CRYSTAL-CHEMICAL ANALYSIS OF MARTIAN MINERALS IN GALE CRATER. S.M. Morrison 1 , R.T. Downs 1 , D.F. Blake 2 , D.L. Bish 3 , R.V. Morris 4 , D.T. Vaniman 5 , E.B. Rampe 4 , C.N. Achilles 3 , D.W. Ming 4 , S.J. Chipera 6 , A.H. Treiman 7 , R. Gellert 8 , T.F. Bristow 2 , J.A. Crisp 9 , J.M. Morookian 9 , P.C. Sarrazin 10 , J.D. Farmer 11 , A.S. Yen 9 , D.J. Des Marais 2 , J. P. Grotzinger 12 , E.M. Stolper 12 , M.A. Wilson 2 , N. Spanovich 9 , R.C. Anderson 9 and the MSL team. 1 U. of Arizona (1040 E 4 th St. Tucson, AZ 85721; shaunnamm@email.arizona.edu), 2 NASA ARC, 3 Indiana U., 4 NASA JSC, 5 PSI, 6 CHK Energy, 7 LPI, , 8 U. Guelph, 9 JPL-Caltech, 10 in-Xitu, 11 Arizona State U., 12 Caltech. Introduction: The CheMin instrument on the Mars Science Laboratory rover Curiosity performed X-ray diffraction analyses on scooped soil at Rocknest [1,2,3] and on drilled rock fines at Yellowknife Bay (John Klein and Cumberland samples) [3,4], The Kimberley (Windjana sample) [3,5], and Pahrump (Confidence Hills sample) [3,6] in Gale crater, Mars. Samples were analyzed with the Rietveld method to determine the unit-cell parameters and abundance of each observed crystalline phase. Unit-cell parameters were used to estimate compositions of the major crystalline phases using crystal-chemical techniques. These phases include olivine, plagioclase and clinopyroxene minerals. Comparison of the CheMin sample unit-cell parameters with those in the literature provides an estimate of the chemical compositions of the major crystalline phases. Preliminary unit-cell parameters, abundances and compositions of crystalline phases found in Rocknest and Yellowknife Bay samples were reported in [1,2,4]. Further instrument calibration, development of 2D-to- 1D pattern conversion corrections, and refinement of corrected data allows presentation of improved compositions for the above samples. Composition as a Function of Unit-Cell Parameters: Unit-cell parameters and chemistry were obtained from the literature for the target minerals, and relationships between cell parameters and chemistry were observed. Some relationships, like that of Fa-Fo olivine (fig. 1), are very well defined, whereas others, such as coupled substitution in plagioclase, are not as well defined. In the case of plagioclase, diffraction patterns from these samples have broad peaks and indications from pattern fitting are that more than one plagioclase phase or zoned plagioclase may be present. Work is ongoing in addressing plagioclase compositions, but at present the use of cell parameters in constraining plagioclase composition is limited. Augite and pigeonite were constrained to the Ca- Fe-Mg system. A simple binary plot, as shown in Fig. 1, does not fully describe the relationship between composition and cell dimensions. Therefore, in both clinopyroxenes, Mg-content was estimated from the b unit-cell parameter because it forms a linear trend (Fig. 2a). Ca and Fe were discriminated by the β angle for augite (Fig. 2b) and by unit-cell volume for pigeonite (not shown). 0.0 0.5 1.0 1.5 2.0 288 293 298 303 308 Mg Content (apfu) Unit-Cell Volume (Å 3 ) Olivine Figure 1. Mg-content of Fa-Fo olivine as a function of unit-cell volume. Individual points obtained from literature data (shown as blue dots). Unit-cell volumes of Rocknest, John Klein and Cumberland samples are represented by a red diamond, blue square and green triangle, respectively. Estimated Mg-content for each CheMin sample was determined using this regression line. Chemical Composition of Olivine and Clinopyroxene Group Minerals: The following chemical compositions were obtained from the regressions illustrated in Figs.1 & 2: Rocknest olivine (Mg0.81Fe1.19)SiO4 augite (Ca0.70Mg0.68Fe0.62)Si2O6 pigeonite (Mg0.97Fe0.67Ca0.36)Si2O6 John Klein olivine (Mg0.98Fe1.02)SiO4 augite (Ca1.02Mg0.69Fe0.29)Si2O6 pigeonite (Mg1.14Fe0.71Ca0.15)Si2O6 Cumberland olivine (Mg0.78Fe1.22)SiO4 augite (Ca0.83Mg0.68Fe0.49)Si2O6 pigeonite (Mg1.10Fe0.68Ca0.22)Si2O6 2506.pdf 46th Lunar and Planetary Science Conference (2015)