Silicification phenomena in fossil Belemnite guards

Most Belemnite guards from-the Upper Senonian 'Belemnite Churchyard' (Maastrichtian Cr3b) show partial silicification. Oriented calcite that constitute the guards is partly replaced by a-quartz. Silicification appears just below the guard surface. Fine quartz needles (20 /tm average in cross-section) follow the radial structure of the calcite prisms. These quartz needles are polycrystalline and consist of domains of preferredly oriented crystals, each domain having a different orientation. The crystallographic orientations of these directions are random. There is no relation between the orientation of the original calcite and that of the quartz replacements. Silicification in this case is probably not a molecule-for-molecule replacement mechanism. Replacement might have taken place by way of dissolution and subsequent filling up with quartz of larger calcite units.