Saw this on the Unmannedspaceflight.com site:

P12A-04
TI: First Low-Iron Materials on Mars and Possibility of a Major Montmorillonite Component.
"Chemical trends implicate Mg sulfates and Ca phosphates are important but minor accessory minerals. Moessbauer analysis indicates some or all Ti to be present as ilmenite. The remaining component has high Al and Si abundances in proportions within the range of classical montmorillonite compositions found at various locations on Earth, including their correspondingly low concentrations of major cations. Typically the result of weathering of basaltic ash and often associated with more arid environments, the formation of montmorillonite or its alteration-product precursor implies that significant aqueous activity to facilitate geochemical separations occurred."

Montmorillonite is a type of clay.

Steve Squyres had mentioned that Independence rock, among those that appear to contain clay, was very unusual:

Mars rovers keep breaking new ground
Opportunity traverses sandy ‘maze’; Spirit studies strange bedrock.
By Leonard David
Updated: 8:56 p.m. ET July 19, 2005
"During its ascent of Husband Hill within the Columbia Hills, the robot came across a “very cool outcrop” of layered bedrock that has been tagged as Independence Rock, Squyres said.
“We’ve thoroughly worked it over with all of the arm instruments now, and it’s very strange stuff,” Squyres reported. He said it was one of the oddest things seen at Gusev.
“I’m not ready to go into much detail here about the chemistry and mineralogy yet, since we’re still chugging through the data,” Squyres noted. The rock is clearly highly altered, sporting an unusually low iron content, he said, “which isn’t something we’ve seen much of before.”
http://msnbc.msn.com/id/8634311/

And clay had been found earlier in the plains by Spirit:

Evidence of Phyllosilicate in Wooly Patch - an Altered Rock
Encountered on the Spirit Rover Traverse [#2327].
Wang A. Haskin L. A. Korotev R. L. Jolliff B. L. de Souza P. A. Jr.
Kusack A. G. Athena Team.
"Rock Wooly Patch on West Spur has several specific physical and
chemical features. Based on our analyses, phyllosilicates of kaolinite,
serpentine, and chlorite types, plus some feldspar and pyroxene are
prime candidates to constitute this rock."
http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2327.pdf

Kaolinite is a type of clay.

I had also speculated some inclusions in Wishstone rock (not among the rocks mentioned in this latest research) might be clays:

Smooth inclusions in Wishstone rock.
Posted by Robert Clark on 12/31/2004 7:26:56 AM
http://www.habitablezone.com/space/messages/363418.html

- Bob Clark