The dark (or ferromagnesian) silicates are minerals containing ions of iron (ferro 5 iron) and/or magnesium in their structure. Because of their iron content, ferromagnesian silicates are dark in color and have a greater specific gravity, between 3.2 and 3.6, than nonferromagnesian silicates. The most common dark silicate minerals are olivine, the pyroxenes, the amphiboles, dark mica (biotite), and garnet.
Olivine, a family of high-temperature silicate minerals, is black to olive green in color and has a glassy luster and a conchoidal fracture.
Transparent olivine is occasionally used as a gemstone called peridot. Rather than develop large crystals, olivine commonly forms small, rounded crystals that give olivine- rich rocks a granular appearance (Figure 1). Olivine and related forms are typically found in basalt, a common igneous rock of the oceanic crust and volcanic areas on the continents, and are thought to constitute up to 50 percent of Earth’s upper man tle.
The pyroxenes are a group of diverse minerals that are important components in dark-colored igneous rocks. The most common member, augite, is a black, opaque mineral with two directions of cleavage that meet at nearly a 90-degree angle. Augite is one of the dominant minerals in basalt (Figure 2A).
The most common member of a chemically complex group of minerals called amphiboles (Figure 2B) is hornblende. Hornblende is usually dark green to black in color, and except for its cleavage angles, which are about 60 degrees and 120 degrees, it is very similar in appearance to augite. In a rock, hornblende often forms elongated crystals. This helps distinguish it from pyroxene, which forms rather blocky crystals. Hornblende is found in igneous rocks, where it often makes up the dark portion of an otherwise light-colored rock.
Biotite is a dark, iron-rich member of the micafamily. Like other micas, biotite possesses a sheet structure that gives it excellent cleavage in one direction. Biotite also has a shiny black appearance that helps distinguish it from the other dark ferromagnesian minerals. Like hornblende, biotite is a common constituent of igneous rocks, including the rock granite.
Garnet is similar to olivine in that its structure is composed of individual tetrahedra linked by metallic ions. Also like olivine, garnet has a glassy luster, lacks cleavage, and exhibits conchoidal fracture. Although the colors of garnet are varied, this mineral is most often brown to deep red.
Well-developed garnet crystals have 12 diamondshaped faces and are most commonly found in metamorphic rocks (Figure 3). Transparent garnets are prized as semiprecious gemstones.