Low-magnesium calcite

Hypothesis 3, Diffusion of DIC together with DOC, sulfate, and cations from confining bed pore waters to the Black Creek aquifer provides sources of electron donor (organic carbon) and electron acceptor (sulfate) for microbial metabolism and additional inorganic carbon to drive low-magnesium calcite precipitation. The combination of magnesium-calcite dissolution from shell material driven by microbially produced carbon dioxide, and the precipitation of more thermodynamically stable low-magnesium calcite cement in the aquifer, can explain major ion and carbon isotope composition of Black Creek aquifer water. 

Johnson W. J. and Goldstein R. H. (1993) Cambrian seawater preserved as inclusions in marine low-magnesium calcite cement. Nature 362, 335-337. 

The Phanerozoic trend (Figure 34) is based on —4,500 samples of low-magnesium calcitic fossils from about 100 localities worldwide. The reasons for believing that it is essentially a primary trend were discussed in detail by Veizer et al. (1999). 

Figure 11.7 Scanning electron microscopy images of marine beachrock and meteoric cayrock cements. (A) Blades of high-magnesium calcite on constituent particles in beachrock. Northeast Sapodilla Cay, southern Belize Barrier Reef. (B) Dissolution cavity is lined with blockyand bladed low-magnesium calcite in cayrock. Cay Bokel, Turneffe Islands, Belize. (C) Low-magnesium calcite needle fibre (whisker) cement in cayrock. Harry Jones Point, Turneffe Islands, Belize.

Planktonic foraminifera secrete tests of Low Magnesium Calcite. Each test consists of a series of... 

High- and low-magnesium calcite minerals, in addition to aragonite and normal calcite, were analyzed by several different methods, including TG (31). The results obtained by TG compared favorably with X-ray diffraction and wet chemical analyses. 

Based on a series of electron microprobe measurements, all samples are composed of low-magnesium calcite (between 0.14 and 1.5 mol% MgCOs). EDX microanalysis... 

Modulated microstructure occurs in calcite, as well as in dolomite, and was first identified in Jurassic oolitic limestone by Gunderson and Wenk (1981). However, chemical analysis of the oolitic limestone showed no compositional differences between areas with the modulation and those without. Since, the modulations could not be the result of compositional fluctuations of Ca-Mg as in dolomite, Gunderson and Wenk (1981) hypothesised that the rotational disorder of CO3 groups that occurred during phase transformation of aragonite to low-magnesium calcite might cause the modulations. 

TEM observations document the presence of distinct areas of modulated microstructure in fossil and Recent brachiopod shells. This is the first record of modulated texture in extant skeletal carbonate. There are unresolved differences of opinion regarding the underlying mechanism of formation of modulated microstructures. Although, most modulated microstructure has been associated with diagenesis, the presence of the modulated structure in Recent brachiopods eliminates a diagenetic environment as a requirement for the formation of the texture in brachiopods. Furthermore, electron microprobe measurements indicate that the calcite studied is composed of low-magnesium calcite. Thus, no major cation substitutions, such as excess Mg+ in the lattice, are responsible for the modulated microstructures in brachiopod calcite. The two most likely explanations for modulated microstructure in brachiopod calcite... 

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