Final answer:
Geologists classify minerals into crystal systems based on observable physical features such as angles and edge lengths of crystals, which differ between the seven crystal systems, along with other properties like grain texture and phenocrysts' presence.
Step-by-step explanation:
Geologists sort out minerals into the crystal systems by examining the physical characteristics that are evident in the structure of the minerals. Each of the seven basic crystal systems has unique geometrical properties in terms of angles between faces and the lengths of edges of the crystal faces, denoted by the parameters a, b, and c for edges, and α (alpha), β (beta), and γ (gamma) for angles.
For example, minerals in the cubic system have equal edge lengths and angles of 90 degrees, making them easy to identify. When examining igneous rocks, geologists look for signs like coarse-grained textures where individual minerals can be discerned by their crystal shapes. A coarse-grained texture indicates that the minerals had the time and space to grow and can often be identified without sophisticated equipment.
Field identification of minerals does not always require high-tech tools. Observable properties such as color, hardness, crystal habit, and the presence of phenocrysts can be indicative of certain minerals. Phenocrysts are larger crystals that are embedded in a finer-grained matrix, providing clues to geologists about the mineral composition.
While these characteristics are insightful, geologists also need to consider the history and formation process of the rocks they are studying. Igneous rocks form from the cooling of magma, which crystallizes into minerals. Sedimentary rocks are formed from weathered pieces of other rocks or the precipitation of minerals, and metamorphic rocks are altered by heat and pressure. These formation processes can influence the growth and orientation of mineral crystals.
Understanding the physical processes, rock types, and crystal systems can significantly aid in the field identification of minerals. Knowledge from introductory geology courses, combined with observable mineral properties, allows geologists to sort minerals into the appropriate crystal systems effectively, even without sophisticated lab equipment.