Mineralogy of some Italian andosols with special reference to the origin of the clay fraction

The mineralogy of four Italian Andosols - derived from volcanic material either oversaturated or undersaturated with respect to silica - has been investigated by XRD, EM and IR. The crystalline clay minerals in all four profiles are essentially similar, consisting of abundant halloysite with moderate illite and 14A intergrade material, minor kaolinite and occasional gibbsite. The soils also contain large amounts of imogolite and proto-imogolite allophane. With the exception of illite all these clay minerals are believed to be of pedogenic origin. Halloysite occurs in the dehydrated form in the surface horizons but becomes progressively more hydrated with depth. At depths of > 1.4 m the clay fraction consists almost entirely of fully hydrated halloysite, supporting the suggestion that halloysite forms best in a stagnant moisture regime where there is a depositional overburden acting as a silica source. EM observations show that the halloysite may have spherical morphology and may be intimately associated with gas vesicles in pumice grains where it probably forms by the transformation of allophanic material. It seems likely that dehydrated halloysite slowly converts to poorly crystallized kaolinite in the upper horizons of these profiles. The origin of the 2/1 minerals is more problematical. Illite is probably inherited from mica in the parent material but the 14A intergrade material is so poorly ordered that a pedogenic origin seems more likely than formation by inheritance or by transformation of pre-existing 2/1 silicates.