Stromatolites are primarily composed of which mineral precipitated on bacterial mats?

Stromatolites are sedimentary formations created by the growth of microbial mats, primarily composed of cyanobacteria. These organisms facilitate the precipitation of minerals, leading to the formation of layered structures. The predominant mineral found in stromatolites is calcite, which is a form of calcium carbonate. This mineral often precipitates in association with the metabolic activity of the bacteria that inhabit the mats.

The process involves the bacteria facilitating the trapping and binding of sediment particles, resulting in a compacted structure that preserves the microbial layers over geological time. The high solubility of calcite in water allows it to precipitate under specific environmental conditions, especially in shallow marine settings where anoxic conditions can promote calcite formation.

While other minerals mentioned in the alternatives may form in different geologic contexts, calcite is specifically associated with the growth and layering seen in stromatolite structures. This makes it the primary mineral of interest in the context of these ancient biological formations.