Abstract: Three kinds of opal-cristobalite, differentiated by the sharpness of the 4·1 Å XRD peak, were isolated from the Helms (Texas) bentonite by selective chemical dissolution followed by specific gravity separation. The δ18O value (oxygen isotope abundance) for these cristobalite isolates ranged from approximately 26–30‰ (parts per thousand), increasing with increased breadth of the 4·1 Å XRD peak. Opal-cristobalite isolated from the Monterey diatomite had a δ18O value of 34‰. These δ18O values are in the range for Cretaceous cherts (approximately 32‰) and are unlike the values of 9–11‰ obtained for low-cristobalite (XRD peaks at 4·05, 3·13, 2·4, and 2·49) formed hydrothermally or isolated from the vesicles of obsidian. The morphology pseudomorphic after diatoms, observed with the scanning electron microscope, was more apparent in the opal-cristobalite from the Monterey diatomite of Miocene age (approximately 10 million yr old) than in the spongy textured opal-cristobalite from the Helms bentonite, reflecting the 40 million yr available for crystallization since Upper Eocene.
The oxygen isotope abundance of Helms montmorillonite (δ18O = 26‰) indicates that it was formed in sea water while the δ18O values of the associated opal-cristobalite indicate that this SiO2 polymorph probably formed at approximately 25°C in meteoric water. Although both cristobalite and mont-montmorillonite in the bentonite were authigenic, the crystallization of the SiO2 phase apparently required a considerably longer period and occurred mainly after tectonic uplift.
In contrast to the results for cristobalite, quartz from the Helms and Upton (Wyoming) bentonites had δ18O values of 15 and 21‰ respectively. Such intermediate values, similar to those of aerosolic dusts of the Northern Hemisphere, loess, and many fluvial sediments and shales of the North Central United States (U.S.A.), preclude either a completely authigenic or a completely igneous origin for the quartz. These values probably result from a mixing of quartz from high and low temperature sources, detritally added to the ash or bentonite bed.