LATE JURASSIC INTERIOR-JEWELRY QUALITY AMMONITES FROM THE ULYANOVSK REGION: THE MINERAL COMPOSITION AND GEMOLOGICAL CHARACTERISTICS

Abstract

Decorative articles made of ammonites are currently in high demand. The mineral composition and gemological characteristics of interior-jewelry ammonites from the Upper Jurassic of the Ulyanovsk Region have first been studied in detail. The ammonites are characterized by a pearl layer showing a bright varycolored iridescence. A complex of ammonite investigations comprised determinations of their microhardness, density, and luminescence; their optical-petrographic analysis; determinations of the mineral and chemical composition; and the electron-probe and electron-microscopic investigations. The ammonites consist mainly of apatite (22-88 wt. % ) and calcite (6-64 wt. %). Quartz, gypsum, pyrite, zeolites, ilmenite, rutile, goethite, zircon, and organic matter are present. Impurity elements recorded in elevated concentrations are (in wt. % ) Sr, 0.237; Ba, 0.011; and Pb, 0.023; and those recorded in low concentrations (less than 0.01 wt. %) are Ni, Cu, Zn, Rb, U, Th, Y, and As. The outer walls and septa of an ammonite shell have completely lost their original aragonite composition and consist of apatite and calcite with inclusions of pyrite and organic matter. The pearl layer iridescence is related to lamellar layers of apatite, that inherited the structure of lamellar layers of aragonite. The iridescence color is determined by the size and order of the lamellars and does not depend on their mineral composition. Ammonites of the Ulyanovsk Region are of commercial interest as interior-decorative specimens. Internal fragments of the shells can be used in jewelry. Ammonites are collected in steep ravine walls, which requires no capital investment and does not violate the environment.