You Gotta Know These Rocks and Minerals
- Quartz is a mineral composed of Si O4 tetrahedra linked in a framework with oxygen atoms shared between neighboring tetrahedra to give an overall chemical formula of Si O2 (silicon dioxide). Inclusions of different elements give different varieties of quartz, including amethyst (purple). Quartz is the second-most abundant mineral in Earth’s crust. Quartz is often used in clocks because it exhibits piezoelectricity. Quartz defines hardness 7 on the Mohs scale.
- Feldspars are a group of silicate minerals that are the most abundant minerals in the Earth’s crust. Feldspars 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 is a stable mineral form of calcium carbonate. Less stable forms of calcium carbonate (called polymorphs), such as aragonite, eventually convert to calcite. Calcite is found in the shells of many marine organisms, including plankton, echinoderms, and bivalves. This makes calcite a major constituent of sedimentary rocks, especially limestone. Many optical devices utilize calcite because it exhibits birefringence (double refraction). Calcite defines hardness 3 on the Mohs scale.
- Diamond is a mineral composed of carbon atoms in a “diamond cubic” crystal structure. Among natural materials, diamond has one of the highest thermal conductivities and highest hardnesses; it defines hardness 10 on the Mohs scale. Natural diamonds are almost always found in igneous rocks called kimberlites.
- Corundum is a mineral composed of aluminum oxide. Common gem forms of corundum include rubies (turned red by inclusions of chromium) and sapphires (which can take a variety of colors from inclusions of iron, titanium, or vanadium). Corundum defines hardness 9 on the Mohs scale.
- Micas are a group of silicate minerals known for their perfect basal cleavage, meaning they can easily split into thin, parallel sheets. Common varieties of mica include black biotite and transparent muscovite. Mica is often found in igneous rocks called pegmatites and also serves as the main constituent of the metamorphic rock schist. Micas generally have a Mohs hardness between 3 and 4.
- Basalt is the most common igneous rock—over 90% of volcanic rocks on Earth are basalt! Basalt 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 basalt. Basalt primarily composed of vesicles is known as scoria.
- Granite is a common igneous rock. Granite is felsic, meaning it is primarily composed of feldspar and quartz, and intrusive, meaning it crystallizes and solidifies underground. Granite can experience large-scale weathering, leading to the formation of structures like Yosemite National Park’s Half Dome.
- Limestone is a sedimentary rock primarily composed of calcium carbonate. Limestone often metamorphoses into marble. Because limestone is made from carbonate minerals, it is soluble in acid, and erosion of limestone can form karst topography. Limestone is commonly used in the production of lime and cement and as a pH buffer in soil conditioners.
- Sandstone is 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 sandstone include arkose (sandstones containing over 25% feldspar), wackes (sandstones with a large amount of clay between mineral grains), and arenites (sandstones with a small amount of clay between mineral grains). Fine-grained sandstones make good aquifers because they are porous enough to allow percolation of water but still filter out pollutants.
This article was contributed by NAQT writer Justin Duffy.