Here's a breakdown:

1. Sandstone: This sedimentary rock is primarily composed of quartz grains, which are cemented together by minerals like silica or calcite.

2. Metamorphism: When sandstone is subjected to intense heat and pressure, often associated with tectonic activity, it undergoes metamorphism.

3. Recrystallization: The heat and pressure cause the quartz grains to recrystallize, forming a new, tightly interlocking texture. The original cementing material also recrystallizes, binding the grains even more tightly.

4. Quartzite: The result of this process is quartzite, a hard, non-foliated metamorphic rock that is mainly composed of quartz.

It's important to note that quartzite often retains some of the original features of the sandstone, such as layering or fossils, but these are often obscured by the intense recrystallization.