WEATHERING OF CHLORITE TO A LOW-CHARGE EXPANDABLE MINERAL IN A SPODOSOL ON THE APENNINE MOUNTAINS, ITALY

The clay fraction of a Spodosol and its parent rock in the Apennine mo untains of central Italy were studied by powder X-ray diffraction (XRD ) and infrared (IR) spectroscopy, to evaluate the possibility of trans formation of chlorite into low-charge expandable minerals. Results ind icated that the main phyllosilicate in the rock was a slightly weather ed trioctahedral chlorite, rich in both Mg and Fe, together with dioct ahedral mica and minor amounts of kaolinite. In the BC horizon, chlori te has undergone partial transformation into 2 vermiculitic components , in 1 of which the interlayer could be removed by hot Na-citrate trea tment; the presence of a regular interstratified mineral (high-charge corrensite) was also observed. Further changes in the structure of chl orite were detected in the Bsl horizon, becoming more evident towards the soil surface. The first stage of weathering of chlorite involved F e oxidation and partial expulsion of Mg from the hydroxide sheet, foll owed by deposition of Al in the interlayer space. Iron is also removed from the interlayer sheet, possibly remaining, in the oxidized state, in the 2:1 octahedral sheet, and so contributing to the lowering of l ayer charge and transformation to a dioctahedral structure. When appro aching the surface, Al removal from the interlayers is enhanced by com plexing agents, and further charge reduction leads to the formation of 2:1 minerals with a smectite nature. Illite, because of its low conte nt in the soil clay fraction, contributes marginally to this weatherin g sequence, forming the high charged expandable component observed in the Bhs horizon. At the soil surface, a randomly interstratified vermi culite/illite was detected, which probably originated from K fixation by the higher-charged expandable minerals. This study of weathering in a natural soil strongly supports the hypothesis, previously ascertain ed by laboratory experiments, that chlorite can transform into a low-c harge expandable mineral.