Earth Materials: Minerals, Rocks, and the Rock Cycle
Domain 3 examines minerals, the three rock classes, and the rock cycle, including how texture and composition record origin.
Core rules and concepts
- A mineral is naturally occurring, inorganic, solid, with an ordered atomic structure and a definite but variable composition.
- Diagnostic properties include hardness (Mohs scale), cleavage versus fracture, luster, streak, density, and reaction to acid. Color alone is unreliable.
- Silicate minerals dominate crustal rocks; Bowen’s reaction series links cooling history to mineral assemblage.
- Igneous rocks are classified by texture (aphanitic, phaneritic, porphyritic, vesicular, glassy) and composition (felsic to ultramafic).
- Sedimentary rocks are clastic, chemical, or organic; grain size and sorting reveal transport energy.
- Metamorphic rocks recrystallize under heat and pressure; foliation and index minerals indicate grade. The rock cycle is recycling, not a one-way ladder.
Worked example
A specimen scratches glass, is scratched by quartz, has two cleavages at nearly 90°, and is salmon pink. Hardness is between 5.5 and 7, consistent with potassium feldspar (about 6). The right-angle cleavages and color support feldspar over quartz, which has no cleavage and is harder. The absence of fizzing with dilute acid argues against calcite. Identification uses multiple properties, not one.
Traps that cost marks
Do not identify a rock from color alone. Crystalline does not automatically mean igneous; metamorphic and chemical sedimentary rocks can be crystalline. Foliation is a metamorphic alignment, not sedimentary bedding. The rock cycle does not require every rock to pass through every stage.