Fringe cementation

Fig. 10. Plot of the Mg/Ca and Fe/Ca ratios of sparry calcite cement, (replaced) carbonate grains and fringe cement of lenses and host rocks. All carbonate is low-Mg calcite. Despite the rather large range in chemical compositions, there is no significant difference in chemical composition between the various carbonate components.

The development of tightly cemented lenses is due to the heterogeneous precipitation of marine fringe cement. That fringe cementation was among the first diagenetic processes is evident from the para-... 

The above example demonstrates the inadequacies inherent in present cement and admixture specifications when cement compositions chemically on the fringe of normal requirements are combined with high doses of chemical admixtures. Minimum as well as the present maximum limits on cement compound composition in ASTM C 150 might best address this problem, but a requirement in ASTM C 494 that admixture compliance be established for the job cement when it is not a Type I or Type II cement might also be useful [138]. 

Mitchell J.T., Land L.S. and Miser D.E. (1987) Modem marine dolomite cement in a north Jamaican fringing reef. Geology 15, 557-560. 

Figure 2.6 End-member types of calcrete microstructure. Alpha fabrics correspond to K fabrics or crystic plasmic fabrics of earlier authors. Corona structures are fringes of bladed or fibrous cement around sediment grains. Displacive structures (see text) are relatively rare. (Based on Wright and Tucker, 1991.)...

Fig. 6. Percentages of carbonate cement (fringe and late calcite cement) and carbonate grains in lenses and host rocks. Host rocks contain only late calcite cement. Percentages were obtained by point counting thin sections. In host rocks, a statistically significant linear relationship exists between the two variables (expressed by the linear function Y= 1.80 A- 0.87 / = 0.88 n = 18). This suggests that late calcite cementation occurred relatively late after most metastable carbonate grains had been dissolved. Cementation only occurred when carbonate grains were available as nuclei for cement precipitation.

If the cement or clinker contains belite only as fringes (coatings) on alite crystals, then the use of... 

The most important uraniferous calcrete deposit is Yeelirrie, Western AustraliaYeelirrie is an area of interior drainage, deep valley fills, abundant evaporite lakes and clay pans in an arid region. Valley calcretes are typically elongate masses of carbonate-cemented alluvium deposited parallel to subsurface valley drainage courses, but some form delta-like deposits that fringe salt lakes. 

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