<P> Observations of rocks on the plains show they contain the minerals pyroxene, olivine, plagioclase, and magnetite . These rocks can be classified in different ways . The amounts and types of minerals make the rocks primitive basalts--also called picritic basalts . The rocks are similar to ancient terrestrial rocks called basaltic komatiites . Rocks of the plains also resemble the basaltic shergottites, meteorites which came from Mars . One classification system compares the amount of alkali elements to the amount of silica on a graph; in this system, Gusev plains rocks lie near the junction of basalt, picrobasalt, and tephite . The Irvine - Barager classification calls them basalts . Plain's rocks have been very slightly altered, probably by thin films of water because they are softer and contain veins of light colored material that may be bromine compounds, as well as coatings or rinds . It is thought that small amounts of water may have gotten into cracks inducing mineralization processes). Coatings on the rocks may have occurred when rocks were buried and interacted with thin films of water and dust . One sign that they were altered was that it was easier to grind these rocks compared to the same types of rocks found on Earth . </P> <P> The first rock that Spirit studied was Adirondack . It turned out to be typical of the other rocks on the plains . </P> <P> Scientists found a variety of rock types in the Columbia Hills, and they placed them into six different categories . The six are: Adirondack, Clovis, Wishstone, Peace, Watchtower, Backstay, and Independence . They are named after a prominent rock in each group . Their chemical compositions, as measured by APXS, are significantly different from each other . Most importantly, all of the rocks in Columbia Hills show various degrees of alteration due to aqueous fluids . They are enriched in the elements phosphorus, sulfur, chlorine, and bromine--all of which can be carried around in water solutions . The Columbia Hills' rocks contain basaltic glass, along with varying amounts of olivine and sulfates . The olivine abundance varies inversely with the amount of sulfates . This is exactly what is expected because water destroys olivine but helps to produce sulfates . </P> <P> The Clovis group is especially interesting because the Mossbauer spectrometer (MB) detected goethite in it . Goethite forms only in the presence of water, so its discovery is the first direct evidence of past water in the Columbia Hills's rocks . In addition, the MB spectra of rocks and outcrops displayed a strong decline in olivine presence, although the rocks probably once contained much olivine . Olivine is a marker for the lack of water because it easily decomposes in the presence of water . Sulfate was found, and it needs water to form . Wishstone contained a great deal of plagioclase, some olivine, and anhydrate (a sulfate). Peace rocks showed sulfur and strong evidence for bound water, so hydrated sulfates are suspected . Watchtower class rocks lack olivine consequently they may have been altered by water . The Independence class showed some signs of clay (perhaps montmorillonite a member of the smectite group). Clays require fairly long term exposure to water to form . One type of soil, called Paso Robles, from the Columbia Hills, may be an evaporate deposit because it contains large amounts of sulfur, phosphorus, calcium, and iron . Also, MB found that much of the iron in Paso Robles soil was of the oxidized, Fe form, which would happen if water had been present . </P>

What are the rocks on mars made of