Vivianite from the Middle Jurassic fossiliferous concretions of Łuków, Polish Lowland

Abstract

Vivianite occurring in fossiliferous concretions dispersed in the Middle Jurassic clays exposed at Łuków (Polish Lowland), and forming rosette-shaped, coarse-crystalline aggregates up to several centimeters in diameter, partly filling the secretion fractures, as it is evidenced by the X-ray powder patterns, DT A, chemical and spectral emission analyses, and measurements of refractive indices, displays composition close to theoretical with very low content of Fe³⁺ ion. The mode of the occurrence of vivianite within septarian-fractured concretions testify its early diagenetic origin.