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Mineralogical and Textural Changes of a Wild 2 Terminal Particle Pentlandite from Capture Heating in Aerogel

Introduction: A primary goal of the Stardust (SD) mission was to return unmodified particles from comet Wild 2 to Earth. Thus far, a wide range of minerals and heated by-products of captured particles extracted from the aerogel tiles have been observed including silicates, sulfides, oxides, metals , glasses and minor carbonaceous matter [1,2]. The various SD particles have a large range of thermal stabilities and depending on their sizes and strengths, and other factors, will undergo variable modifications during capture heating, thus, to fully understand the nature of the grains that were derived from comet Wild 2 it is important to understand any heating effects that may have occurred from capture. Using TEM techniques we studied the terminal particle (TP) from track 59. Analysis of this particle indicates that it has a bulk composition similar to the mineral pentlandite. Individual regions within the TP, however, are composition-ally variable and show large Fe/Ni variations which are not consistent with a pentlandite composition. Texturally, the TP is composed of discrete subgrains or 'islands' of Fe-Ni-sulfides which are often separated by pure silica. Here we show that capture heating of the original particle, believed to be the mineral pentlandite, modified the grain into two high temperature Fe-Ni sulfides, an MSS phase with high Fe/Ni ratio and the high temperature Ni-rich sulfide heazlewoodite. This particle was particularly susceptible to thermal modifii-cation because of its small size of ~ 2 x 3 um and the high thermal conductivity of pentlandites. Techniques: Track 59 is a 0.35 mm long type A track [3] with an optically opaque terminal particle (TP) with sub-micron opaque debris along the track length. To prepare the sample for the TEM, the track was embedded in acrylic resin and flattened between glass slides [4]; the TP was then cut from the flattened track and microtome sections < 100 nm were produced using standard techniques. TEM observations and EDX analyses were done with a Tecnai F20 STEM with attached EDAX Genesis Xray analysis system. Element maps were produced from 51,200 individual spectra that were collected from a 0.5 um x 0.6 um region. To better understand the sulfide compositions, ROIs from irregular areas of differing composition were chosen and the pixels summed to produce quantitative spectra. Petrography: A HAADF STEM image taken of a representative microtome section from the TP is shown in Figure 1a. The image shows that the TP is composed …