Fe Fe

Figure 1. The Mg-Fe-Si (element wt %) diagram showing the compositions of condensed amorphous metastable eutectic solids (dots) [Fe2+,Fe3+ symbolizes variable Fe2+/Fe3+ ratios]. The condensed dusts are (i) smectite (Sm-d) and serpentine (S-d) 'MgSiO' dehydroxylates and high-Si FeO dehydoxylates (incl. Si-rich smectite dehydoxylate labeled A), (2) Fe(Si)0 and Mg(Si)0 dehydroxylates and (5) metal oxide minerals (incl. Si02, silica; solid triangle). Mixing lines (dashed) connect the condensed dusts. The compositions of three Mg-rich 'FeMgSiO' PCs (black diamonds) are shown as well as the average fe (Av.; 0.23) and maximum (max.) fe ratios of these PCs. The solid square is the most Mg-rich composition that possible for Fe-rich ufg 'FeMgSiO' PCs. A mixing line (dashed) that connects the S-d and greenalite (G-d; open circle) metastable eutectic dusts is located in between the stoichiometric olivine (Fo-Fa) and pyroxene (En-Fs) lines. Mixed very-low Si "Mg,FeO" dust is not yet seen in aggregate IDPs but it was detected in the coma of comet Halley [9].

Figure 1. The Mg-Fe-Si (element wt %) diagram showing the compositions of condensed amorphous metastable eutectic solids (dots) [Fe2+,Fe3+ symbolizes variable Fe2+/Fe3+ ratios]. The condensed dusts are (i) smectite (Sm-d) and serpentine (S-d) 'MgSiO' dehydroxylates and high-Si FeO dehydoxylates (incl. Si-rich smectite dehydoxylate labeled A), (2) Fe(Si)0 and Mg(Si)0 dehydroxylates and (5) metal oxide minerals (incl. Si02, silica; solid triangle). Mixing lines (dashed) connect the condensed dusts. The compositions of three Mg-rich 'FeMgSiO' PCs (black diamonds) are shown as well as the average fe (Av.; 0.23) and maximum (max.) fe ratios of these PCs. The solid square is the most Mg-rich composition that possible for Fe-rich ufg 'FeMgSiO' PCs. A mixing line (dashed) that connects the S-d and greenalite (G-d; open circle) metastable eutectic dusts is located in between the stoichiometric olivine (Fo-Fa) and pyroxene (En-Fs) lines. Mixed very-low Si "Mg,FeO" dust is not yet seen in aggregate IDPs but it was detected in the coma of comet Halley [9].

predicted compositions exactly match those of the ultrafine-grained (ufg) FeMgSiO PCs in chondritic aggregate IDPs (Fig. 2). These particular PCs include units known as GEMS (glass with embedded metal and sulfides) that are believed to result from mixing of glassy Mg-pyroxene and an Fe,Ni-sulfide grain that is a source of Fe,Ni-metal. Mixing is thought to be facilitated by prolonged irradiation and sputtering by energetic H and He nuclei, recoil-mixing, vitrification, and magnesium loss but this mixing mechanism was not verified experimentally. Condensed Fe(Si)0 and Fe-oxide could also provide iron for ufg FeMgSiO PC formation ([3] for a review).

6. THERMAL ANNEALING

Heat treatment of amorphous condensates and fused FeMgSiO dust causes crystallization in these chemically closed-systems of secondary mineral phases that are unrelated to condensation. In Mg-rich FeMgSiO PCs the results are coexisting coarse-grained Fe,Mg-olivine and Fe,Mg-pyroxene with identical fe ratio plus an amorphous aluminosilica residue (Fig. 9 in [6]). The fe-ratios of the silicate minerals reflect this ratio of the parent PC. Heat treatment of serpentine-dehydroxylate condensate yields forsterite and enstatite, viz.

Mg3Si207 = Mg2Si04 + MgSi03 or Mg3Si207 = 1.5Mg2Si04 + 0.5Si02.

The reaction products of the latter, olivine + silica instead of thermodynamically stable pyroxene, reflect the kinetic nature of the thermal reactions [7,6]. Hallenbeck et al. [11] monitored the changes in the IR spectra of MgSiO condensates as represented by the above reactions during thermal annealing. They found that the spectral features of samples in a "stall phase" resembled those of "olivine-rich" comets and that when the "stall phase" had ended the abundances of crystalline Mg-silicates continually increased as a function of heat-treatment time and temperature. Similar reaction rates for MgSiO and Mg-rich FeMgSiO samples [11] suggest that Fe occurs as ferrous iron in stoichiometric forsterite and enstatite.

Thermal annealing of metastable eutectic ferrosilica dusts proceeds differently because this dust such as Si-rich FeSiO smectite-dehydroxylate (V\' in Fig. 1) contains ferric iron and the formation pure Fe-olivine is accompanied by formation of Fe3+-bearing Fe-oxides. Trivalent cations will polymerize amorphous silica-rich materials. As a result nucleation and growth in FeSiO condensates will lag behind those in MgSiO and Mg-rich FeMgSiO condensates during heat treatment. This is consistent with the IR spectral development observed during thermal annealing of these different materials by Hallenbeck et al. [11].

7. DISCUSSION

Experimental astromineralogy simulates circumstellar dust formation via vapor phase condensation and post-condensation modification, e.g. thermal alteration [2,5], Because Si, Fe and Mg are the most abundant rock-forming elements we simulated FeMgSiO dust formation in he Mg0-Fe0/Fe203-Si02 system. Using Actualism, the guiding principle for the Earth Sciences, and chondritic aggregate IDPs as ground truth samples to validate the uniqueness of the condensation and thermal annealing experiments, we find that

(1) Kinetically controlled gas-to-solid condensation at high delta-T or supersaturation only produces a limited number of amorphous MgSiO and FeSiO dusts with predictable metastable eutectic compositions, and pure metal-oxides,

(2) Condensed solids have considerable "internal crystallographic and chemical free energy",

(3) Formation of FeMgSiO dusts requires aggregation of condensed dusts,

(4) Compact amorphous Mg-rich FeMgSiO dust with fe - 0-0.35 forms spontaneously by aggregation of Si-rich metastable eutectic MgSiO and FeSiO dusts,

(5) Compact amorphous Fe-rich FeMgSiO dust with a modal composition of fe ~ 0.7 and a range of fe = 0.35-0.85 are the result of aggregation and fusion,

(6) Only heat treatment produces stoichiometric crystalline Mg- and Mg,Fe-silicates, and

(7) Pure crystalline iron silicates, Fe2Si04 (olivine) and FeSi03 (pyroxene) will be rare.

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