Halloysite characteristics and formation in the northern Golan Heights

Abstract

The mineralogy of alteration products of late Pleistocene basaltic scoriae and that of a paleosol from the northern Golan Heights was examined. These products had relatively high specific surface areas (SSAs) and cation exchange capacities (CECs) that were attributed to kaolinite–smectite mixed layers, 10Å halloysite, traces of smectite and illite. The latter is attributed to aeolian deposition. Using specific surface area data, from 18% to 59% clay were calculated to have formed in the weathered scoria. The 10Å halloysite was particularly well preserved in the paleosol. The absence of significant amounts of short-rage ordered minerals (allophane, ferrihydrite) is attributed to the long dry summers and to the relatively old age of the parent materials. SEM observations of weathered rock and of mildly dispersed clay revealed two types of clay particles: (a) hexagonal or prism-like idiomorphic, suggesting kaolinite and (b) rounded, composite, of a diameter approx. 0.5 um, appearing to be made up of spherical segments joined together by one or more flat faces. HRTEM of the 10Å particles in the paleosol showed elongated ellipsoids tapered at one end and with internal layering or spiraling, and spheroids with spiraling layering with a diameter 130–150 nm. Less common are sheet-like, crinkly films. SiO 2/Al 2O 3 molar ratios vary between 1.9 and 3.0. The particles have high Fe (up to 5.5%) and Ti (up to 3.1%) contents. Ti contents were higher in the peripheral parts of the spheroidal and in the crinkly, sheet-like particles. SEM observations suggest that while spheroidal and ellipsoidal 10Å halloysite formed by solid phase transformation of volcanic glass, the crinkly, sheet-like particles formed by precipitation on vesicle walls. Morphological features also suggest transformation of halloysite particles into kaolinite. The absence of significant amounts of 10Å halloysite in the modern soils indicates their xeric moisture regime.