Organic colloids are involved in the early calcification of the modern siliceous sponge Spheciospongia vesparium (Lamarck, 1815) from the Great Bahama Bank. Electron microscopy and in situ fluorescence microspectometry studies indicate that colloids attached within or onto a collagen network promote the precipitation of aragonite crystals in these sponges. Calcification occurs within those portions of the sponge that are buried in the sediment, preferentially in regions of agglutinated sediment particles, with sponge connective tissue being subjected to necrosis and significant degradation of the extracellular collagen matrix. The dismantling of collagen bundles leads to collagen scaffolds, which act as a sorbent and have significant adhesive effects for ions and/or organic colloids. Bacteria and other microorganisms support tissue degradation, but neither act as a substrate for aragonite precipitation or are present in significant numbers at calcification sites. This process of early calcification may explain the origin of fossil calcified siliceous sponges ("sponge mummies" and "tuberoids") as well as the occurrence of patchy calcified sponge materials, thereby calling into question the commonly accepted idea that pelletoidal texture associated with these fossil sponges indicates that bacteria are directly responsible for the calcification.