|Chemical Composition||(Fe,Mg)2SiO4), Iron – Magnesium Silicate|
|Color||Pale olive green to yellow-green, occasionally brown|
|Cleavage||Poor cleavage in two directions at 90o|
|Hardness||6.5 to 7 (very hard)|
|Specific Gravity||3.2 (Mg-rich variety) to 4.3 (Iron-rich variety) (average weight)|
|Luster||Massive or transparent to translucent crystals with a glassy luster|
Most misidentifications of olivine occur as a result of a common tendency to identify all green minerals as olivine. Although olivine is certainly one of the most common green minerals, greenish hues is not limited to this group of rock forming minerals. Olivine is most easily identified by a combination of its color, hardness and close association with mafic igneous rocks.
Varieties of apatite occur that mimic olivine’s green color, but apatite is much softer than olivine (only 5 on Mohs hardness scale). Unlike olivine, it can easily be scratched by a nail and will not scratch a glass plate.
Tourmaline also occurs in green hues, but is typically found in quartz-rich granite pegmatites, where olivine cannot occur.
Chlorite has green varieties, but is very soft compared to olivine (only 2 to 2.5 on Mohs hardness scale) and can be easily scratched by a fingernail. Crystals of chlorite also exhibit the perfect cleavage planes typical of the mica minerals like muscovite and biotite.
Green forms of epidote may be distinguished from olivine by their association with metamorphic rocks. If present in crystal form, epidote forms elongated crystals with one perfect cleavage that is quite distinct from olivine’s typical granular crystal habit. Transparent to translucent epidote crystals are also strongly pleochroic, which means that their color changes (from green to brown) as they are rotated in light.
True emerald, a deep green variety of the mineral beryl, can be confused with peridot as both are green gemstones. However, true emerald is both harder than peridotite and has a deeper green color. When dealing with jewelry or gems it is obviously better to distinguish the two by color, as scratching peridot to test its hardness will greatly reduce its worth!
Serpentine is a name given to both a mineral and a rock, so don’t be confused by its dual use. The mineral serpentine commonly forms from the alteration of the olivine minerals, as water is incorporated into the minerals’ crystal structure. The rock serpentine forms from the metamorphism of olivine-rich ultramafic igneous rocks. As serpentine is a green mineral, serpentine rock typically has a dark green color that makes it a prized building stone. Hardness can be used to differentiate the mineral serpentine from olivine. Olivine is harder than glass and cannot be scratched by a nail, while serpentine only has a hardness of 2.5 on the Mohs hardness scale and is easily scratched by a nail.
Did you know...
One of the ancient Egyptians’ favorite jewels, the green gemstone peridot may be olivine’s most significant contribution to society. Olivine is actually a mineral series, not a single mineral. In a mineral series two or more elements can substitute for one another without changing the crystal structure. In olivine, iron (Fe) and magnesium (Mg) do this, so the Olivine Series has two end members. Favalite is the iron-rich version, while forsterite is the magnesium-rich version, although any combination of magnesium and iron is possible. Minerals in the Olivine Series are important rock-forming minerals in mafic igneous rocks such as basalt, gabbro and peridotite.
Description and Identifying Characteristics
A distinctive olive-green color, translucent luster and hardness (greater than glass) are usually adequate to distinguish olivine from other minerals. Although olivine is an important component of mafic igneous rocks, there really is no single mineral called olivine. Instead, olivine is a minerals series in which iron and magnesium substitute for one another in the same crystal structure. Fayalite and forsterite, respectively the iron-rich and magnesium-rich end members of the series, are difficult to distinguish from one another and the proportions of iron and magnesium can even vary through a single crystal, so for simplicity’s sake, these minerals are collectively called ‘olivine’. Fayalite was named after Fayal (or Faial), an island in the Azores that lies 1500 km west of Portugal close to the Mid-Atlantic Ridge, while fosterite was named after Johann Forster, a German naturalist who accompanied Captain James Cook on his voyages to the Pacific islands. Hence both names indirectly reflect olivine’s abundance in mafic igneous rocks of volcanic islands.
Olivine usually only occurs as part of a rock mass, but when it does form recognizable crystals these are transparent to translucent, olive-green crystals that typically have a granular shape. Minerals in the Olivine Series are quite hard, falling from 6.5 to 7.0 on the Mohs hardness scale.
The term ‘olivine’ is also used for a group of related minerals that all share similar crystal structures and the same general chemical composition of X2SiO4, where X can stand for iron (Fe), magnesium (Mg), calcium (Ca), manganese (Mn) or nickel (Ni). Most of these elements will substitute for one another, so there are several series in the greater Olivine Group of which fayalite and forsterite are the most common members. Olivine minerals primarily occur in mafic igneous rocks, but the calcium-bearing members also occur in marbles that formed from the metamorphism of dolomite or dolomitic limestone.
In Our Earth: The Geologic Importance of Olivine
Named for its olive green color, the mineral olivine is almost exclusively found in igneous rocks (intrusive or volcanic) that are associated with divergent plate boundaries or oceanic hot spots. The olivine minerals are important rock-forming minerals in mafic igneous rocks such as basalt and gabbro, and many peridotite rocks are almost entirely composed of olivine. Olivine also occurs as well-developed crystal masses in xenoliths found in volcanic rocks. These masses originated as fragments of peridotite mantle rock that were brought to the surface during volcanic activity. Fayalite, the iron-rich variety, is also found as inclusions of gem-like crystal masses in many iron-nickel meteorites. Its abundance in these meteorites ranges from only being a trace mineral to making up more than 50% of the meteorites’ volume. Forsterite, the magnesium-rich variety can be found in marble rock formed from the metamorphism of dolomitic limestone.
Olivine does not naturally occur with the mineral quartz. Quartz can only form from magmas that are relative rich in silica, while the olivine minerals only form from magmas that are relatively lean in silica, so quartz and olivine are incompatible minerals. The main minerals associated with olivine are from the pyroxene and plagioclase mineral groups, as well as the mineral serpentine, which forms from olivine as it is metamorphosed and altered.
In Our Society: The Economic Importance of Olivine
Olivine’s best known economic use is the gemstone peridot (pronounced pair-a-doe), often used as a birthstone for August. Peridot has a pale yellow-green color and is sometimes called evening emerald or even mistakenly as chrysolite in the jewelry trade. Although relatively soft for a gemstone, peridot’s beauty allows it to compete with harder gemstones like true emerald. Gem quality peridot is closer to forsterite in composition, and some of the highest quality gems historically came from the Red Sea island of Zagbargad, off the coast of Egypt. These gems were mentioned by Pliny in 1500 B.C. and were mined for nearly four thousand years, although for several centuries knowledge of the mines’ location was lost until they were rediscovered early in the twentieth century. Peridot (‘pitdah’ in Hebrew) is mentioned in the Bible and figured prominently in ancient records. According to legend, peridot was one of Cleopatra’s favorite gemstones.
Apart from its gem form of peridot, the olivine minerals have limited economic value. Olivine minerals are used in industry as abrasives and as refractory sand in steel manufacturing. Mafic igneous rocks that contain olivine may be used as decorative building stones. During metamorphism, olivine is altered to serpentine, which gives the metamorphic rock of the same name its dark green color that makes serpentine rock a valued decorative stone. As with many building stones, serpentine is marketed under a variety of geologically incorrect names, like serpentine marble or verde antigua marble as its color pattern is similar to some true marbles.
A relative lack of economic uses, however, should not diminish olivine’s geologic importance. The primary minerals in ultramafic rocks, olivine minerals may be the most abundant minerals in the Earth as they form the majority of the Earth’s mantle rock. These olivine minerals also have an indirect economic value as they tend to break down (weather) very easily at the Earth’s surface. The iron and magnesium released by this weathering process helps to make volcanic soils very fertile.
Olivine in the Upper Midwest:
No crystal masses of olivine occur in the Minnesota region, but olivine minerals are important components of the basalt and gabbro rock units that make up the North Shore of Lake Superior. Outcrops of these rocks also extend along the St. Croix River Valley, from the Lake Superior area down to Interstate Park on the Minnesota-Wisconsin border.