WEATHERING OF SILICATE MINERALS BY ORGANIC-ACIDS .2. NATURE OF RESIDUAL PRODUCTS

Investigations were done to understand the nature of the residual mate rials as a result of weathering of six different types of silicate min erals by organic acids, primarily oxalic acid. The probes used include chemical analysis, petrographic microscopy, X-ray diffraction, infrar ed spectroscopy and scanning electron microscopy. Results indicate tha t different silicates are affected in different ways. Olivine and biot ite undergo extensive fragmentation by breaking of larger grains; this behaviour is exhibited to a lesser extent by epidote and hornblende, whereas with microcline and tourmaline it is not perceptible. The latt er two minerals show dissolution of ultrafine particles present in the unreacted grains; etching of grain surfaces also occurs. New crystall ine phases are formed with all the silicates. Of these, the most notab le is an unidentifiable phase, probably a silicate, which is common in all residues except biotite. Crystalline oxalate salts are predominan t in olivine and biotite residues. Epidote and hornblende residues als o reveal almost identical prismatic crystal growth on the edges and su rfaces of the grains.