The inner zone of the crystals, which had relative low optical relief, a fluorescent nature and a triangular shape, is dark grey in color under BSEM; the outer zone, which had relatively high optical relief and a non-fluorescent nature, is light grey under BSEM (compare Figs.
Moreover, the elongated triangular shape and "patchy" appearance of the inner zones of the crystals, along with triple junctions at the crystal boundaries, were also observable under BSEM (Fig.
The concentric growth pattern, although less prominent than in longitudinal sections, appears as layers with an overall dark-grey or light-grey color under BSEM (Fig.
However some differences are observed among the FL images, where the concentric pattern is enhanced, if compared with BSEM images where the radial pattern is enhanced; this is probably a consequence of the effect of the thin section's thickness in FL microscopy, in which the incident light cross through the entire thin section, leading to an apparent increase of the thickness of the fluorescent concentric layers.
Microprobe elemental analyses (S, Mg, Sr, Na, Fe and Mn) of the calcite crystals under BSEM (Figs.
The observations that were made under TL, FL and BSEM show that the calcite crystals are internally composed of fluorescent and dark-grey sectors (or zones) and non-fluorescent and light-grey sectors (Figs.
Thus, the fluorescent and dark-grey calcite is interpreted to have formed during belemnite rostrum growth from a biologically controlled mineralization process (sensu Lowenstam and Weiner, 1989 and Mann, 2001), during which the calcite incorporated organic compounds that cause fluorescence and Mg and Na, each of which exhibit less atomic weight than Ca, resulting in the dark-grey color of the calcite under BSEM.
8, 10E-I); 2) the internal and triangular-shaped zoning of the outer zone in the crystals as observed under BSEM and FL (Figs.