Igneous & Metamorphic Rocks: Quick Reference

Classification framework: intrusive vs extrusive; mafic vs silicic (felsic)
Color index: percent of dark mafic minerals; used with color-index circles to estimate composition
Reading composition: estimate mafic percentage, then map onto a line/chart to decide if rock is silicic or mafic
Minerals (simplified):
- Silicic minerals: quartz, potassium feldspar (K-feldspar), plagioclase
- Mafic minerals: olivine, pyroxene, amphibole
Silicic vs felsic (definitions):
- Silicic: silica-rich mineral content; not limited to igneous rocks
- Felsic: feldspar-rich; in igneous rocks usually corresponds to silicic rocks
Granite-style composition (qualitative): common assemblage includes quartz, K-feldspar, and plagioclase
Common rock names by composition (intrusive/extrusive):
- Granite (felsic intrusive)
- Diorite (intermediate intrusive)
- Gabbro (mafic intrusive)
- Andesite (intermediate extrusive)
- Basalt (mafic extrusive)
Ultramafic rocks: typically green due to olivine; olivine is olive-green; gemstone peridot
Komatiite: ultramafic extrusive (rare today; samples are scarce)
Texture overview (igneous):
- Pegmatitic: ultra-coarse grains; intrusive; very slow cooling
- Phaneritic: coarse-grained; crystals visible to the eye
- Porphyritic: large phenocrysts in a finer groundmass
- Pyroclastic/Fragmental: erupted rock fragments cemented by ash; volcanic origin
Quick tip: ultramafic rocks and high-olivine content are strong indicators of mantle-derived compositions

When identifying minerals, silicic minerals appear as quartz and feldspars; mafic minerals include olivine and pyroxenes
For granite-like rocks, expect substantial quartz and feldspars; mineral proportions help distinguish granite, diorite, and gabbro
Graphs showing silicic vs mafic composition can be flipped in arrangement; focus on the relative placement on the chart to classify
Mineral-color cues: quartz is clear/white; K-feldspar often pinkish; olivine green

Textures: foliated vs nonfoliated
Foliation: alignment of minerals due to directed pressure; appears as layers or planes
Fissile: rocks that split into sheets (e.g., slate)
Schistosity: schist-like, mica-rich, shiny appearance (schist)
Common foliated rocks: slate, phyllite, schist, gneiss
Nonfoliated rocks: quartzite (from quartz sandstone), marble (from limestone)
Protoliths (parent rocks): slate/phyllite/schist/gneiss from shale/mud rock; granite gneiss from granite; amphibolite from basalt; quartzite from sandstone; marble from limestone
Metamorphic grades and environment (conceptual): higher grade generally implies greater metamorphic intensity/temperature and pressure; refer to a grade chart for temperatures and associated protoliths

Two identical samples per table to facilitate individual work and discussion prompts
After ID, answer guided questions to think about geologic histories and environments
Sedimentary rocks are not the focus today, but igneous and metamorphic rocks reveal environments and geological history
Use rock names as clues to past conditions (e.g., granite indicates intrusive intrusive activity in a region)
If you’re unsure, focus on textures (pegmatitic, phaneritic, porphyritic, foliated vs nonfoliated) and key mineral assemblages