Mineral composition and gemological characteristics of the interior and jewellery ammonites of the Yaroslavl region

Products from ammonites are currently in high demand. The author for the first time studied in detail the mineral composition and gemological characteristics of the interior and jewellery ammonites of the Yaroslavl region. Late Jurassic ammonites redeposited in the water-glacial Quaternary sediments. Ammonites are characterized by a nacre layer with bright multi-colored iridescence. The complex of ammonite studies included the determination of microhardness, specific gravity, luminescence, optical-petrographic analysis, determination of the mineral and chemical composition, electron-probe and electron-microscopic studies. Ammonites consisted mainly of apatite (42-67 wt. %) and calcite (16-48 wt. %). Quartz, glauconite, kaolinite, hydromica, goethite, pyrite and x-ray amorphous phase were present. The following elements-admixture had high content, wt. %: Sr - 0,170 and Ba - 0,028. The contents of radioactive and carcinogenic elements were at the level of background. The walls and partitions of the ammonite shell completely lost the original aragonite composition and consisted of apatite, calcite with inclusions of goethite and pyrite. The iridescence of the pearlescent layer was associated with lamellar layers of predominantly apatite, which inherited the structure of lamellar layers of aragonite. The color of irisation was determined by the size and orderliness of the plates and did not depend on their mineral composition. Ammonites of the Yaroslavl region are of commercial interest as interior samples. Fragments of the pearl layer with bright iridescence can be used to make ammolite cabochons.