Silicates, formation from calcite

Smith and Robb (2) describe the formation of dolomite, calcite, and aragonite in Lake Uinta. Calcite was formed in the lake s lower layer from the calcium released by chemical digestion of silicates and aluminosilicates. Calcium released to a calcite-saturated sodium carbonate solution immediately formed calcite. 

The stability relationships between calcite, dolomite and magnesite depend on the temperature and activity ratio of Mg " /Ca " (Fig. 5d). Lower Mg/Ca activity ratios are required to induce the dolomitization of calcite and to stabilize magnesite at the expense of dolomite (Fig. 5d) (Usdowski, 1994). Formation waters from the Norwegian North Sea reservoirs have an average log(an g -/ cz- ) - TO to 0.0 and thus fall within the stability field of dolomite. Nevertheless, both calcite and dolomite are common cements in these rocks, indicating that dolomitization is a kinetically controlled reaction. Further evidence of this is revealed from Recent sediments, such as the Fraser River delta in Canada (Simpson Hutcheon, 1995) (log (aMg2+/aca=+) -2.2 to h-1.0), where the pore waters are saturated with respect to dolomite, but it is calcite rather than dolomite that precipitates. Calcite rather than dolomite forms below the deep>-sea floor, yet the pore waters plot at shallow, near sea bottom temperatures in the stability field of dolomite and shift with an increase in depth towards the stability field of calcite (Fig. 5d). This shift is due to a diffusion-controlled, downhole decrease in Mg/Ca activity ratio caused by the incorporation of Mg in Mg-silicate that results from the alteration of volcanic material, a process which is coupled with the release of calcium (McDuff Gieskes, 1976). 

Fig. 4. Localization of authigenic calcite (c) on dissolving silicate grains. Back-scattered electron images. (A) Calcite replaces parts of two K-feldspar grains (K) from the M. Cervarola Formation. (B) Partial calcitization of an epidote grain (e) in the Borello Formation.

The lA Pliocene, Ranzano and Antognola formations were cemented by meteoric water the Bismantova Formation was cemented in part by water with a meteoric component the Loiano and the Borello formations were cemented by slightly modified marine pore water and all the foreland basin units (except the Borello) were cemented by water variably enriched in 0 (8 0 = -2 to +8) generated from silicate reactions. The most 0-enriched values for S 0, a,er are compatible with depths and temperatures of cementation of the three deepest formations, but not for the less deeply buried Loiano and upper part of the Mamoso-arenacea formations. 0-enriched fluids in these latter formations were more probably derived from underlying, more deeply buried rocks apd expelled by compaction. Possibly, the calcite in the deepest buried formations re-equilibrated with hot water after precipitation. 

---