Figure 6. SEM photo of diatomite. Clay-rich Zone of Opal A, 848 ft in well 511S1-1N, North Belridge. Sample is about 40% Opal A (mainly broken and whole diatoms) and minor organics. The remainder is about 15% mixed layer illite-smectite clays (with 30-40% expandable layers), 40% detrital quartz and feldspar, and minor pyrite.