042519c

Small particles in limestone with a complex history. Ovoid particles, probably small pellets, were dissolved away, suggesting they consisted of aragonite (a chemically unstable carbonate mineral common in modern deposits but missing from many ancient ones). Before they dissolved the pallets were coated with a dark microcrystalline rind, often called a "micrite envelope," and formed by boring and encrusting microbes (widespread in modern tropical shallow-water environments). This material is commonly calcite, and it was preserved here. Later, the open space within the rinds was filled with calcite cement, a lot like that which filled in the spaces among the particles (light pink). This deposit, a peloidal grainstone, is typical of shallow water low to moderate energy tropical environments. Jurassic Smackover Formation, subsurface of Alabama, thin section, image 1 mm wide. Calcite stained pink with alizarin red S.