Abstract

Carbonate rhombohedra have been Found in a majority of chert nodules examined. In limestones, they are calcite, sometimes accompanied by a small quantity of dolomite, Fe-dolomite and ankerite; in dolostones only dolomite was found within the cherts. The presence of syntaxial rims on the corroded cores of rhombohedra having a different composition and rhombic pores after dissolved rhomdohedra and silica pseudomorphs suggests changing conditions during the growth of rhombohedra. Syntaxial overgrowth on echinoderm ossicles can be older or younger than silicification. Rhombohedra were formed during the early stages of silica diagenesis. They preserved relics of microspherules (lepispheres ?) which are totally absent in other parts of the chert nodules.

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