One kind of clay or another is fairly common on Earth, where we have liquid water to create more of it. On Mars though, the material is far less common, but still present thanks to its watery past. Recent work by researchers at the Georgia Institute of Technology though has found that Martian clay is more common than previously thought and has been found somewhere quite unexpected.
Using the Mars Reconnaissance Orbiter (MRO) the researchers have determined that Eagle Crater, where the Opportunity rover landed in 2004, contains clays that had previously gone unnoticed. Now the MRO has identified clay deposits at the new location of Opportunity, Endeavour Crater. It is not likely Opportunity will also detect the clay though, as many of its instruments no longer work. However it is possible that Curiosity, the latest and largest Martian rover, will be able to find some where it is currently exploring.
The discovery of the clay minerals is not completely surprising, given that we now recognize Mars had once been covered in water. What is unexpected though is that the minerals have been found above a layer of sulfates, which are believed to have formed during a period of volcanism when the water was acidic. This is contrary to the hypotheses of the history of water on Mars though, as the clays should have formed in basic or alkaline water.