Quartzite is either a metamorphic rock or a sedimentary rock. (The
sedimentary kind of quartzite is described with sand and sandstone on p.
86.) Metamorphic quartzite is made up mostly of quartz. It forms when
heat and fluids below the earth’s surface cause the grains and cement of
a quartz sandstone to recrystallize. When this happens, the grains
interlock and are no longer held together by cement. Metamorphic
quartzite, like sedimentary quartzite, is a hard, firm rock that breaks
through the quartz grains instead of between them.
Ancient Precambrian metamorphic quartzite occurs at the surface in the
Llano uplift area of central Texas, in the Van Horn area of west Texas,
and in the Franklin Mountains north of El Paso in extreme west Texas.