Abstract
ABSTRACT A transmission electron microscope survey of silica phase relationships in layered chert indicates that the phase transformation (opal-CT→ quartz) is complex, even at a sub-micron scale. Preliminary results, utilizing electron diffraction and dark field methods, indicate that the crystallographic relations between phases and the nature of the transformation can be illustrated by these techniques. Previous investigations of siliceous rocks by x-ray diffraction and scanning electron microscope have been restricted to identification of major silica phases in bulk rock samples and recognition of characteristic free-growth morphologies. In this study, identification and location of silica phases is accomplished at the micron scale. Two different kinds of cherts are characterized by dissimilar opal-CT textures-fine-grained groundmass (.01–.1 microns) and bladed crystallites (.1–1.0 microns). Increased crystallinity of opal-CT is not reflected by changes of its bladed morphology. Quartz crystals display...
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