Palagonite Reconsidered: Paracrystalline Illite-Smectites from Regoliths on Basic Pyroclastics

Vadim Berkgaut1, Arieh Singer1 and Karl Stahr2
1 The Seagram Centre for Soil and Water Sciences, Faculty of Agriculture Rehovot 76 100, Israel
2 Institut für Bodenkunde, Universität Hohenheim 70593 Stuttgart, Germany

Abstract: Poorly crystalline authigenic alteration products of basic pyroclastics from the Golan Heights, Israel, were investigated by XRD, DTA, TGA, FTIR and chemical analysis. Modeling XRD patterns with the use of NEWMOD code provided a way to identify these clays as random interstratified illite/smectites (I/S) with ∼70% of illitic interlayers. Their characteristic features were very poor basal reflections, distinct hk bands, high CEC and low (∼2%) K2O content. Crystallite thickness distribution was found to follow Ergun's model with a weight-average thickness of 2.7–2.8 layers. A new method was proposed to calculate the proportion of kaolinite and 2:1 minerals in their mixtures and the average crystallochemical formula of 2:1 minerals in the presence of kaolinite. The method starts from data of chemical analysis and TGA and assumes that the anionic frameworks of kaolinite and 2:1 minerals are exactly O10(OH)8 and O10(OH)2 respectively. The number of OH-groups per ten oxygens not bonded to H in the empirical formula of the mixture is used to evaluate the proportion of kaolinite. Formation of I/S in well-drained environments under humid mediterranean climatic conditions was attributed to long dry seasons. Interstitial water composition was shown to be consistent with authigenic formation of I/S.

Key Words: Basic pyroclastics • Humid mediterranean weathering • Illite/smectite • Poorly-ordered layer silicates

Clays and Clay Minerals; October 1994 v. 42; no. 5; p. 582-592; DOI: 10.1346/CCMN.1994.0420511
© 1994, The Clay Minerals Society
Clay Minerals Society (www.clays.org)