Chemical properties of the graptolite periderm from the holy cross mountains (Central Poland)

Graptolite periderm in the Silurian shales from the Holy Cross Mountains of the Central Poland was examined by means of reflectance measurements and micro-FTIR spectroscopy. Mean graptolite reflectance (R_r) reaches 0.70–0.77%, and the vitrinite reflectance equivalent (VRE) is 0.67–0.72%. Graptolite periderm is composed predominantly of aromatic groups and rings with lesser amount of aliphatic and carbonyl/carboxyl groups. Chemical composition does not vary significantly between the samples from the two considered localities (the Prągowiec ravine and Bardo Stawy), which corresponds to the narrow range of graptolite reflectance. However, the samples from the Prągowiec ravine are characterized by higher hydrocarbon potential. It is found that many similarities occur in the chemical structure of the graptolite periderm and vitrinite within the reflectance range of R_r ≈ 0.7–1.5%. With increasing reflectance the length of the aliphatic chains (as inferred from the CH₂/CH₃ ratio) in the graptolite periderm decreases, and the relative content of the aromatic groups [as indicated by the CH_ar/(CH₂ + CH₃) ratio] begins to increase at • Key R_r ≈ 1.6%. This is accompanied by growth of the coherent domains and improvement in the structural order.