Decorative-jewelry heteromorphic ammonites from Campanian deposits of the Severnaya Sosva River valley, Northern Urals: A new mineral type

The article presents results of a study of ammonites with the heteromorphic shell from Campanian deposits of the Upper Cretaceous, Severnaya Sosva River, as an industrial mineral providing the decorative specimens and jewelry material. The complex of investigation of the ammonite shells included determinations of their density, microhardness, luminescence, mineral and chemical composition and the optical petrographic and electron microscopic studies. The ammonite shells consist predominantly of quartz (more than 79 wt. %) represented mostly (more than 52 wt. %) by an amorphous form. The cherty siltstone filling the destroyed ammonite chambers also contains cristobalite, tridymite, albite, microcline, glauconite, illite and smectite, pyrite, ilmenite, and iron hydroxides. Of the impurity elements, increased contents of Ba (about 0.0129 wt. %) and Cr, V, Zn, Rb, Sr, and Zr (less than 0.01 wt. %) were determined. Based on the mineral composition, the ammonite shells are classified as a new siliceous type. The walls of the shells are characterized by bright colored iridescence, which is associated with preservation of the structure of the original aragonite lamellas completely replaced by quartz in the crystalline and amorphous forms. The bright colored iridescence of the shells makes it possible to use the ammonites as a gem stone material. A new mineral type of ammolite, the siliceous ammolite, has been established. The ammonite shells may be gathered from the Campanian deposits in coastal cliffs or talus without mining workings, which does not require significant material costs and does not disturb the environment.