Olivine is a common silicate mineral found deep within the Earth’s subsurface, making up a large portion of the upper mantle. This mineral is a primary component of mafic and ultramafic igneous rocks, such as basalt and peridotite, which are formed from the cooling of magma. Olivine is classified chemically as a magnesium iron silicate, and it is defined by a characteristic color that gives it both its name and scientific distinction.
The Signature Color of Olivine
The name olivine is derived directly from its observed color, which falls into the olive-green to yellowish-green spectrum. This unique hue is a fundamental property geologists use to identify the mineral in rock samples. While the color can vary, ranging to a pale lime green or brownish-green, a green tint is diagnostic of the olivine mineral group.
Olivine crystals often exhibit a vitreous, or glassy, luster when light reflects off their surfaces. The mineral is usually translucent, allowing light to pass through, which contributes to the brightness of its green color. Its appearance is distinctive, and color and a glassy appearance are often the first clues to its identity in the field.
The Chemical Basis for Olivine’s Hue
The color of olivine is intrinsic to its chemical structure, represented by the formula \((\text{Mg, Fe})_{2}\text{SiO}_{4}\). This formula indicates a solid solution series where magnesium (\(\text{Mg}\)) and iron (\(\text{Fe}\)) substitute for one another within the crystal lattice. The two end-members of this series are forsterite (\(\text{Mg}_{2}\text{SiO}_{4}\)), which is magnesium-rich, and fayalite (\(\text{Fe}_{2}\text{SiO}_{4}\)), which is iron-rich.
The green coloration is caused by the presence of ferrous iron (\(\text{Fe}^{2+}\)) substituting for magnesium in the mineral’s octahedral sites. This iron acts as a chromophore, responsible for absorbing specific wavelengths of visible light. The \(\text{Fe}^{2+}\) ions selectively absorb light in the red and yellow parts of the spectrum, allowing the green wavelengths to be transmitted and reflected.
The specific shade of green is directly related to the ratio of iron to magnesium in the crystal structure. Magnesium-rich olivine tends to be a lighter, purer green, while increasing iron content leads to a deeper, more saturated color. When the mineral becomes extremely iron-rich, nearing the fayalite end-member, the color shifts toward a yellowish or brownish-green hue.
Color Variations and Geological Context
While the idealized color of olivine is bright olive green, geological processes cause deviations from this hue. The purity of the green often depends on the depth at which the mineral formed and its exposure to weathering. Olivine crystals found in deep-seated mantle rocks, such as those brought up in volcanic xenoliths, often display the most vibrant, pure green colors.
Olivine exposed to the Earth’s surface is highly susceptible to chemical alteration, particularly oxidation. The ferrous iron (\(\text{Fe}^{2+}\)) in the crystal lattice reacts with oxygen and water, converting to ferric iron (\(\text{Fe}^{3+}\)). This change in the iron’s oxidation state destroys the green chromophore, causing the mineral to take on a reddish-brown color.
This oxidation process forms secondary iron oxide minerals, such as hematite and magnetite, which cause the reddish-brown discoloration. The altered product is often called iddingsite, appearing along the fractures and edges of the original olivine grains. Trace elements other than iron, such as nickel or manganese, can also substitute into the lattice, contributing to slight variations in the green tint.
Peridot The Gemstone Form
Peridot is the name given to the gem-quality variety of olivine, representing the most appealing and transparent manifestation of the mineral’s color. The color of Peridot is its defining characteristic, with the most prized stones exhibiting a vibrant, intense green without brownish or yellowish undertones. The stone’s color is entirely inherent to its chemical composition, making it one of the few gemstones whose color is not caused by trace impurities.
The highest-valued peridot possesses a perfect balance of iron content, which provides rich green saturation without the color darkening or turning too yellow. This desired hue is often described as a lime green or a rich bottle green. Peridot is unique because it occurs in only this one color, though intensity and saturation vary considerably from a pale, almost yellow-green to a deeper olive.
The iron concentration is directly responsible for the depth of the green. However, the most desirable gems typically have minimal iron content compared to the fayalite end-member, which prevents the unattractive brownish mask associated with excessive iron or early signs of oxidation. The color of peridot remains constant in both natural and artificial light, a property that historically led to it being called “the evening emerald.”