Kristalgreining og efnasamsetning leirs í gufuborholum á Hengilsvæðinu | Phase analysis and chemical compsition of clay in geothermal wells from the Hengill area

Kristalgerð og efnasamsetning leirsteinda í borsvarfi úr gufuborholum á Hengissvæðinu var greind og borin saman við efnasamsetningu grannbergsins. Montmorillonít/vermikúlít leir er ríkjandi í efstu 600 m holanna, þá blandleir niður á um 1000 m þar sem klórít verður ríkjandi leirsteind. Efnasamsetning leirsins er nokkuð óregluleg í efri hluta holanna en verður regluleg í klórít beltinu. Lækkandi vatnsmagn, magnesíum, járn og kísill innan klórít beltisins myndar nokkuð reglulegan feril með vaxandi dýpi. Efnasamsetning grannbergsins hefur ekki teljandi áhrif á efnasamsetningu leirsteindanna og má af því ráða að leirsteindinar falli út úr jarðhitavatni. Reglulega fylgni dýpis og efnasamsetningar leirsins í klórít beltinu mætti hugsanlega nýta sem jarðhitamæli.

Crystal structure and chemical composition of clay minerals in cuttings from geothermal wells in the Hengill geothermal system were analyzed and compared with the chemical composition of the host-rock. Montmorillonite/vermiculite clay dominates the uppermost 600 m of the wells followed by mixed-layer minerals down to about 1000 m where chlorite becomes the dominating clay mineral. The chemical composition of the clay is slightly irregular in the upper half of the wells but becomes regular within the chlorite zone. Decreasing water content, and variation in magnesium, iron and silica within the chlorite zone from a fairly regular trend with increasing depth. Chemical composition of the host rock has insignificant effect upon composition of the clay minerals, indicating their precipitation from hydrothermal fluid. Coherent variation between depth and chemical composition of the clay minerals within the chlorite zone might be used as a geothermometer.