Phosphorus and clays.
Clay is known to be highly absorptive of phosphorus because of its small grain size.
Reducing phosphorus in aquatic systems using modified clays.
http://www.eidn.com.au/phos98.html
This page discusses the absorption of phosphorus on clays and other sedimentary materials:
13.3 Binding of phosphorus in sediments.
http://lepo.it.da.ut.ee/~olli/eutr/html/htmlBook_98.html
However, I've been informed that the APXS, Pancam, and mini-TES do not have sufficient resolution to see if the phosphorus is concentrated in the dark, smooth inclusions.
But perhaps the mini-TES could measure them if it were placed closer to the target rock than is usually done.
Bob Clark