Figure F27. XRD patterns of clay minerals and zeolites in the 2-µm grain-size fraction measured on textured mounts in the air-dried state, after ethylene glycolation, and after heat treatment at 550°C. Samples from the uppermost red silty clays of Unit I include detrital illite and generally show broad low-angle peaks related to admixtures of various expandable clay minerals. Because chlorite is missing (no 14-Å peak after heat treatment,) the 7- and 4.77-Å peaks may indicate minor proportions of vermiculite as one of the expandable clay minerals besides poorly crystallized smectite. Below 240 mbsf, the expandable clay minerals consist of pure, well-crystallized smectite as indicated by the relatively sharp 17-Å peaks in the glycolated state. The sample of green sandstone displays a 8.9-Å peak of clinoptilolite/heulandite that disappears after heat treatment and thus provides a positive indication of clinoptilolite instead of heulandite, which would survive the heat treatment.