Radiaxial-fibrous calcites: A new look at an old problem

Marine radiaxial-fibrous and fascicular-optic calcites are very common but poorly understood pore-filling cements in Paleozoic and Mesozoic marine neritic and upper bathyal limestones. The main diagnostic feature of these cements is their converging or diverging crystal c-axis, respectively. The reasons for this anomalous texture are at present unknown. Another controversy is due to the relative lack of occurrences in Cenozoic strata and their apparent absence in Quaternary marine limestones. Despite these uncertainties, marine fibrous cements are thought by some to be amongst the best proxies for the geochemistry of ancient oceans because of the absence of metabolic effects during their precipitation. Applying electron backscatter diffraction analysis, we here for the first time document radiaxial-fibrous and subordinate fascicular-optic fibrous biominerals from well-preserved Jurassic and Cretaceous low-Mg calcite belemnite rostra. The finding of fibrous biogenic calcites in combination with the recent description of Holocene and modern fibrous calcites in stalagmites represents—in the view of the authors—a significant advance in carbonate research. Here, these findings are placed in their wider, processes-oriented context and the significance of biogenic and speloan fibrous calcite for their marine counterparts is assessed. Comparing the physico-chemical and organomineralic properties of different precipitation sites indicates that the diagnostic variations in the crystal c-axis orientation are not related to a specific nucleation environment or substratum. In contrast, preliminary crystallographic analyses suggest that kinetic factors during nucleation or subtle gradients in the statistical replacements of Ca2+ by Mg2+ in the crystal structure may lead to local strain resulting in converging or diverging crystal c-axes.