Here's why:

* Quartzite is a metamorphic rock, meaning it's formed from the transformation of an existing rock (the protolith) under heat and pressure.

* Sandstone is a sedimentary rock composed primarily of quartz grains.

* When sandstone is subjected to intense heat and pressure, the quartz grains recrystallize and become tightly interlocked, forming the hard, dense, and often glassy texture characteristic of quartzite.

So, the process is:

1. Sandstone (protolith) is formed from the deposition and cementation of quartz grains.

2. Metamorphism (heat and pressure) causes the quartz grains in sandstone to recrystallize and interlock.

3. Quartzite is formed.