CRYPTOCRYSTALLINE RELATIONSHIPS OF SILICA PHASES IN CHERT

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 distinctive lineations (.01 –.02 microns spacing) in image contrast which often occur in association with streaked diffraction patterns or evidence of twinned crystals. Boundaries between laminations of opal-CT and quartz are not sharp, as previously described in the literature, but are composed of a 5 micron thick mixed-phase mosaic.

Selected area diffraction, tilting experiments, and dark field techniques with the transmission electron microscope permit the characterization of opal-CT and quartz relationships at a scale that allows us to address the mechanisms of transformation.

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Notes on contributors

ANNEMARIE MEIKE

Present address: Earth Sciences Department L-219, Lawrence Livermore National Laboratory, University of California, California 94550