Iceland spar (calcite) crystals, placed in the shallow subtidal marine environment at Discovery Bay, Jamaica, are infested by the filamentous endolithic (boring) alga Ostreobium sp. Within 25 days after the crystals are placed in the sea, filaments project from the substrate into the sea; between 65 and 95 days the exposed filaments become completely calcified by low Mg calcium carbonate, both internally and externally. The submarine precipitation of calcium carbonate occurs only upon dead filaments and takes place at a geologically very rapid rate. The breakage of exposed calcified filaments is estimated to produce about 1 cm 3 /m 2 /yr of micrite-size carbonate. This is a source of mud-size materials, not only in the back reef and reef lagoon, but also within the reef itself. The coalescence of dense populations of exposed calcified filaments could produce a micrite envelope about a grain, without the destruction or alteration of the grain periphery associated with the classical mechanisms of micrite envelope formation Such micrite envelopes would be wholly constructive in origin, the product of precipitation of calcium carbonate on algal filaments outside the substrate, rather than within it.