An HRTEM investigation of the metastable low-temperature silica phase opal-CT in cherts and porcelanites from the Monterey Formation, CA

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5721711

Cady, S L; Wenk, H R ^[1]

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1. Univ. of California, Berkeley, CA (United States)

High resolution transmission electron microscopy (HRTEM) is used to investigate the metastable low-temperature silica phase opal-CT in cherts and porcelanites from the Miocene Monterey Formation of California. Low-dose imaging techniques developed to image highly beam sensitive proteins were used in this study and have resulted in good phase contrast images of this hydrous silica phase. Detailed X-ray powder diffraction studies of stratigraphically equivalent rocks along the Santa Barbara coast indicate that the primary d-spacing of newly formed opal-CT differs in rocks with different ratios of silica and detrital minerals. Opal-CT forms progressively later and with a smaller primary d-spacing in rocks with increasing amounts of detrital minerals. In siliceous cherts opal-CT occurs as long needles that most often form dense spherulitic fiber bundles which are randomly dispersed within the rock matrix. The random orientation of fiber bundle nucleation centers does not appear to be associated with any obvious nucleation site, unlike the length-slow opal-CT fibers known as lussatite. Opal-CT needles produce optical diffractogram patterns that are compatible with tridymite and crystobalite. Streaking in the diffraction pattern of individual needles is attributed to a high density of planar defects parallel to their length. Planar defects are not as abundant in opal-CT needles formed in detrital-rich rocks suggesting the rapid growth of opal-CT in highly siliceous environments results in a greater proportion of stacking disorder in the needles. HRTEM provides a method for investigating the development of the microstructure of opal-CT during diagenesis.

• OSTI ID: 5721711
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360602 -- Other Materials-- Structure & Phase Studies
58 GEOSCIENCES
580000* -- Geosciences
CALIFORNIA
CHALCOGENIDES
CHEMISTRY
COHERENT SCATTERING
DEVELOPED COUNTRIES
DIAGENESIS
DIFFRACTION
ELECTRON MICROSCOPY
GEOCHEMISTRY
GEOLOGIC FORMATIONS
MICROSCOPY
MICROSTRUCTURE
MINERALS
NORTH AMERICA
OPALS
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PHASE STUDIES
SCATTERING
SILICA
SILICON COMPOUNDS
SILICON OXIDES
TRANSMISSION ELECTRON MICROSCOPY
USA
X-RAY DIFFRACTION