Final answer:
Quartzite is harder and denser than its parent rock because it has undergone metamorphism, resulting in a tightly interlocked crystal structure with less porosity and high durability.
Step-by-step explanation:
Quartzite is very hard and denser than its parent rock due to metamorphism. The process of metamorphism involves the transformation of quartzose sandstones or siltstones under high pressure and temperature conditions, leading to recrystallization and the formation of a denser, interlocking crystal structure.
During metamorphism, the grains of quartz are fused together, resulting in a rock that is both hard and dense. Quartzite has gone through a significant change wherein the original sand grains might still be visible, but are often interlocked with coarser-grained quartz. This means that the mineral content is more tightly packed than in its sedimentary precursor, making the rock harder and less porous. The rock's high silica content, represented by the formula SiO2, contributes to its hardness, which can scratch glass and does not bubble in acid, distinguishing it from carbonate rocks.