Calcite accretion

Fedoroff, N., Courty, M.A., Lacroix, E. Oleschko, K. (1994) Calcitic accretion on indurated volcanic materials (example of tepetate, Atliplano, Mexico). Proceedings of the XVth World Congress of Soil Scientists, Acapulco, Vol. 6A, pp. 459-472. 

At the Toca do Serrote da Bastiana rock shelter in Brazil, a red, iron ochre pictograph of an anthropomorphic figure had become coated with a calcite accretion over time. Using X-ray diffraction and Fourier transform infrared spectroscopy, we determined that the accretion also contains whewellite, the monohydrate of calcium oxalate, in addition to calcium carbonate. An AMS radiocarbon date on the oxalate formed from contemporaneous carbon yielded a minimum date of... 

Efflorescence. The solvent properties of water also causes efflorescence, a phenomenon whereby soluble or slightly soluble substances migrate from the interior of porous solids to the surface, where they precipitate. Efflorescence is an important factor in the decay and disintegration of many rocks, and of human-made porous materials such as ceramics, and even of some types of glass. On archaeological objects, efflorescence generally occurs mostly as a white, powdery, but sometimes consolidated accretion on the surface of the objects. Calcite, a form of calcium carbonate, is one of the most common substances to effloresce on archaeological ceramics. 

Surface nuclei are to be distinguished structurally from mere surface clusters. For surface nuclei, accretion and rearrangement of constituent ions are needed to present a kernel on which a surface precipitate can grow successfully (7). A case in point is the formation of calcium phosphate nuclei on the surface of calcite after rearrangement of adsorbed phosphate clusters (68). The transition from surface complexes to clusters to precipitate was reviewed in detail by Charlet and Manceau (62). They stressed the important interre-... 

The calcite-cemented lenses are stratabound and parallel to the large-scale, low-angle bedding or to coset boundaries of storm-built ridges and tidal-delta lobes. In addition, smaller lenses occur as tightly calcite-cemented foresets of metre-scale dunes, lateral-accretion sets and channel lags. Such small lenses are concordant to, or partly coincide with, primary sedimentary structures. Most of the lenses are discontinuous, with a lateral continuity of... 

Next, low-temperature (<150°C), low-pressure torr) oxygen plasmas oxidized organic components of the sample to CO2. Decomposition of inorganic carbon present (dolomitic limestone rock and calcite/calcium oxalate accretions) was prevented by running the plasmas at low-temperature. Carbon dioxide from the sample was flame-sealed into a glass tube cooled to liquid nitrogen temperature (-194°C), after water had been frozen out with a dry-ice/ethanol slurry (-58°C), and finally sent for radiocarbon analysis at the Center for Accelerator Mass Spectrometry at the Lawrence Livermore National Laboratory (LLNL-CAMS). It was necessary to utilize an AMS measurement due to the small sample size. 

Re-crystallization and deposition processes most likely formed the calcite portion of the accretion when water containing soluble calcium carbonate coated the shelter wall. Formation of individual calcite layers would have occurred with multiple events throughout the entire history of the painting even up until the present time. ESR and TL measure accumulated electron traps produced as the newly formed calcium carbonate accretion is exposed to ionizing radiation. If the environmental radiation dose rate is known, then the time of crystal formation or when the accretion was deposited can be determined. 

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