Abstract
The types of clay minerals formed by hydrothermal alteration of basaltic rocks in geothermal fields in Iceland have been a matter of interest for some time. The fine-grained nature and poor crystallinity of the minerals make investigation difficult. X-ray-diffraction analyses suggest the presence of numerous different minerals. DTA analyses, supported by infrared-absorption analyses, show the mineralogy to be much simpler. It is suggested that a gradual transformation of a smectite rich in iron and magnesium into chlorite occurs at temperatures of 200–230°C.

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