Fe Fe

Figure 2. Same Mg-Fe-Si diagram as Fig. 1 showing compositions of Fe-rich ufg 'FeMgSiO' PCs (solid squares; Rietmeijer, unpublished data) and GEMS (triangle; [10]). These compositions are defined by mixing lines (heavy solid lines) between the (1) smectite dehydroxylate (Sm-d)-Fe(Si)0 and Fe-oxides; (2) enstatite (En)-Fe(Si)0 and Fe-oxides, and (3) Mg-rich 'FeMgSi0'-Fe(Si)0 and Fe-oxides. Preferred composition occur at intersections of these mixing lines with the serpentine dehydroxylate (S-d)-(G-d) mixing line. The light solid line marks the maximum amount of ferrous iron in the silicates; towards the apex only iron metal exists.

Figure 2. Same Mg-Fe-Si diagram as Fig. 1 showing compositions of Fe-rich ufg 'FeMgSiO' PCs (solid squares; Rietmeijer, unpublished data) and GEMS (triangle; [10]). These compositions are defined by mixing lines (heavy solid lines) between the (1) smectite dehydroxylate (Sm-d)-Fe(Si)0 and Fe-oxides; (2) enstatite (En)-Fe(Si)0 and Fe-oxides, and (3) Mg-rich 'FeMgSi0'-Fe(Si)0 and Fe-oxides. Preferred composition occur at intersections of these mixing lines with the serpentine dehydroxylate (S-d)-(G-d) mixing line. The light solid line marks the maximum amount of ferrous iron in the silicates; towards the apex only iron metal exists.

Predictable condensation and aggregation of FeMgSiO dust characterize dust formation in oxygen-rich circumstellar environments. Some implications of our work are:

(1) Circumstellar outflows contain only a limited number of compositionally distinct condensed grain types with relative proportions that may vary among different stars. Our simulations match the observations at far-IR wavelengths by the Infrared Space Observatory (ISO) satellite showing condensation of nearly pure amorphous MgSiO grains that anneal to crystalline Mg-silicates without detectable iron as well as the absence of crystalline Fe-silicates in certain relatively-rare high-mass-loss-rate stellar outflows, and

(2) "Silicate" dust in the jets emanating from the nucleus of comet Halley (Fig. 4a in [9] is silica-rich compared to the CI abundance. It resembles FeMgSiO PCs in chondritic aggregate DDPs.

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