Miscellaneous Features in Argillaceous Rocks

Abstract

“Framboid” describes spheroidal aggregates of pyrite microcrystallites (Rickard, 1970). They are common in organic rich shales and give clues to geochemical and microfabric conditions. Rickard indicates the spheroidal shape may be due to pseudomorphism of a pre-existing spherical body. It may represent replacement of an immiscible organic liquid globule by iron sulfide or infilling by microcrystalline pyrite of spherical gaseous vacuoles or even by pseudomorphism of single-celled microorganisms. Framboid morphology is also attributed to pyritization of microflora (Javor and Mountjoy, 1976). Amstutz et al. (1967) indicate the framboid is inherited from the colloidal glob of iron monosulfide formed authigenically in sediment. Authigenic pyrite is found with organic matter and is an indicator of anaerobic sulfide diagenesis (Berner, 1970). Field and laboratory observations (Berner, 1969; Love, 1967) show pyrite forming at shallow sediment burial depth (10 cm to 3 m) under low temperature (<80° C). A good brief summary of their origin is given by Kalliokoski (1974). Observations suggest framboid formation in wet, unconsolidated sediment, probably associated with a flocculated clay of soupy consistency. Thus, their presence offers clues to the early burial diagenesis history of clayey sediment.