Minerals generally form under specific sets of conditions reflective of equilibrium.  Therefore, the details of a mineral assemblage (i.e., rocks) can often be used to infer rock formation conditions and the history of mineral formation and alteration.  For this reason, since the late 19^th century when scientists believed that Mars surface was covered by canali, mankind has sought to learn the mineralogical makeup of planetary bodies.  Virtually all of our knowledge of Earth’s neighbors (apart from our Moon and Mars) has been obtained remotely via spectroscopic or photographic methods.  Not unlike Percival Lowell, who suggested in 1895 that martian canals had been constructed by an intelligent race tapping melting polar ice for water to irrigate equatorial crops, understanding the mineralogy of other planetary bodies has often required imagination. Although considerable chemical and spectroscopic data exist for the surface of Mars, these do not provide unambiguous mineralogic information, thereby generating considerable speculation about the mineralogy of the martian surface.  Orbital data show the widespread occurrence of common basaltic or andesitic minerals, but it has proven more difficult to unravel Mars’ alteration mineralogy.  Spectroscopic and chemical data obtained by the martian landers have provided tantalizing suggestions of a secondary minerals, such as phyllosilicates (e.g., clay minerals) and evaporite minerals, but there are few relatively unambiguous mineralogical identifications, including jarosite, KFe₃(SO₄)₂(OH)₆ (based on Mössbauer spectroscopy), gypsum, and a variety of less well-constrained sulfates and silicates.  CheMin is a miniature XRD/XRF instrument designed for mineralogical analyses on extraterrestrial bodies (e.g., planets, moons, asteroids and cometary nuclei).  CheMin uses a Co X-ray source in transmission mode with a 2-D CCD detector, operating in single-photon counting mode, capable of spatial and energy resolution of X-ray photons.  Sample preparation requires only crushing to <150 μm by virtue of its sonic vibration sample movement technique applied during analysis for enhanced particle statistics.  A 2-D image of fluorescent and diffracted photons is obtained on the CCD, and imaging only the characteristic Co Kα photons produces Laue rings.  Circumferential integration of the rings produces a conventional diffraction pattern.  The instrument has been lab and field tested and was used to obtain in situ XRD and XRF analyses in Death Valley, Rio Tinto, Spain, the Arctic, and the Antarctic.  The mineralogy of rock and soil samples is easily identified in the field, with analysis times as short as 5 min, and data were analyzed on-site, including Rietveld refinements to compute the quantitative compositions of mineral mixtures.  In situ analysis largely eliminates the concern with changes in sample mineralogy after field sampling, either by hydration/dehydration or mineral-mineral reactions. CheMin is scheduled to fly on the 2009 NASA Mars Science Laboratory landed mission to Mars, where it will perform mineralogical and elemental analyses of rocks, sediments, and regoliths to provide the first definitive mineralogical data for Mars, including assessing the possible role of water in mineral formation and searching for indicators of past habitable environments.

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