Silicified spicules

The term spicule encompasses various extracellular mineral secretions exhibiting a needlelike morphology, formed of calcite or silica. Silicified spicules can... 

Opal is related to the very common Si02 mineral species, quartz. Oceans are at present undersaturated with respect to opal (Broecker, 1971) possibly because of the biological formation of animals with silicified skeletons such as the diatoms. These delicate structured creatures, which proliferate in the upper photic zone, dissolve at depth. Therefore, only robust siliceous skeletons such as sponge spicules are retained in sediments that accumulate in deep waters, although some diatoms survive on the continental shelf under zones with high productivity. The initial deposition of the amorphous hydrated silica, opal, converts first to opal-CT and eventually to crystalline quartz (Kastner, 1981). 

Figure 19.5 The silicified structure of sponge spicule, (a-c) Demosponge. (d-f) Hexactinellid. (Reproduced with permission from Ref. [235], Copyright 2012, FEES.)...

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