What is the primary characteristic of minerals?

The primary characteristic of minerals is their definite chemical composition. This means that a mineral has a specific formula that defines its elemental makeup, which is consistent and predictable. For example, quartz always has the chemical composition of SiO2, indicating that it is composed of silicon and oxygen in a fixed ratio. This characteristic is crucial for identifying and classifying minerals, as it allows geologists to distinguish between different minerals based on their unique chemical structures.

In contrast, other options describe attributes that are not universal to all minerals. Irregular structure is not a defining characteristic since many minerals, like halite or quartz, can have a crystalline structure. Low density varies widely among minerals; some are very dense like galena, while others, such as talc, are much less dense. Flexibility is also not a defining characteristic since most minerals are not flexible but rather brittle. Thus, the definite chemical composition is the most reliable and fundamental feature that sets minerals apart in the field of geology and mineralogy.