COLLOIDAL CALCITE IN FORAMINIFERAL TESTS - CRYSTALLIZATION AND TEXTURE OF THE TEST

Observations during this study and those reported in previous works sh ow the consistent presence of calcitic globular crystallites in the te sts of all foraminiferal groups. The globular crystallites (primary [o r first order] elements) can be grouped into rows or needles (secondar y [or second order] aggregates), columns (tertiary [or third order] ag gregates), tablets (quaternary [or fourth order] aggregates) or rhombo hedral crystals (quinternary [or fifth order] aggregates). They may be discernible directly or only after a surface dissolution of the aggre gates. This grouping into more or less complex arrangements has been r eported for inorganic minerals, but it has never been taken into accou nt for biomineralization in foraminiferal tests. The presence of these colloidal crystallites can be compared with similar crystallites obse rved in mineralized tissues of other organisms, and with colloidal cry stallites that are frequent in inorganic minerals. These observations suggest that both nucleation and arrangement result from a double set of processes: physicochemical processes under an energetic control and biological processes under a strict biological control. The texture o f the test depends on these controls and on the location at which the nucleation occurs: in vesicles (porcelaneous tests) or on an organic m embrane (hyaline tests). The various textures of the foraminiferal tes ts that result from biological control may be related to phylogenetic evolutionary processes.