MICROSTRUCTURAL INDICATIONS FOR PROTOENSTATITE PRECURSOR OF COMETARY MgSiO3 PYROXENE: A FURTHER HIGH TEMPERATURE COMPONENT OF COMET WILD

Introduction: Comets are small bodies composed mainly of a mixture of ice and dust particles and were formed during accretion of the planetary system. Their reservoirs are the Kuiper belt (30-55 AU) and the Oort cloud (50,000 AU). The classical picture about the formation of short-period comets (e.g. Comets Borrelly, Temple 1, Wild 2; [1]) is condensation of material beyond the frost line in the cold Kuiper Belt extending from the orbit of Neptune to 55 AU from the Sun. Here we present evidence for the former existence of the high temperature MgSiO 3 polymorph protoenstatite (PEN) as a precursor for the formation of clino-(CLEN) and minor orthoenstatite (OREN). Five well characterized enstatite polymorphs are known. PEN is only stable at very high temperatures above ca. 1275K and low pressures. CLEN can exist in the C2/c structure at high temperatures (HT-CLEN, >1450K) or high pressures (HP-CLEN, >6GPa) and is stable at low temperatures and low pressures in the P21/c-modification (LTLP-CLEN). OREN (Pbca) show a large stability field at moderate temperatures and pressures. Most natural enstatite crystals are a mixture of CLEN and OREN and therefore show 9Å or 18Å periodicities along [100]. Due to periodic repeats of stacking sequences a number of different polytypes are possible (e.g. [2], [3], [4]). LTLP-CLEN can be formed by at least five transformation paths. [2] described several differences between those various reaction paths and present a basic table with the most striking features (Table 1). Results: The composition of the enstatite is measured along a traverse through the microtomed section of the particle. We found almost pure enstatite (Mg/Si=0.858) with some minor element concentrations of Fe, Ca, Al and Cr (see also [5]). In most of the studied areas alternating CLEN and OREN with a dominance of CLEN lamellae were found (Fig. 1). The variable stacking sequence of the 9Å CLEN and 18Å OREN lamellae lead to a number of polytypes on a scale less than 100 nm. CLEN fields showing odd and even numbers of lamellae were observed with no preference to one or the other. Field widths of 27Å and 45Å are shown in Figure 1 which corresponds to (n+1)9Å repeat (n=3, 5) and also field widths of 54Å and 90Å are visible in the