Conversation so far:
What was the Spirit's main discovery?
Analysis shows that the rocks have been slightly altered by tiny amounts of water.
Which Martian rocks were analysed?
All the rocks contain a fine coating of dust and one or more harder rinds of material.
What materials do Martian rocks contain?
They contain the minerals olivine, pyroxene, plagioclase, and magnetite, and they look like volcanic basalt as they are fine-grained with irregular holes.
Where were these rocks found?
There are a variety of rocks in the Columbia Hills, some of which have been altered by water, but not by very much water.

Question: How did water alter the Martian rocks?
Self-contained rewrite: How did water alter the Martian rocks?