Mineralogy of fossilized ammonites

The composition and structure of mineral substance in shells of three Jurassic ammonites (Cadoceras elatmae) from the Makar’ev South outcrop in Kostroma oblast, Russia, have been studied. The studied shells differ in degree of preservation. The shell of one ammonite with partly retained internal structure has been replaced with quartz, carbonate-bearing apatite-(CaF), and newly formed aragonite. Other shells without retained internal structure underwent phosphatization rather than replacement with quartz. This difference serves as the basis for a discussion on fossilization conditions. Silicification of the shell supports preservation of its wall and elements of the internal structure. It is suggested that quartz might be biogenic in origin and was formed in cavities of phosphate substance, which had replaced the soft tissue of ammonites.