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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 65A (1995)No.
4. (October), Pages 633-638
Moganite as an Indicator for Vanished Evaporites: A Testament Reborn?
Peter J. Heaney
ABSTRACT
Although lutecite, or length-slow silica, frequently is used as an indicator
for evaporites, its occasional presence in non-evaporitic regimes has shed
suspicion on its reliability. This study provides evidence that lutecite
actually is a newly recognized mineral called moganite. Powder X-ray diffraction
studies reveal that moganite is abundant in but not restricted to cherts
that formed in evaporitic environments. Quantitative analysis using Rietveld
refinement methods indicates that non-evaporitic silica typically contains
between 5 and 15 wt % moganite, whereas evaporitic specimens may contain
between 20 and 75 wt % moganite. No chert from non-evaporitic settings
has been found to contain more than 25 wt % moganite. Consequently, enhanced
moganite concentrations (> 20 wt %) in m crocrystalline silica may prove
a valuable indicator for vanished evaporites. The frequent association
of moganite with "Magadi-type" chert suggests that moganite is a diagenetic
alteration product of the hydrous Na-silicate magadiite. The transformation
of magadiite to moganite occurs over hundreds to thousands of years, whereas
the inversion of metastable moganite to quartz often requires tens of millions
of years.
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