Dehydroxylation behavior of heat-treated and steam-treated homoionic cis-vacant montmorillonites

• Published in: Clays and clay minerals 1999-10, Vol.47 (5), p.591-604
• Main Authors: EMMERICH, K, MADSEN, F. T, KAHR, G
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Mineralogy; Silicates
• ISSN: 0009-8604; 1552-8367
• DOI: 10.1346/CCMN.1999.0470506

Li + , Na + , Ca 2+ , Sr 2+ , Cu 2+ , or Zn 2+ -saturated samples of a cis -vacant montmorillonite from Linden, Bavaria, were heated to temperatures between 200–700°C. Half of each heated sample was subsequently autoelaved under steam at 200°C (∼1.5 MPa) to promote rehydroxylation. The smectites were characterized by cation-exchange capacity (CEC), determination of exchangeable cations, infrared (IR) spectroscopy, and thermoanalytical investigations of evolved water in a thermobalance linked with a mass spectrometer. Changes in the montmorillonite structure and dehydroxylation behavior are related to three respective mechanisms: type of the interlayer cation, interlayer cation radius, and the movement of the interlayer cation. The migration of the smaller Li + , Cu 2+ , and Zn 2+ ions after heating produces a strong reduction of CEC due to the Hofmann-Klemen effect before the initiation of dehydroxylation. Thereafter, the CEC of these smectites remains constant over a large temperature interval during dehydroxylation. After rehydroxylation, Cu 2+ and Zn 2+ -rich samples release 16–23 meq/100 g of Mg 2+ from the structure. No Mg 2+ release is observed for the Li + -rich montmorillonite. Also the dehydroxylation behavior after rehydroxylation differs between the Cu 2+ , Zn 2+ , and Li + -rich samples. The mass curves of the evolved water during thermoanalysis of the rehydroxylated Cu 2+ and Zn 2+ -rich smectites show a peak doublet between 480–700°C. For the Li+, Na + , Ca 2+ , and Sr 2+ -rich montmorillonites, the second peak disappeared and a third peak at ∼760°C developed after rehydroxylation. The resulting structure after rehydroxylation of all samples is celadonite-like.