New Rock Type Found at Mars

In 2005, NASA's rover named Spirit found a rock (later to be named "Peace") in an exposure of bedrock within the Gusev Crater. The new rock, Peace, houses more sulfate salt than any other rock previously encountered by the rover. The rock appears to be made up of volcanic rocks that were crumbled, then cemented together by magnesium-sulfate salt. This could mean that liquid water carrying dissolved magnesium sulfate salt ran through the rock, and afterward the salt was left behind. Or, weathering by dilute sulfuric acid is another possibility regarding the rock's formation. Both rock-forming processes need water to work, so the new rock is another sign that the Mars of the past had water.

http://www.space.com/scienceastronomy/new_rock_050215.html