Enstatite phase compositions

Figure 8. Figure (a) after Clayton et al. (1976, 1977). The scales are as in Figure 1. The O isotopic compositions of the different meteorite classes are represented ordinary chondrites (H, L, LL), enstatite chondrites (EFl, EL), differentiated meteorites (eucrites, lAB irons, SNCs) and some components of the carbonaceous chondrites. As the different areas do not overlap, a classification of the meteorites can be drawn based on O isotopes. Cr (b) and Mo (c) isotope compositions obtained by stepwise dissolution of the Cl carbonaceous chondrite Orgueil (Rotaru et al. 1992 Dauphas et al. 2002), are plotted as deviations relative to the terrestrial composition in 8 units. Isotopes are labeled according to their primary nucleosynthetic sources. ExpSi is for explosive Si burning and n-eq is for neutron-rich nuclear statistical equilibrium. The open squares represent a HNOj 4 N leachate at room temperature. The filled square correspond to the dissolution of the main silicate phase in a HCl-EIF mix. The M pattern for Mo in the silicates is similar to the s-process component found in micron-size SiC presolar grains as shown in Figure 7.

The listed chemical formulae are ideal and most of these minerals contain trace and minor elements which undoubtedly affect the CL. Several of these minerals have polymorphic or compositional varieties which also may, or do, show CL (e.g. the silica polymorphs quartz, cristobalite, tridymite phosphate compositional varieties apatite, whitlockite, farringtonite, buchwaldite carbonate compositional varieties calcite, dolomite, magnesite). Glass and maskelynite (shock modified feldspar), although not strictly minerals, are relatively common. Below are described the CL observations for the most common phases including enstatite, feldspar and forsterite and they are related to their use for interpreting the mineralogy of meteorites. The observations for the other minerals are sporadic and many details have yet to be studied. 

The composition of talc is situated close to the eutectic, so that the amount of liquid phase suddenly formed would deform the product. However, in a talc-day mixture, one may choose a proportion of components providing an amount of meJt just suitable for satisfactory sintering (about 30%) and for inhibition of the proto-enstatite-clinoenstatite inversion. This amount corresponds to approximately 10% clay. The composition of such a mix which roughly corresponds to commercial steatite compositions is indicated in Fig. 189 by point A. The diagram then allows equilibrium amounts of the melt to be evaluated for various temperatures, as shown in the diagram in Fig. 190. The shape of curve A indicates that about 30 % of melt is formed rapidly at 1345 °C. This is the temperature of the tridymite-enstatite-cordierite eutectic. The amount of melt increases with temperature, so that the sintering interval is comparatively narrow. 

The main crystalline phase is cordierite surrounded by a glassy phase. Further crystalline substances may be present, such as mullite, spinel, cristobalite, forsterite, enstatite, according to the initial composition and the raw materials employed. A composition may be chosen in order to bring about spontaneous formation of a glaze on the ware surface by firing the melt obviously does not wet the solid particles and is thus forced out towards the surface. An example of such composition is shown in Table 26 (Sample 5). 

The diagram indicates that the mixes for the manufacture of steatite ceramics lie away from the theoretical composition of enstatite, which is a binary phase in the system MgO —Si02- The maximum enstatite content should be attainable through... 

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