Mineralogy of interior jewellery ammonites of the Samara region

Abstract

Background. Ammonite products are increasingly gaining in popularity. Ammonites are characterised by a variety of colour shades, shapes and sizes, making them suitable for manufacturing a wide range of souvenir and jewellery products. Russia enjoys significant resources of ammonites of interior and jewellery-ornamental quality; however, these materials remain insufficiently studied from the mineralogical point of view. This article presents the mineral composition and gemological characteristics of the Upper Jurassic interior jewellery ammonites of the Samara region for the first time.Aim. To describe the stratigraphic position and mineral composition of interior jewellery ammonites of the Samara region.Materials and methods. Late Jurassic ammonites (45 samples) were collected from a number of their main locations. The characteristic types of ammonites of interior-jewellery quality were distinguished and studied. Experiments included the determination of microhardness (25 determinations), density (12 determinations), luminescence features (10 samples), optical and petrographic analysis (5 thin sections), as well as the quantitative determination of mineral and chemical composition (3 samples), and electron probe studies (2 samples).Results. Ammonites are characterized by the presence of a mother-of-pearl layer with multi-coloured iridescence. The ammonites under study consist mainly of apatite (21—70 wt. %) and calcite (21—87 wt. %) along with quartz, aluminosilicates, gypsum, pyrite, zeolites, goethite, ilmenite and organic matter. The chambers of ammonite shells are made of phosphorite and calcite. The walls and partitions of shells have completely lost their original aragonite composition and consist of apatite and calcite with pyrite inclusions. The increased content of Sr (up to 0.17 wt. %) and Ba (up to 0.01 wt. %) are recorded as trace elements in ammonites. Calcite contains the following trace elements (wt. %): Mg — 0.17, Mn — 0.17, Fe — 1.14, Sr — 0.11, and Y — 0.17.Conclusion. The mineral and chemical (including microinclusions and trace elements) composition of interior-jewellery ammonites was established for the first time. The ammonites of the Samara region are of commercial interest as interior samples. Their shell fragments can be used in jewellery production. The ammonites under study can be collected along the coastal cliffs and beach area of the Volga river, thereby requiring no capital investments and producing no environmental pollution.