Mg-rich minerals

A representative sample of the isotopic data from terrestrial standards is given in Table 1 and plotted in figure 6 together with data from Mg-rich minerals (spinel and Ti-pyroxene) from Allende inclusions. Raw isotopic data are presented, corrected... 

Table 1. llnnormalized Mg Isotope Data from Terrestrial Samples and Mg-rich Minerals from Allende... 

Al/24Mg ratio for each sample. Data for Allende Ti-Al-pyroxene and spinel fall within the range of the terrestrial standards 625Mg = 0 7 permil. 626Mg = 0 14 permil. It is important to recognize that the spread in the data for Allende Mg-rich minerals due to instrumental fractionation is the same as that observed for... 

From a theoretical point of view, the gel layer is a barrier that reduces further hydrolysis of the silicate network, and is supposed to be more stable than the glass matrix, thus reducing the overall rate of corrosion. However, gel exfoliation may momentarily re-activate corrosion, at least locally. No clear trend was observed for the presence of the crystalline secondary phases identified at the surface of the corroded HT samples. The most abundant minerals are aluminosilicates, calcium phosphates, Fe- and Mg-rich minerals, and zeolites their role in the scavenging or release of metals remains ambiguous, although many mineral phases identified bear traces of metals. 

The nature of the lunar crust is imperfectly known. It is on a firm basis inasmuch as it was derived mainly from studies of samples. The samples, however, were collected from eight locations in a region on the lunar frontside that is not characteristic of the lunar highlands. An important characteristic of the lunar crust is its enrichment, relative to the remainder of the moon, in the constituents of Ca-rich feldspar (CaAl2Si20s). Presumably, feldspar-rich materials crystallized from the molten lunar exterior along with more mafic (Fe, Mg rich) minerals and collected selectively at the lunar surface. Samples of nearly pure feldspar rock, called anorthosite, were included with the lunar samples. Lanthanide distributions (fig. 21.14) in these rocks reflect the strong affinity of feldspar for Eu(II) and the relative preference for lighter members of the lanthanide series. 

Iwao, S. and Minato, H. (1959) Occurrence of Mg-rich clay minerals around some hydrothermal ore deposits in Japan. Int. Geol. Congress XX (Mexico 1956), Com. Int. Est. Arcilas, pp. 31-42. 

Figure 6. Three-isotope correlation diagram for Mg using S-notation fsee text) for unnormalized data from terrestrial samples and Mg-rich Allende minerals.

Dolomites compose a significant portion of ancient evaporites. This mineral is essentially a Mg-rich calcite, having variable Mg content. Its empirical formula is commonly given as MgCa(C03)2. Having no modern examples of dolomite precipitation beyond those of a few salinas and sabkhat, geochemists have been unable to determine conclusively how the ancient deposits must have formed. 

Perkins D. (1983). The stability of Mg-rich garnet in the system Ca0-Mg0-Al203-Si02 at 1000 to 1300°C and high pressure. Amer. Mineral, 68 355-364. 

After the experiments, significant quantities of newly formed minerals were observed at the cold extremity of the tube, pointing to a fast material transport by diffusion from the hot to the cold end of the tube. The following spatial distribution of newly formed phases, reflecting the temperature profile, was observed in both runs (Fig. 8) quartz + K-feldspar + plagioclase + Mg-rich saponites (hot extremity) quartz + K-feldspar + plagioclase (middle of the tube) and alkaline or Ca-rich clays + quartz + plagioclase (cold extremity). The cation composition of the phyllosilicates was similar in both experiments. Some newly fonned quartz crystals... 

Si-Al-R coordinates and Figure 31a shows the compositional range in Fe-Al-Mg coordinates for grains in ten rock samples. It is evident that these berthierines are iron and alumina-rich minerals. 

It is important for the discussion below to distinguish between direct and indirect process routes. Direct carbonation is the simplest approach to carbonate production (or mineral carbonation see Section 14.4) and the principal approach is that a suitable feedstock-for example, serpentine or a Ca/Mg-rich solid residue-is carbonated in a single process step. For an aqueous process this means that both the extraction of metals from the feedstock and the subsequent reaction with the dissolved C02 to form carbonates takes place in the same reactor. 

In some pipe deposits in geothermal power plants, arsenic is associated with clays or other silicate minerals rather than sulfides or (oxy)(hydr)oxides. Pascua et al. (2005) found that about 80 % of the arsenic in pipe scales from a Japanese geothermal power plant was associated with Mg-rich smectite clays. The arsenic (mostly III) was probably located in the crystalline structures of the clays and/or present as submicron inclusions. 

Mn2 +. The clay minerals tend to be concentrated in three relatively discrete areas representing Mg-rich sheets, R3+ (both A1 and Fe 3+)-rich sheets and Fe2+-rich sheets. Due to the large size of the Fe2+, octahedral sheets containing only this ion do not normally occur in the clays. 

Mineralogically the sediments of the laminite series consist of carbonates (mainly dolomite and calcite), various clay minerals, zeolites, opal, quartz and rare gypsum (8). The occurrence of gypsum, based on our X-ray diffraction (XRD) data, is restricted to the marl unit. Deposition, according to Jankowski (8), occurred in a periodically evaporitic, stagnant lake. The high bitumen concentrations most probably were responsible for the preservation of unusual minerals such as Mg-rich calcites (up to 25% Mg 9) and bituminous smectites (Muller, G., University of Heidelberg, personal communication, 1989). 

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