Occurs in Pinal County. Common in Arizona.
Chemical Formula: (Mg,Fe)2SiO4
Specific Gravity: 3.2-4.4
The Peridot of Earth’s Mantle
Olivine is a silicate mineral and one of the most abundant components of the Earth’s mantle. Its unique greenish-yellow hue has earned it the nickname “peridot” in its gemstone form. Beyond its role as a gem, olivine plays a crucial role in understanding the Earth’s geology, its formation, and its behavior under various conditions.
Geological Origins of Olivine
Olivine is a common mineral in the Earth’s mantle, the layer of the planet below the crust we inhabit. It forms under high-temperature and high-pressure conditions. It is a magnesium iron silicate, represented by the chemical formula (Mg,Fe)2SiO4. Olivine is found in igneous rocks, such as basalt and gabbro, which are formed from the cooling and solidification of magma.
Olivine can be found in various geological settings across Arizona. One notable occurrence is associated with ultramafic rocks, such as peridotite. These ultramafic rocks are remnants of ancient volcanic or mantle-derived processes. Olivine’s characteristic green color and gem-quality transparent varieties, known as peridot, have attracted attention from mineral enthusiasts and even the gemstone industry, making Arizona a significant location for olivine exploration and appreciation.
Peridot Mesa on the San Carlos Apache Reservation, Arizona is a renowned locality for gem-quality peridot. The mesa’s geological setting provides ideal conditions for the formation of this vibrant green gemstone, attracting collectors and researchers. Current collecting has been prohibited by the tribal government.
Mineralogical Composition and Crystal Structure
Olivine’s chemical composition consists of magnesium (Mg) and iron (Fe) cations, combined with silicon (Si) and oxygen (O) anions in a 2:1 ratio. The varying proportions of magnesium and iron give rise to different compositions of olivine, such as forsterite (Mg2SiO4) and fayalite (Fe2SiO4), with intermediate compositions known as “hydrous olivine” containing trace amounts of water.
The crystal structure of olivine belongs to the orthorhombic system. Its primary building block is a SiO4 tetrahedron, with one silicon atom surrounded by four oxygen atoms. These tetrahedra are linked together through the sharing of oxygen atoms, forming a three-dimensional framework.
Olivine in its gemstone form is known as peridot. Peridot is a transparent, greenish-yellow to olive-green gemstone, valued for its vivid color and exceptional brilliance. The intensity of its color is determined by the proportion of iron in the crystal structure. Peridots with higher iron content exhibit a more intense green hue.
Peridot is a magnesium-rich variety of olivine, and its gem quality is relatively rare compared to other gemstones. It has been treasured for centuries, with historical sources indicating its use in jewelry as early as ancient Egypt.
Occurrences of Olivine
Olivine is a widespread mineral found in many rock types. It is abundant in mafic and ultramafic igneous rocks, such as basalt, gabbro, and peridotite. Peridotite, a rock composed mainly of olivine and pyroxene, is one of the dominant rock types in the Earth’s upper mantle.
Olivine is also found in some metamorphic rocks, where the original mineral assemblages have been altered by heat and pressure. However, its presence is more common in igneous rocks due to its formation under specific conditions in the Earth’s mantle.
Unique Features of Olivine
Olivine possesses several unique features that set it apart from other minerals
Olivine is relatively susceptible to weathering when exposed to water and air. The alteration of olivine leads to the formation of secondary minerals, such as serpentine, which contributes to soil formation.
High Melting Point
Significance in Earth’s Geology
Xenoliths of mantle rocks, including olivine-rich peridotites, are brought to the surface during volcanic eruptions, providing geologists with valuable insights into the composition and processes of the Earth’s mantle.
Earthquakes and Seismic Activity
The presence of olivine in the mantle has implications for seismic activity and the study of earthquakes, as it affects the propagation of seismic waves. This is especially important in California, where one of olivine’s derivatives, serpentine, is abundant.
Industrial and Scientific Uses
Olivine has several industrial and scientific uses.
Olivine is used in the production of refractory materials, which can withstand high temperatures and are used in furnaces and kilns.
Olivine is added to blast furnaces in steelmaking to help remove impurities from the molten metal.