Rocks and Minerals

A rock is a natural solid made of one or more minerals or mineral-like materials, and a mineral is a naturally occurring, inorganic solid with a definite chemical composition and an ordered atomic structure. Geologists sort rocks by how they formed, into igneous, sedimentary and metamorphic, and sort minerals by their chemistry. This hub explains those schemes and links every specimen in the CRV Science Geology Guide.

🪨 Geology Guide: Browse all 124 rocks and minerals in the CRV Science Rock & Mineral ID Guide, filter them by subgroup, and test your identification skills in LithoClade.

No rock is permanent. Magma cools into igneous rock; weathering and erosion break rock into sediment, which is buried and cemented into sedimentary rock; and heat and pressure transform either into metamorphic rock, which can melt to make new magma. The cycle is driven by two engines. Heat from the interior moves plates and melts rock, as described in plate tectonics, and the climate and the water cycle at the surface weather rock and move sediment, as described in silicate weathering.

Igneous rocks form from molten rock. Two properties do most of the sorting: how fast the melt cooled, which sets crystal size, and how much silica it contained, which sets the minerals. Slow cooling underground gives coarse, visible crystals (plutonic, or intrusive, rocks), while rapid cooling at the surface gives fine grains or glass (volcanic, or extrusive, rocks). Silica content runs from felsic (rich in silica, light colored) through intermediate and mafic to ultramafic (poor in silica, rich in iron and magnesium). The Geology Guide groups them into seven subgroups.

• Felsic plutonic (granitoids): Granite, Pegmatite, Granodiorite, Aplite
• Felsic volcanic and glassy: Rhyolite, Obsidian, Pumice, Dacite
• Intermediate: Diorite, Andesite, Syenite, Trachyte
• Mafic plutonic (gabbroids): Gabbro, Diabase, Anorthosite, Norite
• Mafic volcanic (basaltoids): Basalt, Scoria, Pillow Basalt, Pahoehoe Basalt
• Ultramafic and mantle: Peridotite, Dunite, Komatiite, Kimberlite
• Pyroclastic (tephra and tuffs): Welded Tuff, Volcanic Breccia, Volcanic Bomb, Lapilli Tuff

Sedimentary rocks form at the surface. Clastic rocks are made of grains of older rock, sorted by size on the Wentworth scale: gravel (larger than 2 millimeters), sand (0.0625 to 2 millimeters) and mud (smaller than 0.0625 millimeters). Chemical and biogenic rocks form by precipitation from water or by the accumulation of the remains of organisms. The Geology Guide groups them into six subgroups.

• Coarse clastic (rudites): Conglomerate, Sedimentary Breccia, Tillite, Puddingstone
• Medium clastic (sandstones): Quartz Arenite, Arkose, Greywacke, Glauconitic Greensand
• Fine clastic (mudrocks and shales): Black Shale, Siltstone, Mudstone, Oil Shale
• Carbonates (limestones and dolostone): Fossiliferous Limestone, Coquina, Chalk, Oolitic Limestone, Travertine, Dolostone
• Evaporites (saline precipitates): Rock Salt, Rock Gypsum, Anhydrite, Trona
• Siliceous, iron and organic: Chert, Banded Iron Formation, Diatomite, Bituminous Coal, Lignite

Metamorphic rocks are existing rocks recrystallized in the solid state by heat, pressure or chemically active fluids. When directed pressure aligns platy minerals, the rock is foliated, and foliation coarsens with rising grade from slate to phyllite, schist and gneiss, ending in migmatite where partial melting begins. Non-foliated rocks such as marble, quartzite and hornfels form where heat or the chemistry of the starting rock leaves no preferred direction. Subduction zones make the high-pressure blueschists and eclogites, and hot water and fault movement make serpentinite, soapstone and mylonite. The Geology Guide groups them into six subgroups.

• Low-grade foliated (slate and phyllite): Slate, Phyllite, Chlorite Green Slate, Spotted Slate
• Medium-grade foliated (schists): Garnet Muscovite Schist, Biotite Quartz Schist, Chlorite Actinolite Schist, Staurolite Mica Schist
• High-grade foliated (gneiss and migmatite): Banded Orthogneiss, Augen Gneiss, Migmatite, Granulite
• Non-foliated (granoblastic and contact): Quartzite, Marble, Hornfels, Skarn, Anthracite
• High-pressure subduction: Eclogite, Blueschist, Jadeitite, Whiteschist
• Hydrothermal and fault zone: Serpentinite, Soapstone, Mylonite, Suevite

Minerals are sorted by their dominant anion or anion group, an idea that underlies both the Dana and Strunz classification systems. The Geology Guide uses eight classes: the silicates (built from silicon-oxygen tetrahedra and making up most of the crust), carbonates, oxides and hydroxides, sulfides, sulfates, halides, phosphates and vanadates, and native elements.

• Silicates: Quartz, Orthoclase, Labradorite, Muscovite, Olivine, Almandine Garnet, Beryl, Tourmaline, Kyanite
• Carbonates: Calcite, Aragonite, Dolomite, Malachite, Azurite, Rhodochrosite
• Oxides and hydroxides: Hematite, Magnetite, Corundum, Rutile, Cassiterite
• Sulfides: Pyrite, Galena, Chalcopyrite, Sphalerite, Cinnabar, Stibnite
• Sulfates: Gypsum, Barite, Celestite, Chalcanthite
• Halides: Fluorite, Halite, Sylvite, Atacamite
• Phosphates and vanadates: Apatite, Turquoise, Pyromorphite, Vanadinite
• Native elements: Native Gold, Native Copper, Native Sulfur, Diamond, Graphite, Native Bismuth

Rocks are the record of Earth's history and the link between its interior and surface. They show where oceans and ice sheets were, how hot the mantle was, and how carbon moved between air, ocean and rock, as in the carbonate-silicate cycle. For the larger picture see geology, plate tectonics and the lithosphere.

• Geology
• Plate tectonics
• Lithosphere