Here's why:

* Chemical Formula: Minerals have a specific chemical formula that defines the elements present and their proportions. For example, quartz is always SiO2 (silicon dioxide).

* Consistency: This chemical composition remains consistent within a specific mineral type. While there may be minor variations due to impurities, the fundamental chemical structure remains constant.

* Exceptions: There are a few exceptions, such as some minerals that exhibit solid solutions, where the chemical composition can vary within certain limits. However, these variations are still predictable and follow specific rules.

Therefore, a definite chemical composition is a fundamental defining characteristic of minerals.