FORMATION OF MULTIPLE HALLOYSITIC PHASES IN A NEOTROPICAL FLUVIAL TERRACE SEQUENCE

Chemical weathering of basaltic and andesitic volcanic detritus in three flights of Holocene and Pleistocene (?) fluvial terraces in the Atlantic Coastal plain of Costa Rica has produced a complex series of kaolin and associated dioctahedral minerals that include 7 Å, 10 Å and 14 Å forms. Gibbsite and goethite are also present as secondary minerals. The occurrence of 14 Å, 10 Å and 7 Å 'halloysite' suggest a complex sequence of mineral transformations that are apparently progressing toward 7 Å kaolin.

Quantitative XRD analysis of B-horizon soil samples indicates rapid dissolution of plagioclase feldspar, from 37 % (by wt.) in the youngest terrace to <5% in middle and upper terraces. Plagioclase weathering is paralleled by the formation of a complex mixture of halloysitic kaolins that undergo an erratic progression from 10 Å to 7 Å with increasing age. Formamide intercalation of samples from older terraces and residual soils indicates the presence of halloysitic phases and discrete kaolinite. In all halloysitic specimens, ethylene glycol solvation produces increased integrated peak areas at 3.5-3.6 Å, suggesting alteration to the kaolin structure that enhances the 002 repeat distance. A weak 14 Å peak associated with dioctahedral clay (based on lack of trioctahedral 060) is present in most samples and generally increases with increasing soil age. The nature of the 14 Å material is unclear. Ethylene glycol treatment indicates that expandable phases occur only in the modern floodplain, where a weak 17 Å peak indicates traces of smectitic clay. The 14 Å peak is weakened and broadened by heating to 250C (1hr) and is destroyed by heating to 550C for 1hr. This indicates that the 14 Å peak is not the result of a dioctahedral chlorite-like 2:1:1 structure. However, stepwise destruction of the 14 Å peak by heating may indicate loss of (1) interlayer water at 250C, and (2) structural water in the form of hydroxide layers at 550C. ICP-AES analyses indicate retention of Mg to a much greater extent than Na, Ca or K, suggesting that it may be incorporated into hydroxide layers in a predominantly dioctahedral 14 Å aluminous mineral. ICP calculations (from 060 peaks) produce systematically higher % halloysite than QXRD, indicating that amorphous phases such as allophane are also present in significant quantities (~12-30%).