Question

A mountain stream drains an area that is predominately a coarse grained granite. the granite has the basic composition of 15% quartz and 85% feldspars. however, a few miles from the mountains, an inspection of the stream sediments reveal that the composition of the sand is transporting is about 75% quartz and only 25% feldspars. why is there such a difference between the source of the sand (granite) and what is found in the stream that drains the granite mountains?

Answer 1

The quartz is more resistant to abrasion and chemically more resistant than the feldspar.  The result is that the quartz remains in the stream but the feldspar breaks down to clay or is dissolved.

(this is from coursehero.com)