Calcite

Abstract

Biogenic calcite occurs as chambered foraminifera tests and test fragments (Figs 13–15), ovoid coccolith and stellar discoaster plates (Figs 16–9). Foraminifera chamber forms and arrangements are extremely varied (see Appendix 5). Most of the formanifera tests accumulating on the deep seafloor are of planktonic species which, in life, inhabit mainly the upper 200–300 m of the water column. Most planktonic foraminifera have multichambered tests, the chambers being spirally and/or cyclically arranged. Test walls are frequently perforated by numerous pores. Coccoliths are minute oval plates of calcite produced by unicellular planktonic algae. Most are formed of minute rhombohedral crystals, although a few have hexagonal prisms. Discoasters are tiny star or rosette-shaped plates. They have tabular crystalline forms and are more coarsely constructed than coccoliths. High magnifications (×1000) will be needed to resolve individual coccolith and discoaster plates. Non-skeletal calcite debris may occur as lumps, grains, ooids, aggregates and pellets (Fig. 20).