I had thought quartz wasn't a likely example because I thought of quartz as a soft mineral. However, it is listed as a 7 on a Moh's scale of hardness of 1 to 10, with diamond being the hardest at 10.

Quartz can also appear as white outcrops at the surface:

Quartz sandstone (orthoquartzite), Cambrian.
http://www.earth.ox.ac.uk/~oesis/nws/nws-a98-16.html

This though is a type of sedimentary sandstone, so wouldn't work as an non-sedimentary explanation for the hard, white material seen on Mars.

Other examples of white quartz outcrops:

1.) Megalithomania - Slievebawn - County Carlow
http://www.megalithomania.com/show_site.php?site_id=330&new_scheme=original

I don't know the origin of this white quartz.

2.) Pegmatite, Lewisian Gneiss Complex.
http://www.earth.ox.ac.uk/~oesis/nws/nws-s98-18.html

This might be an example of white igneous quartz.

3.) Images of the Sam Knob Hike.
http://www.hikewnc.info/gallery/sknobimg10.html

I don't know if this is sedimentary or not.

Bob Clark