Trilobite thin section analysis clarifies matrix separation properties

Abstract

Mechanical preparation of trilobite and other calcitic fossils in carbonates depends on fossil-matrix separation properties. In this pilot study, thin sections with trilobite fossils from the Ordovician of Norway and the Devonian of Morocco were investigated to identify the mineralogical basis for separation differences, using petrographic microscopy as well as semi-quantitative elemental analysis and element mapping under the scanning electron microscope. The results show that the matrix is composed of fine-grained calcite (CaCO3) together with silicates, i.e., impure limestone with a clastic component. The trilobite specimens are composed of calcite, often in the form of large euhedral crystals. A thin layer of different composition was found along the boundaries between trilobite fossils and their host rock in well-separating specimens. Silica and sometimes barium sulphate were identified in these thin layers. We conclude that partial silicification at the surface of fossils facilitates separation during mechanical preparation.