PALAGONITE RECONSIDERED - PARACRYSTALLINE ILLITE-SMECTITES FROM REGOLITHS ON BASIC PYROCLASTICS

Poorly crystalline authigenic alteration products of basic pyroclastic s from the Golan Heights, Israel, were investigated by XRD, DTA, TGA, FTIR and chemical analysis. Modeling XRD patterns with the use of NEWM OD code provided a way to identify these clays as random interstratifi ed illite/smectites (I/S) with approximately 70% of illitic interlayer s. Their characteristic features were very poor basal reflections, dis tinct hk bands, high CEC and low (approximately 2%) K2O content. Cryst allite 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 mi xtures and the average crystallochemical formula of 2:1 minerals in th e presence of kaolinite. The method starts from data of chemical analy sis and TGA and assumes that the anionic frameworks of kaolinite and 2 :1 minerals are exactly O-10(OH)8 and O-10(OH)2 respectively. The numb er of OH-groups per ten oxygens not bonded to H in the empirical formu la of the mixture is used to evaluate the proportion of kaolinite. For mation of I/S in well-drained environments under humid mediterranean c limatic conditions was attributed to long dry seasons. Interstitial wa ter composition was shown to be consistent with authigenic formation o f I/S.