Abundant precipitation of aragonite is suggested by well-preserved fabrics in dolostones of the lower Proterozoic (1.9 Ga) Rocknest Formation. Fabrics include dolomite and silica after botryoidal aragonite in tepee structures and seafloor cement fans, and cement crusts in cryptalgal tufas. Aragonite was common on the 1.9-Ga Rocknest shelf, as it is on modern tropical shelves. A search for similar marine cement fabrics in other Precambrian carbonates should help resolve whether aragonite was an important precipitate from Precambrian tropical seas (implying an ocean carbonate chemistry similar to today), or if aragonite precipitation was restricted to rare times in the past, calcite (and even dolomite) being the main carbonates precipitated (as recently suggested by some authors).

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