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Experimental astromineralogy: Circumstellar ferromagnesiosilica dust in analogs and natural samples

Frans J.M. Rietmeijera and Joseph A. Nuth IIIb institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA.

laboratory for Extraterrestrial Physics, MS 691, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.

Hutton's principle of Actualism guides all Earth Sciences research and includes experimental verification of natural processes whereby collected samples are the ground truth to validate experiments. Chondritic aggregate IDPs serve to validate the uniqueness of astromineralogy experiments of kinetically controlled condensation, post-condensation aggregation and thermal alteration of ferromagnesiosilica circumstellar dust. Laboratory experiments constrain the crystallographic and chemical properties of metastable eutectic ferrosilica and magnesiosilica dusts and make very specific predictions about the compositions of ferromagnesiosilica condensates.

1. introduction

The first time silicates were postulated to exist in dusty space environments, Astronomy became linked to Earth Sciences. The notion that this dust was initially formed by gas-to-solid condensation of chondritic vapors has no analog in terrestrial environments wherein we can observe this particular formation of silicate minerals. In this paper we will explore the implications to Astronomy of the mineralogical literature. First, when minerals, such as olivine [(Mg,Fe)2SiC>4] and pyroxene [(Mg,Fe)SiC>3], are identified in space there is an extensive database on the physical (crystallographic) and chemical properties of these silicates, their occurrences and conditions of formation. In this context terms such as "dirty silicates" or "astronomical silicate" have no meaning although the latter qualifies a non-geological use. This is a matter of nomenclature where two different Sciences will have to meet. Second, the presence of minerals in space can only be interpreted using the founding principle of Geology put forward by James Hutton (1726-1797): "No powers are to be employed that are not natural to the globe, no action to be admitted except those of which we know the principle". He called it Actualism but Sir Charles Lyell re-christened this as Uniformitarianism. Earth Scientists have to accept that the processes operating in the past on Earth proceeded exactly as we can observe their operations today on Earth. The presence of minerals in space can constrain the processes in their environments but this formulation of Actualism causes an obvious problem for astronomical application.

The second part of Hutton's original formulation was later interpreted to include experimental verification under laboratory conditions. This change paved the way to attempt experimental quantification of geological processes, such as experimental petrology to determine the pressure and temperature of formation of minerals and mineral assemblages. Earth Scientists literally have the sample in hand that they want to simulate in the laboratory and this provides ground truth to evaluate the uniqueness of the laboratory experiments. This situation does not exist for space dust wherefore there are only infrared spectral data available. By the very nature of the signal such data represent a measurement of 'secondary' dust properties. The primarily properties are determined by the crystallography and chemistry of the solids. Unique mineral determinations based on IR spectra require additional independent information that is generally unavailable to Astronomers. This situation improved when interplanetary dust particles (IDPs) became available. The porous chondritic aggregate IDPs are unique solar system materials from asteroids and periodic comets [1], Aggregate IDPs offer ground truth to examine the uniqueness of laboratory simulations of space dust whereby the IR spectral information of the analogs can be used to explore dust compositions and physical processes in oxygen-rich astrophysical environments [2],

2. INTERPLANETARY DUST PARTICLES (IDPS)

Classification of unmelted IDPs shows that the early solar system contained a limited number of chondritic and non-chondritic dust types, viz. (1) ferromagnesiosilica principal components (PCs), (2) Fe,Mg- and Ca,Mg,Fe-silicates. (3) Fe,Ni-sulfides (mostly Ni-free and low-Ni pyrrhotite), (4) Fe-metal and -oxides and (5) rare refractory Ca,Ti,Al-rich dust [3,4], The textures of chondritic aggregate IDPs 10-15 jam in size show that hierarchical dust accretion began with the formation of a fractal matrix of PCs, 90-1,000 nm in diameter [2,3],

The PCs are (1) carbonaceous PCs (refractory hydrocarbons, amorphous, poorly graphitized, pre-graphitic turbostratic carbons, 'buckytubes' and 'buckyballs'[5]; (2) carbon-bearing ferromagnesiosilica PCs of ultrafine (2-50 nm) Fe,Mg-olivines and -pyroxenes, Fe,Ni-sulfides, Fe-oxides in a carbonaceous matrix, and (3) ferromagnesiosilica PCs. The ferromagnesiosilica PCs are (1) Mg-rich, coarse-grained (10 - 410 nm) PCs with Fe/(Fe+Mg) (fe) = 0-0.35 [6] and (II) Fe-rich, ultrafine-grained PCs,/e = 0.35-0.85 (modal fe = 0.67) with an amorphous ferromagnesiosilica matrix and embedded Fe,Mg-olivines and -pyroxenes, Fe,Ni-suIfides and Fe,Ni-metal grains (<50 nm) [3]. The PCs are the smallest textural entities in the matrix of aggregate IDPs. They have their analogs in terms of particle size (mass) and composition among the CHON, 'mixed' and 'silicate' particles in comet Halley's dust [6],

This paper emphasizes ferromagnesiosilica (FeMgSiO) PCs that are among the oldest dust in the solar system. We accept that all dust was initially formed by gas-to-solid condensation although there is a discontinuity in size (mass) and composition between-the condensates and PCs in aggregate IDPs. The compositions of the FeMgSiO PCs provide ground truth to test the results of our condensation experiments.

3. GAS-TO-SOLID CONDENSATION

Circumstellar dust analogs produced by the kinetically controlled gas-to-solid condensation experiments of Mg-Si0-H2-02 and Fe-Si0-H2-02 vapors yielded amorphous MgSiO and FeSiO grains a few nanometers to 150 nm in diameter. However, pure Fe-, Mg-and Si-oxide condensates were crystalline solids (i.e. minerals). The most significant finding was that the non-stoichiometric MgSiO and FeSiO compositions uniquely matched those of metastable eutectics in the Mg0-Si02 and Fe0/Fe203-Si02 equilibrium phase diagrams [6]. The existence of predictable metastable eutectic compositions is the best proof yet that kinetically controlled gas-to-solid condensation is not a chaotic process but with predictable chemical order. During nucleation in a cooling vapor, the first stable nuclei/molecules that form become seeds for grain growth during continued vapor condensation. Given sufficient activation energy and time, the condensed solids will become arranged into the least-energetic physical and chemical configurations until they equilibrate with the ambient environment. Kinetic factors at high condensation rates might favor metastable high-energy states [6] while surface energy considerations could change the relative stability fields for small grains [7,8]. Condensation in a Mg-Fe-Si0-H2-02 vapor mimicked the metastable eutectics in the separate binary systems. Condensation in the Mg0-Fe0/Fe203-Si02 system did not lead to the formation of MgO-FeO and Mg0-Fe203 solids because due to complete solid solution in these binary systems there is no metastable eutectic.

Mixed FeMgSiO dust can not form by condensation alone. Although formation of Mg-olivine and Mg-pyroxene is predicted by equilibrium thermodynamics, the condensation of Fe-silicates is not. Instead, iron is predicted to condense as a metal until T<600K when it becomes incorporated into earlier condensed Mg-silicates. Kinetically controlled condensation yields amorphous magnesiosilica and ferrosilica dust with predictable metastable eutectic compositions and pure metal-oxide grains. The experimental data make specific predictions about FeMgSiO dust compositions that are possible as a result of aggregation.

4. AGGREGATION OF MG-RICH FERROMAGNESIOSILICA DUST

The amorphous MgSiO grains are metastable Mg3Si207, serpentine dehydroxylate, and Sirich Mg6Si8022, or 6Mg0-8Si02 (smectite dehydroxylate). We note that the chemical composition of an amorphous material is properly presented as a mixture of metal-oxides and not as a mineral structural formula as used here; but using the latter makes it easier to display post-condensation mineralogical evolution]. Amorphous FeSiO solids have two Si-rich metastable eutectic compositions because the Fe0/Fe203 to Si02 ratio is sensitive to the Fe oxidation state during condensation. There are also two very-low silica Mg(Si)0 and Fe(Si)0 metastable eutectics. The condensed dusts that formed in close proximity can mix along tie lines connecting the metastable eutectic MgSiO and FeSiO condensates. The most likely compositions of the resulting mixed FeMgSiO aggregates are uniquely determined by the intersections of these tie lines (Figure 1). The relative positions of the metastable eutectics will restrict spontaneous aggregation to Mg-rich FeMgSiO compositions with fe = 0-0.35. These experimentally constrained compositions exactly match those of the coarse-grained FeMgSiO PCs in aggregate IDPs [6], The amorphous metastable dusts in the aggregates contain a significant amount of internal free energy. As a result, dust aggregates can become compact chemically homogenized amorphous solids. The Mg-rich FeMgSiO PCs (fe = 00.35) could form spontaneously via aggregation while formation of Fe-rich FeMgSiO dust probably requires an external energy source to initiate fusion of aggregated dust.

5. AGGREGATION OF FE-RICH FERROMAGNESIOSILICA DUST

Metastable MgSiO condensates and Mg-rich FeMgSiO PCs will continue to react with the low-silica metastable Fe(Si)0 dust and pure Fe-oxides. This mixing process also occurs along tie lines in the Mg0-Fe0/Fe203-Si02 ternary system whereby preferred FeMgSiO compositions occur at the intersection of experimentally determined metastable eutectic serpentine dehydroxylate and the Mg6Sig022 (Sm-d) - Fe(Si)0 mixing-lines (Figure 2). The predicted modal composition of Fe-rich FeMgSiO dust is fe ~ 0.7 with a range fe = 0.35-0.85.

Condensed dust in the aggregates may still contain internal free energy but efficient fusion into compact amorphous Fe-rich FeMgSiO dust will require an external energy source. The

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