Australia has recently published high resolution surface mineral maps using data from ASTER see here. Does anyone know of similar free products available world-wide?
In particular you may be interested in the Mineral Resources Data System (MRDS).
In addition to interactive maps, they also offer a list of API web-services.
Just a couple of the services:
In addition to Phil's answer I have found two other resources.
GLiM is a 0.5 degree grid of surface rock properties. The categories here are very broad,
AD Alluvial deposits
DS Dune sands
There is also OneGeology which is a collection of geological surveys from different countries that can be viewed in a single portal. The drawback of OneGeology is that there is no single file format with consistent fields and scale etc. It is just a collection of separate resources.