The Stardust Mission: Returning Comet Samples to Earth

the clay formation in Nakhla or Lafayette, which occurred at <180°C [1,2). Similarly, studies of Lafayette and Nakhla here and elsewhere [7) have not revealed any obvious alteration features associated with this phase. It is believed therefore that the 0 isotopic composition of the feldspar reflects equilibration at high temperatures, rather than pervasive lower temperature exchange with water of either heavy or light 0 isotopic composition [3]. In contrast, 8 180 values of olivine and pyroxene do not provide realistic temperatures consistent with equilibration [ 4). This is probably due to iddingsite within the olivine, the effect of which is to increase the oiso value [3]. Further mineralogical and isotopic data on mineral separates including illite/nontronite are being gathered in order to help characterize the effects and nature of the alteration fluids. It is also hoped to provide in-formation about theo 170 value of the clay. References: [I) Ashworth J. R. and Hutchison R. (I 975)Nature, 256, 714-715. [2) Gooding J. L. et al. (1991) Meteoritics, 26, 135-43. [3) Romaneck C. S. et al. (1996) LPS XXVII, 1099-1100. [4) Franchi I. A. et al. (1997) LPS XXVI//, 379-380. [5) Chiba H. et al. (1989) GCA, 53, 2985-2995. [6) Matthews A. (1980) GCA, 44, 387-402. [7) Treiman A.H. et al. (1993) Meteoritics, 28, 86-97.