Is Rock A Mixture?

Yes rocks are mixture of different elements and compound.
It may made up of clay slit contain sand particle and some other elements depending upon environment and location.
 

A rock is almost always a mixture of elements.  It's possible in rare cases, to find rocks that are almost pure, such as a iron rock (most likely a meteor, which would be a lucky find), or a gold rock (which would be a very lucky find!)