Forsterite

Chemical formula: Mg₂SiO₄

Forsterite is a magnesium-rich mineral from the olivine group, valued for its well-formed, often transparent crystals with a characteristic green color.

## Characteristics Forsterite is a magnesium silicate and the magnesium-rich end-member of the olivine isomorphic series, the other end of which is iron-rich fayalite (Fe₂SiO₄). Pure forsterite is colorless, however, iron impurities, common in natural conditions, give it characteristic shades of green – from light green, through olive, to yellowish-green. It forms crystals with a tabular or short prismatic habit, often with distinctly striated faces. It also occurs in granular or massive aggregates. It is an important rock-forming mineral, constituting a key component of many igneous rocks (e.g., peridotites, dunites) and metamorphic rocks (dolomitic marbles). ## Physical Properties Forsterite crystals are characterized by a vitreous luster and significant hardness, reaching 7 on the Mohs scale, which makes it a scratch-resistant mineral. It is usually transparent to translucent. Its density is approximately 3.2-3.3 g/cm³, increasing with the iron content in its structure. ## Colors and Varieties The most well-known and valued gemological variety of forsterite is peridot. This name refers to transparent crystals with a beautiful, olive-green or emerald-green color, conditioned by the presence of iron(II) ions. Pure, colorless forsterite is rare in nature. Depending on the amount and oxidation state of impurities, the color can range from yellow, through various shades of green, to brownish. ## History and Name The name forsterite was given in 1824 by the French mineralogist Armand Lévy in honor of the English banker, collector, and mineral dealer Adolarius Jacob Forster (1739-1806). However, this mineral was known and used since antiquity under the name peridot. ## Applications Transparent and beautifully colored forsterite crystals (peridots) are a valued gemstone, used in jewelry making. Due to its high melting point, forsterite also has industrial applications as a refractory material in foundries and the steel industry. It is also an object of scientific interest as a key mineral forming the Earth's upper mantle.

Properties

Mohs hardness: 7
Color: Green, pale yellow or white.
Luster: Vitreous
Streak: White
Density: 3.275
Cleavage: Perfect on {010} imperfect on {100}
Fracture: Conchoidal
Transparency: Transparent,Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Forsterite is most easily recognized by its characteristic olive-green color, vitreous luster, and relatively high hardness (scratches glass). It often occurs as granular aggregates in dark igneous rocks or as single, well-formed crystals in marbles. It does not exhibit fluorescence under UV light. ## Distinguishing from Similar Minerals Forsterite (peridot) is sometimes confused with other green minerals. It is most often distinguished from diopside and epidote by its crystal habit and higher hardness. It may resemble green tourmaline (verdelite), which, however, often exhibits stronger pleochroism and has a different, more columnar crystal habit. It is distinguished from green garnet (demantoid) by its lower refractive index and the lack of the characteristic "fire" of demantoid. ## Crystal Forms It crystallizes in the orthorhombic system. It most often forms short, prismatic or tabular crystals, often flattened. Crystal faces can be striated parallel to the longest axis. It also occurs as rounded grains in rocks (xenoliths) or as granular and massive aggregates.

Geological environment

## Genesis Forsterite is a high-temperature mineral. It forms mainly in magmatic processes, crystallizing from mafic and ultramafic magmas rich in magnesium. It is a major component of rocks such as peridotite, dunite, and gabbro. It also occurs in some basalts. Another important environment for its formation is contact metamorphism of magnesium-contaminated limestones or dolomites, where it forms well-developed crystals in skarns and marbles. It is also found in some meteorites (pallasites), where it forms beautiful aggregates within an iron-nickel matrix. ## Mineral Associations In igneous rocks, forsterite most commonly co-occurs with pyroxene group minerals (diopside, enstatite), chromite, magnetite, and plagioclase. In metamorphic rocks (marbles), it is accompanied by calcite, dolomite, spinel, phlogopite, diopside, and humite. In pallasites, it occurs with kamacite and taenite (iron-nickel alloy). ## Localities The world's most famous deposits of gem-quality forsterite (peridot) are found on the volcanic island of Zabargad (St. John's Island) in the Red Sea, Egypt, exploited since antiquity. Another important historical locality is Mogok in Myanmar (Burma). Currently, beautiful crystals come from the Suppat plateau in the Kohistan region of Pakistan, from the San Carlos Apache Reservation in Arizona (USA), from China, Vietnam, and Norway (near Åheim). In Poland, trace amounts of forsterite have been found in basalts of Lower Silesia.

Rarity

Not very common

For collectors

## Quality Criteria The most highly valued forsterite (peridot) specimens are those with gemstone status. Key criteria include the intensity and hue of the color – a pure, vibrant green without brownish or yellowish tones is most desirable. Equally important are transparency and the absence of eye-visible inclusions or internal fractures. Crystal size matters, with large, clean stones over 5 carats being rare and valuable. For mineralogical specimens, crystal habit, luster, and an attractive association with the host rock (matrix), e.g., with basalt or white marble, are important. ## Market Prices Peridot prices vary. Small, cut stones weighing 1-2 carats can be purchased for tens of dollars per carat. Prices increase with quality (color, clarity) and weight. Large, clean stones over 5 carats can fetch prices of several hundred dollars per carat, and exceptional specimens even more. Well-formed crystals on matrix, especially from classic localities, are also highly valued in the collector's market. ## Popular Localities Collectors and jewelers most seek specimens from historical localities: Zabargad in Egypt and Mogok in Myanmar. Currently, the best in the world are considered to be intensely green, large, and clean crystals from Pakistan (Kohistan region). Peridots from Arizona (USA), which often have a slightly lighter, yellowish-green hue, are also very popular.

Care and storage

## Cleaning Forsterite specimens are best cleaned with lukewarm water with a mild soap and a soft brush, such as a toothbrush. After washing, rinse thoroughly with clean water (preferably distilled) and dry with a soft, lint-free cloth. Cleaning in ultrasonic or steam cleaners is risky and not recommended, as it can lead to cracks, especially in specimens with inclusions. ## What to Avoid Forsterite is sensitive to acids, including hydrochloric and sulfuric acid, which can etch it and dull its surface. Avoid sudden temperature changes (thermal shock), which can cause internal fracturing. Prolonged exposure to strong sunlight is not advisable, although its color is generally stable. ## Storage Due to its hardness (7 on the Mohs scale), forsterite is relatively scratch-resistant, but it should be stored separately from harder minerals such as corundum or diamond. It is best to keep it in separate display boxes or pouches to prevent mutual scratching of specimens. When displaying, protect it from dust and chemical contaminants.