Photomicrograph of a thin section showing a quartz sand grain (Q) in an oolitic grainstone from the Mesozoic of Brazil. Pores are filled with rimming cement of inclusion-rich elongate crystals (small arrows in upper left point to some examples) and clear blocky pore-filling calcite cement (C). Note that rimming cement is present on the quartz grain where the quartz grain has a micritic coating (at thick large arrows) but not present where the quartz grain has no coating (at thin large arrows).

     This illustrates the criticality of substrates and nucleation to cementation. Where a substrate provided many pre-existing crystals (the micritic coating), many small cement crystals grew. Where the substrate provide few or no pre-existing carbonate crystals (at the large thin arrows), one calcite crystal overgrew the entire quartz surface because no new calcite crystal nucleated. The implication is that substrates and nucleation can control fabrics of cements. Thus the inferences about diagenetic fluids and diagenetic environments that carbonate petrologists make from cements have to be tempered by the recognition that the presence or absence of a given cement type may have been controlled by the substrate. The implication for sandstone petrologists is that sandstones would be much more likely to have marine cements if a few minute carbonate particles were present in or on the original sediment.

    Width of field of view is about 2.4 mm. The thin section and hand sample were provided by Dr. Gilles O. Allard (Professor Emeritus, Department of Geology, University of Georgia).