Rocks and Minerals

Quartz

a mineral composed of SiO₄ tetrahedra linked in a framework with oxygen atoms shared between neighboring tetrahedra to give an overall chemical formula of SiO₂ (silicon dioxide). Inclusions of different elements give different varieties, including amethyst (purple). This is the second-most abundant mineral in Earth’s crust. This is often used in clocks because it exhibits piezoelectricity. This defines hardness 7 on the Mohs scale.

Feldspars

a group of silicate minerals that are the most abundant minerals in the Earth’s crust. These are categorized based on whether they contain potassium (orthoclase) or sodium and calcium (plagioclase). The continuous branch of Bowen’s reaction series covers the transition from calcium-rich to sodium-rich plagioclases. Orthoclase defines hardness 6 on the Mohs scale.

Calcite

a stable mineral form of calcium carbonate. Less stable forms of calcium carbonate (called polymorphs), such as aragonite, eventually convert to this. This is found in the shells of many marine organisms, including plankton, echinoderms, and bivalves. This makes this a major constituent of sedimentary rocks, especially limestone. Many optical devices utilize this because it exhibits birefringence (double refraction). This defines hardness 3 on the Mohs scale.

Diamond

a mineral composed of carbon atoms in a “[namesake] cubic” crystal structure. Among natural materials, this has one of the highest thermal conductivities and highest hardnesses; it defines hardness 10 on the Mohs scale. Natural ones of this are almost always found in igneous rocks called kimberlites.

Corundum

a mineral composed of aluminum oxide. Common gem forms of this include rubies (turned red by inclusions of chromium) and sapphires (which can take a variety of colors from inclusions of iron, titanium, or vanadium). This defines hardness 9 on the Mohs scale.

Micas

a group of silicate minerals known for their perfect basal cleavage, meaning they can easily split into thin, parallel sheets. Common varieties include black biotite and transparent muscovite. This is often found in igneous rocks called pegmatites and also serves as the main constituent of the metamorphic rock schist. These generally have a Mohs hardness between 3 and 4.

Basalt

the most common igneous rock—over 90% of volcanic rocks on Earth! This is mafic, meaning it is rich in magnesium and iron, and extrusive, meaning it cools and solidifies on the Earth’s surface. Lava containing gas bubbles may cool into vesicular [this]. When primarily composed of vesicles, this is known as scoria.

Granite

a common igneous rock. This is felsic, meaning it is primarily composed of feldspar and quartz, and intrusive, meaning it crystallizes and solidifies underground. This can experience large-scale weathering, leading to the formation of structures like Yosemite National Park’s Half Dome.

Limestone

a sedimentary rock primarily composed of calcium carbonate. This often metamorphoses into marble. Because this is made from carbonate minerals, it is soluble in acid, and erosion of this can form karst topography. This is commonly used in the production of lime and cement and as a pH buffer in soil conditioners.

Sandstone

a clastic sedimentary rock, which means it is composed of pre-existing fragments of other rocks and minerals (usually quartz and feldspar). Common subgroups of this include arkose (containing over 25% feldspar), wackes (with a large amount of clay between mineral grains), and arenites (with a small amount of clay between mineral grains). Fine-grained ones make good aquifers because they are porous enough to allow percolation of water but still filter out pollutants.