Distinguishing celadonite from glauconite for environmental interpretations:a review

Celadonite and glauconite are comparable in terms of physical,chemical and mineralogical characteristics.Formation of both these minerals requires slightly oxygen-depleted conditions in a semiconfined micro-environment,facilitating the uptake of Fe into the structure.Although glauconite is ubiquitous in marine deposits,celadonite forms in both marine and non-marine environments,more commonly by altering intermediate to mafic rocks.A review of data across the geological column shows that both these minerals are more common in the Mesozoic and Cenozoic,which may be related to extensive volcanism.The composition of celadonite and glauconite is highly variable and is controlled by the availability of cations within the pore water micro-environment.The major element composition of celadonite overlaps with that of evolved to highly evolved glauconite to a large extent.Existing data and representative samples of celadonite and glauconite reveal subtle differences in X-ray diffraction parameters and Fourier transform infrared(FTIR)spectra.Celadonite shows sharper basal and hkl reflections than glauconite.It characteristically exhibits distinct 111-021,003-022,023 and 130-131,and 060 reflections at less than 1.51A,indicating a betterordered crystallographic structure than glauconite.The FTIR spectra of celadonite and glauconite show similar absorption bands although Si-O bending,Si-O stretching and OH-stretching bands are sharper and well resolved in celadonite than that of glauconite.For palaeoenvironmental interpretations,it is,therefore,necessary to examine the subtle differences in X-ray diffraction parameters and FTIR spectra to confirm the identification of these two minerals.The composition of celadonite relates closely to the host rock and depositional environment.The celadonite associated with mafic rock shows relatively high Fe,Mg and low Al contents.In contrast,those associated with felsic to intermediate composition rocks show slightly higher contents of Al and lower content of Fe,Mg.The mineral chemistry of celadonite varies from marine to continental settings.Marine celadonite contains higher Fe,Mg and lesser Al than their non-marine counterpart.Celadonite with relatively higher Al may show slightly lower interlayer K than Fe and Mg-rich celadonite for charge balancing.

Origin and geochemical characterization of the glauconites in the Upper Cretaceous Lameta Formation, Narmada Basin, central India

This study presents geochemical characteristics of glauconites in estuarine deposits within the Maastrichtian Lameta Formation in central India. Resting conformably over the Bagh Group, the Lameta Formation consists of ~4-5 m thick arenaceous, argillaceous and calcareous green sandstones underlying the Deccan Traps. The sandstone is friable, medium-to coarse-grained, well-sorted and thoroughly crossstratified, and contains marine fossils. Detailed petrography, spectroscopy and mineral chemistry indicates unique chemical composition of glauconite with high KO, MgO, AlOand moderate TFeO. Glauconite is formed by the replacement of K-feldspars, initially as stringers in the cleavages and fractures of feldspars. Incipient glauconite subsequently evolves fully, appearing as pellets. Fully-evolved glauconite pellets often leave tiny relics of K-feldspar. XRD exhibits characteristic peak of 10A from basal(001)reflection of glauconite, indicating the "evolved" character. The KO content of glauconites in the Lameta Formation varies from 5.51% to 8.29%, corroborating the "evolved" to "highly-evolved" maturation stage.The TFeOcontent of glauconite varies from 12.56% to 18.90%. The PASS-normalized-REE patterns of glauconite exhibit a "hat-shape" confirming the authigenic origin of glauconites. The slightly-negative to slightly-positive Ce anomaly value and the moderate TFeOcontent of glauconite agree well with a suboxic,estuarine condition. The replacement of K-feldspar by the glauconite contributes towards the high KO content. Compositional evolution of glauconites in the Lameta Formation is similar to those observed in many Precambrian sedimentary sequences.