Magnesite formation

PuEYO Mur, J.J. Ingles Urpinell, M. (1987) Magnesite formation in recent playa lakes, Los Monegros, Spain. In Diagenesis of Sedimentaiy sequences (Ed. Marshall, J.D.), Spec. Publ. geol. Soc. London, 36, 119-122. 

The amount of hardness present in natural surface and groundwaters depends to a large extent on the action of dissolved carbon dioxide in rainwater on the watershed s geological formations (such as limestone, dolomite, gypsum, or magnesite). The dissolved hardness levels remain relatively low because of the sparingly soluble nature of the salts formed. Typically, MU water sources initially contain anywhere from 5... 

Therefore the formation of magnetite in that way could hardly be of essential importance in the metamorphism of iron-formations, and martitiza-tion is still less hkely. However, in deposits of other genetic types, for instance skam deposits, oxidation of iron silicates to magnetite at the contact with large masses of carbonate rocks (dolomite, magnesite) can be considered an ore-forming process. The last conclusion is still feasible because the carbon dioxide released in the dissociation of carbonates probably had an undisturbed CO O2 ratio. 

Koziol A. M. and Newton R. C. (1995) Experimental determination of the reactions magnesite -F quartz = enstatite -F CO2 and magnesite = periclase -F CO2 and the enthalpies of formation of enstatite and magnesite. Am. Mineral. 80, 1252-1260. 

Many carbonate minerals are found in the earth s crust. As a result, the waters of several lakes, rivers, and even oceans are in contact with these minerals. CaC03 is the primary component of limestone and marble, while dolomite (CaMg(C03)2) and magnesite (MgC03) are minerals found in other rock formations. 

Values of reported by Kubas et a/. [31], 140 to 175 kJ mol, were comparable with those cited above, but (as with calcite) in the presence of CO2, E was increased to 234 kJ mol. Addition of ZnO or NiO to MgCOj decreased the value of E slightly. The much greater decrease (to ii, = 54 kJ mol ) which resulted from the addition of CaO was ascribed to the formation of CaCOj as an intermediate. Doi and Kato [32] identified MgO.MgCOj as an intermediate in the decomposition of magnesite in N2 or CO2. The effects of procedural variables on the non-isothermal decompositions of magnesite and of dolomite have been reported by Sharp et al. [12]. 

Eogenetic magnesite cement in sandstones is relatively rare because its formation requires pore waters to be enriched in Mg " " and depleted in Ca " ", S04 and Cl". These conditions may occur in arid climates in which marine pore waters evaporate and become successively saturated with respect to calcium carbonates, calcium sulphates and halite, such as in sabkha settings (Kinsman, 1969 Morad et al., 1995). Continental brines enriched in Mg + are also suitable for the formation of eogenetic magnesite due to the low sulphate and chloride ion concentrations. Most recent magnesite cements form in the fine-grained sediments of alkaline/saline lakes (Last, 1992 Warren, 1990) and, less commonly, in freshwater lacustrine sediments (Zachmann, 1989). 

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). 

Zachmann, D.W. (1989) Mg-carbonate deposits in freshwater environment. In Magnesite Geology, Mineralogy, Geochemistry and Formation of Mg-Carbonates, (Ed. Mbller, P.), Monograph Series on Mineral Deposits, 28, 61-94. 

Dissolution kinetics of single carbonates such as calcite, aragonite, and magnesite exhibit simple dependence with respect to a limited number of reactants, specifically, H +, H2C03, and H20. It is thus easy to identify the elementary steps leading to the formation of the surface activated complex and the nature of the products of the detachment process following the decomposition of the activated complex. 

Sedimentary deposits of magnesite have been described as having either a biogenic origin or a chemical route via direct precipitation. These deposits are cryptocrystalline in form. Actinomycetes bacteria, which belong to the genus Streptomyces, are thought to have played a role in the formation of... 

---