X-ray determination of the silica minerals in submicroscopic intergrowths

When tridymite and cristobalite are found in isolated grains or in relatively coarse intergrowths, they can easily be identified with the microscope. But, when they occur in submicroscopic intergrowths with feldspar, as often found in spherulites and the groundmass of lavas, they defy the ordinary means of identification; and whether a given rock is reported to contain cristobalite or tridymite may be largely a matter of personal opinion or prejudice. The purpose of this paper is to outline a method of determining the silica minerals when found in these intimate intergrowths, and to show the applicability of the method in analyzing the submicroscopic groundmass of lavas.