Terrestrial planetary surfaces, other than Earth, are overwhelmingly basaltic in character. Because of this, among the most intriguing and significant results from recent studies of Mars are the occurrences of high-silica rocks at the Pathfinder site and orbital thermal emission spectroscopy evidence for a global petrological dichotomy with andesites dominating the entire northern hemispheric plains. However, during the alteration of basalt under a wide variety of pressure, temperature, and fluid conditions, silica is highly mobile, and chemical and/or mineralogical relationships among Martian soils and other surface deposits suggest the likelihood that sedimentary silica phases would have formed during surficial alteration processes. Accordingly, the redistribution of sedimentary silica needs to be considered in interpreting at least some of the high-silica features seen on Mars.

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