Nodular Fraction of Phosphatic Sand from the Namibia Shelf

Abstract

Morphology, microstructure, and chemical composition of the nodular fraction of phosphatic sand from the outer Namibia shelf are considered. The nodules are divided into three varieties: concretions, phosphatic casts of shells pelecipod and gastropod, and phosphatized sea lion coprolites. A part of nodules shows massive homogeneous structure, whereas other nodules contain sand-size phosphatic grains derived from the host phosphatic sand. All nodules display the same spectrum of microstructures, including colloform, granular-globular, and crystallomorphic ones. The crystallomorphic nodules are composed of scattered and aggregated crystallites of isometric, rod-shaped, and dumbbell-shaped forms, as well as crystallized shells on grains incorporated into nodules. Phosphatized remains of diatoms, foraminifers, and embryonic gastropods, as well as vermicular and filamentous bacterial forms occur among biogenic components of the nodules. Framboidal pyrite is present in all nodules. In terms of phosphate microstructure, character of organic remains, and chemical composition, the nodules are similar to phosphate grains of the host sand, recent phosphorites from the inner Namibia shelf, and some ancient phosphorites.