Chalcedony with or without agate banding occurs as amygdaloidal and vein fillings besides zeolites in the ∼65 Ma old Deccan flood basalts. Thin-section study has shown the occurrence of length-slow fibrous quartz in association with length-fast fibrous chalcedony in samples collected from a depth of 122 meters in a borehole passing through the Deccan basalts at Killari (18°03'N and 76°33'E) in Maharashtra, India. X-ray powder-diffraction studies show at least seven diffraction peaks corresponding to a monoclinic cell of the silica polymorph moganite, with lattice parameters α = 0.876(2) nm; b = 0.688(1) nm; c = 1.0715(2) nm; β = 90.07° in addition to α-quartz. Thermogravimetry indicates moganite contents up to 70 ± 10 wt% in moganite-rich samples and 50 ± 10 % moganite in chalcedony of Killari borehole samples. Both thermogravimetry and FT-IR spectroscopy point to the presence of a hydrous component in moganite. Solid state 29Si MAS-NMR and 1H MAS-NMR spectroscopic studies show characteristic peaks at -110 ± 2 ppm and 5.28 ± 0.05 ppm respectively, confirming the presence of moganite in the Deccan samples. Moganite of Killari is associated with amygdaloidal flows containing the zeolites—chabazite, natrolite, heulandite and mordenite, but is absent in flows with laumontite, indicating the stability of moganite up to the laumontite zone.

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