Ferrosilite

Chemical formula: Fe<sup>2+</sup><sub>2</sub>Si<sub>2</sub>O<sub>6</sub>

Ferrosilite is an iron-rich pyroxene, forming a series with enstatite, found mainly in meteorites and some terrestrial rocks.

## Characteristics Ferrosilite is a mineral from the pyroxene group, belonging to the orthorhombic pyroxene solid solution series, whose other, magnesium-rich end-member is enstatite. Pure ferrosilite is rare in nature, and most specimens represent an intermediate member between these two minerals. It forms prismatic or tabular crystals, but most often occurs as granular aggregates or rock masses. Its presence is an important indicator of the physicochemical conditions under which the rock formed. ## Physical properties Ferrosilite is characterized by a hardness ranging from 5.5-6.5 on the Mohs scale. It has a vitreous to pearly luster. It is a relatively dense mineral, with a specific gravity of approximately 3.9-4.0 g/cm³. It exhibits distinct cleavage in two directions, intersecting at an angle close to 90 degrees, which is typical for pyroxenes. ## Colors and varieties It usually ranges in color from greenish-brown, through dark green, to almost black, depending on the iron content and impurities. Pure, synthetic ferrosilite is light green. No named varieties are distinguished, and classification is based on chemical composition within the enstatite-ferrosilite series. ## History and name The name "ferrosilite" directly refers to its chemical composition – a high content of iron (Latin: *ferrum*) and silicon (Latin: *silex*). The mineral was first described in 1935 by Henry Washington. For a long time, it was believed that pure ferrosilite does not occur in nature and is stable only under high pressure; however, its occurrence in terrestrial rocks was later confirmed. ## Uses Ferrosilite has no direct industrial application. Its significance is primarily scientific, as a rock-forming and indicator mineral in petrology. It is also an object of interest for collectors specializing in rock-forming minerals and mineralogical systematics.

Properties

Mohs hardness
5.5-6.5
Luster
Vitreous, Pearly
Streak
Gray, Greenish-gray
Density
3.9-4.0
Cleavage
Good on {210}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification A key diagnostic feature of ferrosilite, as with other orthopyroxenes, is the presence of two cleavage directions intersecting at an angle of approximately 87-88°. Its dark, greenish-brown to black color, vitreous luster on fracture surfaces, and relatively high density aid in identification. It often occurs as grains embedded in rock. ## Distinguishing from similar minerals Ferrosilite can be confused with other pyroxenes (e.g., augite) or amphiboles (e.g., hornblende). It is distinguished from augite (a clinopyroxene) by a slightly different cleavage angle (for augite, closer to 90°). It is distinguished from hornblende by its cleavage angle – in amphiboles, it is approximately 120°/60°, which is often visible in crystal cross-sections. Definitive differentiation from other orthopyroxenes (like hypersthene) requires chemical analysis. ## Crystal forms Ferrosilite crystals are rare. They usually have a prismatic, columnar, or tabular habit. Most often, however, it occurs as irregular grains, granular aggregates, or as a significant component of rock masses.

Geological environment

## Genesis Ferrosilite crystallizes from iron-rich and silica-poor magmas under reducing conditions. It is a component of some igneous rocks, such as gabbros, norites, and anorthosites. It also forms as a result of high-grade metamorphism in iron-rich rocks, such as banded iron formations (BIFs), creating eulysites. It is also an important component of some meteorites (chondrites, achondrites). ## Mineral associations On Earth, it most commonly co-occurs with other pyroxenes (hedenbergite, augite), plagioclase (especially calcium-rich varieties), olivine (fayalite), ilmenite, magnetite, and garnets (almandine). In metamorphic rocks, it may be accompanied by quartz. ## Localities Significant occurrences of ferrosilite in terrestrial rocks are rare. It has been identified, among others, in the Bushveld Igneous Complex in South Africa, the Skaergaard intrusion in Greenland, Mont-Saint-Hilaire in Canada, and the Obsidian Cliffs area in Oregon, USA. It also occurs in rocks from the Moon and Mars, and in numerous meteorites.

Rarity

Rare

For collectors

## Quality criteria For collectors, well-formed, freestanding ferrosilite crystals are most desirable, though they are extremely rare. Rock fragments rich in ferrosilite with clearly visible mineral grains also have collector value, especially if they come from classic or rare localities. Extraterrestrial specimens (in meteorites) are highly prized, but their value primarily stems from the provenance of the entire object. ## Popular localities The most valued specimens by specialized collectors come from classic petrological localities, such as Greenland (Skaergaard intrusion) or Canada (Mont Saint-Hilaire), where better-formed varieties of this mineral have been found.

Care and storage

## Cleaning Ferrosilite specimens can be cleaned with a soft, dry brush to remove dust. If necessary, distilled water may be used, followed by gentle drying. It typically does not require intensive cleaning. ## What to avoid Avoid contact with strong acids, which can damage the mineral's surface. Ferrosilite is susceptible to chemical weathering and oxidation, which can lead to a brownish color change and the formation of secondary iron minerals. It is not particularly sensitive to light, but prolonged exposure to moisture can accelerate oxidation processes. ## Storage Store in dry conditions to minimize the risk of oxidation. It is best kept in closed boxes or display cases, away from specimens that may shed dust or react chemically. Due to its cleavage, it should be protected from impacts.

External references

Sources

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