Final answer:
Geologists primarily use chemical composition to classify minerals into different classes. This is because chemical composition is a defining and consistent characteristic, unlike color, hardness, or cleavage, which may vary.
Step-by-step explanation:
The main answer to what characteristic geologists use to separate minerals into classes is D) Chemical composition. While properties like color, hardness, and cleavage can aid identification, they can be variable within a mineral and are not reliable for classification. Chemical composition, on the other hand, defines the specific structure and make-up of a mineral. Minerals with the same chemical composition can be grouped into the same class regardless of variations in other properties such as color, which may vary due to impurities, or hardness that is measured on Mohs' scale.For example, quartz can come in various colors, and while elements like amphibole and pyroxene may share physical properties, their cleavage is different. However, it is their chemical composition that ultimately classifies them. A mineral's chemical formula, such as halite's NaCl or olivine's (Mg,Fe)2SiO4, reveals its consistence or variations, indicating which class it belongs to. This property provides the most definitive classification of minerals and is used extensively by geologists, especially when field conditions do not allow for complex laboratory analyses.