Crystallinity of Subsurface Clay Minerals in the Te Mihi Sector of the Wairakei Geothermal System, New Zealand

Abstract

Lacustrine sediments of the Falls Formation in the Te Mihi sector of the Wairakei geothermal system occur at depths between and where the measured drillhole temperatures range from 90 to 220?C. The sediments contain clay minerals of variable composition and structure, including discrete illite and chlorite. The crystallinity of the (001) reflections for layer clays increases with increasing host rock temperature and depth, with most (Kubler Index) values ranging from 1.40 to The Kubler Indices of the illites are from 0.35 to An empirical relationship between the Kubler Index of the (001) peak (air-dried) and the measured drillhole temperatures (T) from 90 to 230?C is, T ("C) = 249 - where C is the Kubler Index The K content per component of of increases with increasing drillhole temperature and correlates negatively with their Kubler Indices. Chlorite crystallinity as represented by the (002) reflectios, varies only slightly with the drillhole temperature but shows consistently high values throughout (average = 0.28). There is an approximate linear correlation between the (002) of the air-dried and ethylene glycolated chlorites in both the and size fractions.