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 semi-confined 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 11-021, 003-022, 023 and 130-13, and 060 reflections at less than 1.51, indicating a better-ordered 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 identifification 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.