Allophane clays are derived primarily from the in situ weathering of volcanic ash, although they may be derived from other volcanic material. This parent material may be either basic or acidic in nature. It appears that the primary condition for Allophane formation is that the parent material be of non-crystalline (or poorly ordered structure) composition. Volcanic ash meets this criteria; it is formed by the rapid cooling of relatively fine-grained pyroclastic material, the cooling process being too rapid for the formation of well ordered crystalline structures.