Thursday 4th October Sol 58

Previous missions suggest that the ‘soil’ on Mars is roughly
basaltic in composition.  However, on
Earth sand is mainly composed of quartz (silica).  That reveals a fundamental difference between
Mars and the Earth.  The Earth has
experienced plate tectonics, leading to crustal melting and the formation of
silica-rich rocks like granites over much of the continental surface.  When that is eroded by rivers and the wind it
leads to silica-rich sand.  Mars has
experienced a different evolution involving the formation of great volcanic
regions like the Tharsis bulge, but these are melts which take their
composition from the underlying mantle rather than from extensive crustal melting. 

Thats the current theory.

However, one of the things we are keeping a look out for in
Gale Crater is any sign that some rocks have different compositions.  That will tell us about the processes,
perhaps unique to Mars, that have altered its crust.