Spheroidal Halloysite in a Guatemalan Soil

Abstract Mineralogical analyses of clays from two Alotenango soil horizons were made by utilizing transmission and scanning electron microscopy, infrared, X‐ray diffraction, and chemical methods. This report of spheroidal halloysite demonstrates the presence of this mineral form in an area of important volcanic ash‐derived soils where it has not been previously identified. The presence of spheroidal halloysite in Central American volcanic soil and its presence in volcanic soils of Hawaii, Japan, and New Zealand indicate its wide geographic occurrence in soils of volcanic origin. Oblate halloysite spheroids tend to settle preferentially, offering an end‐view along the axis of some particles and correspondingly, reduced 00 l X‐ray reflections from portions of layers not parallel to the plane of the powder mount. Halloysite spheroids appear to be composed of roughly concentric layers with occasional interlayer separations. The interior structure suggests inclusion of amorphous cores and interlayers which appear to be removed by 0.5 N NaOH. The exterior layers often retain a shell‐like appearance after NaOH treatment. The apparent association of spheroidal halloysite and allophane interlayers suggests that the crystalline component forms alternately with the amorphous one. The crystalline exterior may protect amorphous interior material from rapid alteration and cause allophane to persist in a soil where minerals representing more advanced stages of weathering have accumulated.