Original paper

Occurrence of moganite-rich chalcedony in Deccan flood basalts, Killari, Maharashtra, India

Parthasarathy, Gopalakrishnarao; Kunwar, Ajit Chand; Srinivasan, Ramaswamiah

Abstract

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 a = 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.

Keywords

moganitechalcedonydeccan basaltszeolites