CRYSTAL CHEMISTRY

Dolomites are calcium rich and have the general chemical formula Ca_1.04-1.13(Mg_0.83-0.94Fe_0.01-0.04)_0.84-0.98(CO₃)₂. Although CL spectra indicate that CL is Mn-activated, Mn is below the detection limits of the microprobe and is therefore not included in the general chemical formula. Weakly luminescent dolomicrites contain as much as 3.2x the amount of Fe as luminescent varieties (Table T2). Blocky dolomite can contain ~1.4x the amount of iron as weakly luminescent dolomicrites and 4.4x the amount of iron as luminescent dolomicrites (Table T2). The high iron contents of these dolomites reflects their precipitation in a reducing environment (McHargue and Price, 1982).

X-ray diffractograms of semilithified and lithified authigenic layers are shown in Figure F4 with hexagonal reference indices (Lippmann, 1973). Diffractograms show sharply defined peaks, including superstructure reflections. The presence of the (101), (015), (021), and (009) superstructure reflections (Reeder, 1990) indicates that both lithified and semilithified horizons are formed of well-ordered dolomite.

Dolomite samples from the Angola Basin, Walvis Ridge and Basin, and the Northern Cape Basin show extreme enrichment in ¹³C values with values as great as 16.85 and an average 9.39 (PDB) for 18 samples (Table T1). Following the models of Claypool and Kaplan (1974) and Irwin et al. (1977), we interpret the "heavy" ¹³C values of dolomite horizons (Table T1) as evidence that after the initial precipitation of dolomite within the zone of sulfate reduction, authigenesis continued with progressive burial into the zone of methanogenesis, where the bulk of dolomite was formed. The ¹⁸O signature varies between 2.87 and 6.50 (PDB).