ABSTRACT

Two contemporaneous Middle Cambrian glauconies from different depositional environments in southern Baltoscandia (the Acrothele granulata Conglomerate of Oland and the Exsulans Limestone Formation of Bornholm) have been studied by means of XRD, EMP, and SEM.

These glauconies show a characteristic high silica (> 50%), high alumina (> 10%), and low iron (< 18%) content. The total iron values, as well as substitution of aluminum by iron, indicate a gradual transition between illitic and glauconitic minerals, and support the layer-structure theory to some extent. There is no correlation between interlayer cations, and aluminum/iron.

The chemical composition is environmentally induced and can be related to formation and deposition in shallow, cool water.

All progressive stages of weathering from glaucony to goethite are found in the Kalby Member of the Exsulans Ls. Fm.

A new morphological type of glaucony is described: molds closely replicating the (inorganic) pore space where they occur, having imprints of crystal faces from the surrounding calcite cement.