Magnesian calcites biogenic

Walter and Morse (1984) were able to document the relative importance of microstructure for the dissolution of biogenic carbonates. Biogenic magnesian calcites are structurally disordered and chemically heterogeneous. Both these factors play a role in the reactivity of these minerals in natural systems. 

In Figure 3.10 the unit cell volume and cell edge c/a ratio of biogenic magnesian calcites are plotted as a function of bulk MgC03 content. The relationship between unit cell volumes and the c/a ratio versus composition for... 

Figure 3.10. Unit cell edge ratio c/a (A) and volume (B) versus composition for biogenic magnesian calcites. Solid line is quadratic least-squares fit to data for synthetic calcites, and dashed line represents straight line between calcite and disordered dolomite. (After Bischoff et al., 1985.)...

Bischoff W.D., Sharma S.K., and Mackenzie F.T. (1985) Carbonate ion disorder in synthetic and biogenic magnesian calcites A Raman spectral study. Amer. Mineral. 71, 581-589. 

Walter L.M. and Hanor J.S. (1979) Effect of orthophosphate on the dissolution kinetics of biogenic magnesian calcites. Geochim. Cosmochim. Acta 43, 1377-1385. 

Sjoberg, E.L. Kinetics and mechanism of calcite dissolution in aqueous solutions at low temperatures, Acta Univ. Stockholmlensis. Stockholm Contrib. Geol. 32, 92 p. (1978). Walter, L.M. and Hanor, J.S. Effect of orthosphosphate on the dissolution of biogenic magnesian calcites, Geochim. Cosmochim. Acta (in press). 

Bischoff, W. D., F. T. Mackenzie, and F. C. Bishop (1987), "Stabilities of Synthetic Magnesian Calcites in Aqueous Solution Comparison with Biogenic Materials," Geochim. Cosmochim. Acfa51, 1413-1423. 

The solid carbonate can also present several potential difficulties in solubility studies. These can be broken down into two major areas heterogeneity in composition, and excess free energy associated with lattice strain or defects and surface free energy. The problem of solid heterogeneity presents itself in most sedimentary carbonates, and is especially important in biogenic carbonates such as magnesian calcites. The problem of lattice strain and high defect density is most... 

Many organisms secrete shells high in Mg, rather than low magnesian calcite (see Milliman, 1974 for a summary and Chapter 5). The Sr concentration of these shells varies considerably, but is generally about a factor or two higher than that of low magnesian biogenic calcite. As Ohde and Kitano (1984) have shown, this difference is readily explained by the observation of Mucci and Morse (1983) that... 

Figure 3.14. Stabilities of calcite, and synthetic (closed squares) and biogenic (closed circles) magnesian calcites as a function of composition. Stabilities are expressed as -log IAPMg-Calcite- The curve is a hand-drawn "best" fit to the synthetic data. Also plotted are the results of precipitation experiments by Mucci and Morse (1984, open squares) and biogenic dissolution experiments by Walter and Morse (1984a, open circles). (After Bischoff et al., 1987.)...

The results of Walter and Morse (1984a) and Bischoff et al. (1987) on cleaned, annealed and size-fractionated magnesian calcite skeletons best describe the reactivity of biogenic materials with chemical and physical... 

Figure 3.15. Infrared absorbance of the O-H bond as a function of the magnesium content in magnesian calcites after drying at 110°C. All samples are biogenic except for two hydrothermally synthesized magnesian calcites indicated by the letter S. (After Mackenzie et al., 1983.)...

Submarine lithification and precipitation of cements in deep sea carbonate sediments are relatively rare processes in typical major ocean basin sediments. Milliman and his associates have summarized much of the information on these processes (Milliman, 1974 Milliman and Muller, 1973,1977). The cements are of both aragonitic and magnesian calcite mineralogies, and are largely restricted to shallow seas such as the Mediterranean and Red seas, and sediments in the shallower parts of major ocean basins in which biogenic aragonite is also present. The formation of carbonate cements will be discussed in detail in subsequent chapters. 

Bischoff W.D. (1985) Magnesian calcites Physical and chemical properties and stabilities in aqueous solution of synthetic and biogenic phases. Ph.D. dissertation, Northwestern Univ.. 

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