Here's why:
* Mineral composition directly reflects the chemical makeup of the magma or lava from which the rock formed. Different minerals crystallize at different temperatures and pressures, and their presence indicates the specific conditions under which the rock formed.
* Mineral composition dictates the rock's physical properties. For example, rocks rich in quartz and feldspar are typically lighter in color and harder than rocks rich in olivine and pyroxene.
* Mineral composition provides insight into the rock's origin and evolution. Certain minerals are indicative of specific tectonic settings or magmatic processes.
While other chemical properties like silica content are also important, mineral composition is the primary classification criterion because it is directly linked to the rock's formation and physical characteristics.
