Geochemical Scenarios for the Precipitation of Biomimetic Inorganic Carbonates
Juan Manuel García-Ruiz, 2000. "Geochemical Scenarios for the Precipitation of Biomimetic Inorganic Carbonates", Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World, John P. Grotzinger, Noel P. James
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The precipitation of carbonate into alkalinc silicate solutions results in the formation of self-assembled crystal aggregates with noncrystallographic morphologies. These precipitates emulate biologically induced mineral textures as well as display forms typical of primitive microfossils. The precipitation behavior varies with pH, i.e., as a function of the species created by dissociation of the silicic acid under alkaline conditions. Calcite single crystals and crystal aggregates precipitated in these media display complex forms derived From the specific inhibition of some crystal faces, and eventually, noncrystallographic shapes such as sheaf-of-wheat with self-organized banding develop. When strontianite and witherite precipitate in these environments at pH higher than 10, their crystal aggregates display in addition very specific morphologies, such as target patterns, scrolls, twisted ribbons, spirals, fingers, etc., with typical sizes ranging from microns to millimeters. The crystallites of the metal carbonate are embedded in a silicate matrix and are co-oriented and parallel to each other, suggesting that both the loci for nucleation and the orientation of the carbonate groups are controlled by the silica phase.
The silica concentration (>250 ppm SiO2), ionic force, and pH values (>8.5) required for the phenomenon to be observed are well within the range of values measured in contemporary alkaline lakes. A number of geological scenarios where the phenomenon could occur have been identified, among which are: a) contemporary lakes and thermal springs associated with alkaline magmatism such as those in the African rift valley; b) Precambrian (particularly Archean) terranes where cherts formed as a result of direct precipitation of silica; and c) a scenario on Earthlike planets where the existence of a silica-rich environment derived From hydrolysis of alkaline rocks is predicted.
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Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.