones.