Mineral Composition and Stratigraphic Position of Interior Jewelry Ammonites from the Middle Aptian of the Republic of North Ossetia-Alania

The stratigraphic position and mineral composition of ammonite shells of interior and jewelry quality from the Republic of North Ossetia-Alania have been studied for the first time. Ammonites were collected from the middle reaches of the Urukh River and originated from middle Aptian sediments accumulated in anoxic environments. The set of ammonite studies included determination of microhardness, density, luminescence; optical–petrographic analysis; quantitative determination of chemical and mineral composition; and electron probe studies. Ammonites consist mainly of calcite, pyrite, and aragonite. The walls and septa of the shells retained the original aragonite composition. The residential and destroyed hydrostatic chambers of the shells are composed of marl. Micron inclusions of ankerite and apatite have been found from accessory minerals. Sr and Ni have been identified as impurities. The mineralization of the shells began with the formation of pyrite layers. The mineral composition of the ammonites is calcite–pyrite.