THE IRRADIATION-INDUCED OLIVINE TO AMORPHOUS PYROXENE TRANSFORMATION PRESERVED IN AN INTERPLANETARY DUST PARTICLE

The very low sulfur IDP L2011K7, 22 × 17 μm in cross section, is dominated by areas of highly vesicular, mostly ferromagnesiosilica glass and elongated and rounded (spherical) grains of the same materials. Clusters of rounded grains (100–200 nm in size) are generally somewhat porous and enclose micrometer-scale minerals and silica-rich glass. The minerals are diopside and its irradiation-induced transformation products (Rietmeijer 1999), Mg, Fe-olivine, rare enstatite (Mg₂Si₂O₆), and rare Ni-bearing (6 at% Ni) pyrrhotite. Olivine grains, and glass grains with pyroxene compositions and compositions intermediate between olivine and pyroxene (see below), range from 190 nm to 435 nm in diameter for spherical grains. These grains forming flattened spheroids (aspect ratios of ∼0.5) range from 280 nm × 135 nm to 1060 nm × 615 nm. Most of these grains are between 270 nm in diameter (spheres) and 275 nm × 136 nm in size. They occur as individual grains or in small clusters, and it is possible that the largest glass grains are fused clusters. Very few olivine grains still show their characteristic external shape. The trend is toward rounding off of grain edges and ultimately spherical and flattened spherical grains. Few grains have core of a remnant olivine grain. Typical grains have a smooth amorphous core, which often contains small (∼5 nm to ∼15 nm), rounded relict grains, that gradually changes into a mélange of fine-grained glass grains, small crystals and many voids (pores), typically a few nanometers in size but up to 10–20 nm sized vesicles (Figure 1). These changes in petrographic properties are consistent with the gradual transformation of Fe-rich olivine mineral grains to glass grains with pyroxene compositions caused by irradiation-induced amorphization.

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The mean (at) ±S.D. major abundances for all olivine and pyroxene glass compositions define Gaussian, distributions for Si = 17.2 ± 4.5, Fe = 6.7 ± 3.85, except Mg = 16.65 ± 6.4 that shows a skewed (S) distribution (mode = 19.11; S = −0.38) indicating the grains are dominated by high Mg compositions. The average atomic Mg/(Mg+Fe) (mg) ratio, mg = 0.71, shows that the grains in this IDP are Fe-rich. The data are ordered in discrete size bines according to the Mg contents expressed as the mg ratio. It allows tracking of any effects that could be related to the differential susceptibility of olivine minerals, i.e., the fayalite (Fe₂SiO₄) component of the grains being more susceptible to amorphization than forsterite (Mg₂SiO₄). The olivine crystals in the irradiation study by Carrez et al. (2002a) contained a lower amount of iron, viz., mg = 0.9.

The full range of olivine and glass grains in IDP L2011K7 ranges from mg = 0.80 to mg = 0.31 (Table 1). Few olivine and pyroxene glass grains have a pure Mg, Fe-composition. Most contain heterogeneously distributed, minor amounts of Al (μ = 1.0 ± 0.7 at %) or Ni (μ = 0.08 ± 0.07 at %), or both. The amount of Al increases slightly from olivine to stoichiometric pyroxene glass. These low contents will not affect the olivine susceptibility to amorphization (Wang et al. 1998a, 1998b). Iron and nickel will behave in a similar manner in silicates. It is not anticipated that these low Ni contents will affect the amorphization process. These low Al and Ni contents are normal for olivine and Ca-free pyroxene in IDPs (Rietmeijer 1998).

The co-linear decreases in the O/Si and Mg/Si ratios in low-energy helium irradiation experiments (Carrez et al. 2002a) imply stoichiometric Mg and O loss during the transformation of olivine to amorphous pyroxene. The data sets for the mg groups 0.8, 0.7, and 0.59 (Table 1) are large enough to draw meaningful conclusions. Only eight grains represent the mg groups 0.48, 0.42, and 0.31. The data for the olivine to pyroxene glass transformation are shown in two complementary diagrams, viz. Mg/Si versus O/Si (at) (Figure 2) and Fe/Si versus O/Si (at) (Figure 3). The observed Mg/Si versus O/Si correlations conform to the experimentally determined trend (cf. Carrez et al. 2002a). In olivine, the stoichiometric O/Si (at) = 4, in stoichiometric pyroxene O/Si (at) = 3. Olivine in the mg = 0.8 group with a smooth grain surface, i.e., no discernable irradiation-induced voids, include two different O/Si populations, viz., (1) O/Si = 4.09 ± 0.17, i.e., statistically stoichiometric olivine and (2) O/Si = 4.28 ± 0.04. The latter matches the solar ratio, O/Si = 4.32, which seems coincidental. Similar high O/Si ratios for GEMS (Table 2) were explained as excess oxygen due to the amorphization process, although no complete explanation was offered. In IDP L2011K7, excess oxygen occurs in Fe-rich, crystalline olivine, and thus from a mineralogical point of view the reaction may have been of the type

$$\begin{eqnarray*} &&{\rm (Mg,Fe)}_2 {\rm SiO}_4 + {\rm O}^{2 - } = {\rm (Mg,Fe)}_2 {\rm SiO}_5 \;\; \\ &&{\rm with}\;{\rm O/Si} = 5. \end{eqnarray*}$$

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Oxidation of a fraction of ferrous iron in the fayalite component of olivine could introduce excess oxygen, viz. ²⁺Fe_2−x³⁺Fe_xSiO_4+0.5x with x ≤ 1. The observed average O/Si ratio of 4.28 suggests that 30% of the fayalite component was stoichiometric ²⁺Fe_1.4³⁺Fe_0.6SiO_4.3 but with excess oxygen. If this oxidation reaction is an accurate explanation for this excess oxygen in Fe-rich olivine in this IDP, the cause for this unusual mineralogical reaction must also be unusual. Irradiation-induced olivine amorphization qualifies as an unusual mineralogical process.

The sequence of events that resulted in natural samples for laboratory analyses is often difficult to unravel, such as variations in oxidation/reduction conditions during amorphization of Fe-rich olivine. Following the original amorphization process several different and independent processes might have affected the mineral and glass grains in this IDP, viz., (1) dynamic annealing of the fayalite component in olivine during irradiation at elevated temperatures (∼600 °C) causing nucleation (crystallization) of quartz (SiO₂) and magnetite (Fe₃O₄) nanocrystals in amorphized material (Wang et al. 1995), (2) oxidation caused by flash-heating of the grains in this IDP the few seconds of during atmospheric entry up to ∼600° C (Rietmeijer 1999) or both. The low Fe/Si versus O/Si linear correlation coefficient (0.6) of the mg = 0.8 group (Figure 3) might point to the actions of untraceable oxidation/reduction conditions, or it simply shows a larger error of measurement in this very low-Fe glass.

Many grains have a core-rim texture whereby the core has a (relict) olivine composition. Selected core-rim examples are shown in Figure 4. In general, the compositions of the amorphous rims in most show a trend toward higher silica (SiO₂) contents, that is, glass with (1) a smectite dehydroxylate composition or (2) pyroxene composition (Figure 4; arrows 1 & 2). In only one zoned grain the olivine core has an amorphous pyroxene rim that is more Mg-rich than the core suggesting the loss of Fe and O (Figure 4; arrow 3). This trend is consistent with heating olivine to temperatures close to its melting point. It is an anomaly among the data that might indicate another process than irradiation-induced amorphization or a secondary process that affected the most Fe-rich olivine in this IDP. In two other grains the rim was pure silica glass on a smectite dehydroxylate core (no relict olivine) or pure silica glass on pyroxene glass with a small relict olivine core (Figure 4, arrows 4). They were the two smallest grains in IDP L2011K7, which suggests that irradiation-induced element loss is volume diffusion-controlled. The smectite dehydroxylate (see below) composition suggests kinetically control on the irradiation-induced amorphization process in this IDP.

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The solar system sources of chondritic aggregate IDPs remain unknown but since their textures and petrological properties are entirely different from these properties of any meteorites in our collections, they are from small solar system bodies that are not parent bodies of these meteorites (Mackinnon & Rietmeijer 1987). It suggests a cometary reservoir for these particles. If this connection can be made, IDP L2011K7 accreted from dust that was modified prior to the accretion of comets. Once accreted in a comet nucleus, this IDP and its constituents were preserved in their original mineralogical state that may have included amorphous interstellar silicate grains and silicates that were modified by irradiation-induced amorphization in the solar nebula.

Min et al. (2007) modeling the shape and compositions of amorphous interstellar silicate grains concluded that these silicates are on average Mg rich (mg > 0.9). The defining parameters in this model (Min et al. 2007) are well matched by those for the average amorphous silicates in IDP L2011K7 here reported (Table 2). The average mg ratio is smaller than the value in the model of Min et al. (2007) but observed compositions in the IDP silicates are dominated by high-Mg compositions. The data presented in a O/Si versus (Mg+Fe)/Si (at) diagram show that the composition of the amorphous interstellar silicate (cf. Min et al. 2007) perfectly matches a serpentine-dehydroxylate, (Mg, Fe)₃Si₂O₇, composition for the average glass composition in IDP L2011A9 (Figure 5). The most Si-rich glass composition observed in this IDP matches a smectite-dehydroxylate, (Mg, Fe)₆Si₈O₂₂, composition (Figure 5). These two particular compositions point to a lack of thermodynamic equilibrium during the olivine amorphization process. Highly disordered amorphous solids with these deep metastable eutectic compositions formed preferentially in vapor phase condensation experiments (Nuth et al. 2000a; 2000b; Rietmeijer et al. 2002) and they match many of the compositions of glass grains in chondritic aggregate IDPs (Rietmeijer 2002; Rietmeijer et al. 1999). The physiochemical conditions necessary to form these metastable dehydroxylate glasses are rapid quenching to well below its solidus of a super-heated melt or a vapor. How exactly these conditions can be connected to high-energy induced amorphization of silicates is not yet clear but I recall that the susceptibility to amorphization of silicates was linked to their glass-forming ability. Studies of irradiation-induced amorphization showed that amorphization of pure forsterite is much more difficult to achieve than for Fe-containing olivine minerals (cf. Wang et al. 1998a, 1998b). Amorphization of 200 nm diameter forsterite grains in the interstellar medium during 10⁷ − 10⁸ yr produces a negligible fraction of amorphous forsterite grains (Brucato et al. 2004). When crystalline forsterite were the precursor of the amorphous interstellar silicate discussed by Min et al. (2007), it places rigorous constraints on the energies and times available for the amorphization process. The data presented here suggest that the olivine grains entering the interstellar medium include pure forsterite and Fe-rich olivine minerals. The latter will readily succumb to amorphization, which yields a mixed population of crystalline forsterite and Fe-rich amorphous silicate (glass) grains, unless extreme conditions also produced forsterite glass. Amorphization will be sensitive to grain size that is also a sensitive parameter to reduce infrared spectroscope data.

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The compositions of the crystalline olivine to pyroxene glass transformation (Figure 5) are all for ∼200 nm to ∼1 micron, Fe-rich olivine mineral and glass grains with a wide range of mg ratios. Whether or not they are detectable using infrared spectroscopy is another matter. If these serpentine dehydroxylate glass grains are the amorphous interstellar silicate grains, it raises two questions: (1) could there be more than one type of interstellar silicate grains and (2) is the hypothesized GEMS interstellar silicate origin correct? It is not yet proven that all GEMS in the chondritic aggregate IDPs studied to date are pre-accretionally irradiated grains and could not include evolved non-equilibrium condensate grains (Rietmeijer 2009). There is no physical resemblance between GEMS and the dehydroxylate glass grains in IDP L2011K7 but both show the vesicular textures that developed in radiation-induced amorphization experiments. There are considerable chemical differences between GEMS and the amorphous interstellar silicate and the glass grains from this study (Table 2). It is remarkable then that the average O/Si (≈2.7) and (Mg+Fe)/Si (≈0.7) ratios for 144 GEMS (Keller & Messenger 2004) match the smectite-dehydroxylate glass composition in IDP L2011K7 (Figure 5). A possible scenario could be that interstellar Fe-rich ferromagnesiosilica glass grains from the crystalline olivine to pyroxene glass transformation continued to lose Mg with concomitant reduction of Fe and Ni from the silicate fraction to Fe, Ni-metal grains. The answer to these questions are then, yes, there are different types of amorphous silicate grains that formed by the process hypothesized for pre-accretionally irradiated GEMS. The chondritic aggregate IDP L2011K7 is the first primitive extraterrestrial sample that preserved evidence for the natural, irradiation-induced olivine to amorphous pyroxene transformation. If, as I hypothesize, the grains from this transformation represent amorphous interstellar silicate grains, then this metastable interstellar silicate has an average Mg, Fe-serpentine-dehydroxylate composition with rare grains having a Mg, Fe-smectite dehydroxylate composition.