Abstract

Growth of spar calcite crystals in rootcretes from the Bluff Formation of Grand Cayman was mediated by spores (1 mu m diameter) through a three-stage paragenetic sequence. Stage I involved calcification of the spore walls and, in some cases, precipitation of minute anhedral calcite crystals around the spores. In Stage II spar calcite grew around the spores which acted as nuclei for precipitation. Stage III was marked by the development of overgrowth zones around clusters of spores that occurred along the edges of smaller, previously formed crystals. Precipitation of calcite inside the spores occurred at any stage. Spore identity was progressively lost as the crystals grew. The resultant crystals appear identical to abiogenic crystals, because they contain little or no evidence of the spores that mediated their formation.

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