Opal Sponge Spicules

Spicules are accumulated by a variety of organisms (i.e., echinoderms, holothurians, Porifera) and generally result from genetically controlled biomineralization processes. Within sponges (Porifera), fresh-water sponges (Spongillidae, class Demospongiae) produce only opal A spicules (SiO2 n(H2O)), whereas marine sponges (Demonspongiae and Hexactinellida) accumulate opal or calcareous (CaCO3) spicules. This chapter deals with opal sponge spicules. Sponge spicules have been observed by soil micromorphology in archaeological or man-made soils and in archaeological stratigraphic units. Opal spicules are colourless and optically isotropic with a low negative relief. They have been reported in earth-based construction materials, in herbivore dung and as temper in pottery. Micromorphology only allows for a generic identification of opal spicules. Determination of species has not so far been carried out from soil thin sections. When reworked from older deposits, spicules are sometimes converted into more stable forms of silica (i.e., low cristoblalite) or are subject to epigeny.