ABSTRACT

Non-skeletal aragonite and Mg-calcite is formed in the interior of hollow particles (chambers) in West Indian and Mediterranean nearshore calcareous sediments. The calcite has 15 to 17 mole percent MgCO₃ in solid solution. The intragranular fillings are best developed in sediments from turbulent environments; the carbonate is an addition to the particles, not re-organized initial grain substance. The growth processes are not related to the activity of organisms; they are supposed to represent slow precipitation under supersaturated conditions. The time needed for precipitation is estimated to 100-1,000 years; the radiocarbon age of a Mediterranean chamber assemblage was 890 ± 115 years.

Four kinds of fabric are found: ordered arrangement in monomineralic fringes of aragonite, resp. Mg-calcite; random arrangement in Mg-calcite micrite, and in aragonite clusters. Changes in mineralogy during growth are from Mg-calcite to aragonite; changes in fabric are from ordered to random arrangement. Mineralogy and fabric vary on a microscopic scale, and the two mineral polymorphs are evidently formed from natural seawater under the same general conditions. One important controlling factor is the growth substrate; biogenic aragonite induces nucleation and growth of aragonite, while biogenic calcite favors precipitation of Mg-calcite. The organic calcification matrices in the biogenic carbonates are probably active in the process.

The intragranular precipitate is related in fabric and ultrastructure to various other shallow-marine carbonates, such as: beachrock cement; shallow submarine cement; and internal cement in reef-algal frameworks. Also the secondary carbonate in micrite envelopes appears to have the same properties.