The Effect of Acid and Heat Treatment on Montmorillonoids

Abstract
H-montmorillonite, -beidellite, and -nontronite were found to change spontaneously into Al-clays. Rates of conversion of H- to Al-clay were slow at 0 ~ C, but at tem- peratures of around 100 ~ C, moist H-montmoriUonite changed to Al-saturated montmo- rillonite within 24 hours. It appeared that Al-ions moved from lattice positions to exchange positions, with octahedral A1 moving more rapidly than tetrahedral A1. Treatment of montmorillonoids with HCI solutions at 80 ~ C resulted in the removal of large quantities of Fe, Mg, and AI. The residue after hot acid treatment appeared to be a mixture of essentially unaltered montmorillonoid and SiO~, rather than a new mineral. On heat treatment, H- and Al-montmorillonites became largely non-expanding of 300 ~ C, and lost the bulk of the CEC which could be attributed to permanent lattice charge. H- and Al-nontronite and -beidellite, on the other hand, did not suffer irre- versible dehydration and loss of CEC tmtil heated to sufficiently high temperatures to cause the expulsion of lattice OH. Montmorillonites With largely octahedral charge had smaller CEC's after heating to 500 ~ C than did montmorillonites with appreciable tetrahedral charge.

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