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 microcrystalline 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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