Hibbingite
Chemical formula: Fe<sup>2+</sup><sub>2</sub>(OH)<sub>3</sub>Cl
Hibbingite is a rare iron(II) hydroxychloride that rapidly oxidizes and darkens in air, which is its main diagnostic feature.
Description
## Characteristics Hibbingite is a halide mineral, specifically an iron(II) hydroxychloride with the chemical formula γ-Fe₂(OH)₃Cl. When fresh and protected from oxygen, it forms very small, hexagonal crystals in the form of plates or lamellae, as well as granular aggregates. Its most characteristic feature is extreme instability under atmospheric conditions – it rapidly oxidizes in the presence of oxygen, changing its color from its original to a much darker hue. ## Physical Properties The hardness of hibbingite on the Mohs scale is approximately 3.5. It has a vitreous luster, and fresh, unoxidized fragments are transparent to translucent. The mineral's density is about 3.16 g/cm³. It exhibits perfect cleavage in one plane, consistent with the basal plane of the hexagonal crystal. ## Colors and Varieties Freshly exposed hibbingite ranges in color from pale green to greenish-yellow. Due to the rapid oxidation of iron(II) to iron(III), its color changes to brown, and eventually almost black. There are no distinct color varieties or commercial varieties. ## History and Name The mineral was first described in 1991 by E.H. Dahlberg and B. Saini-Eidukat. Its name comes from the city of Hibbing in Minnesota (USA), near which its type locality, the Duluth geological complex, is located. It was discovered there in drill cores from depths where it was protected from the oxidizing effects of the atmosphere. ## Applications Hibbingite has no industrial applications. It is solely an object of scientific and collecting interest, mainly due to its rarity, specific formation conditions, and instability.
Diagnostic features
## Identification The most important diagnostic feature of hibbingite is its rapid color change from pale green to brown or black soon after exposure to air. This observation, combined with its occurrence in a specific environment (e.g., drill cores from iron formations), allows for its preliminary identification. ## Differentiation from similar minerals Hibbingite can be confused with other greenish secondary iron minerals. Key to differentiation is the rate and intensity of the oxidation process, which are exceptionally rapid for hibbingite. Akaganeite, another iron hydroxychloride, is an oxidation product and is immediately yellowish-brown. Other green iron chlorides and sulfates are usually more stable under atmospheric conditions. ## Crystal forms Hibbingite most often occurs as microscopic hexagonal plates and lamellae. It also forms fine-grained, powdery, or massive aggregates in rock fissures and voids.
Geological environment
## Genesis Hibbingite forms under strongly reducing (anoxic) conditions and with elevated chloride ion concentrations. Its typical environment includes deep-seated parts of iron formations and serpentinized peridotites, where it forms as a result of hydrothermal or metamorphic processes involving saline solutions. It is mainly found in drill cores. It has also been identified as a component of some meteorites and a corrosion product of ancient iron artifacts that lay in anoxic, saline sediments. ## Mineral associations This mineral coexists with magnetite, siderite, iowaite, greenalite, quartz, and minerals from the serpentine group. ## Localities The most important hibbingite localities worldwide include the type locality in the Duluth Complex in Minnesota (USA) and occurrences in ultramafic intrusions in the Norilsk region and on the Taymyr Peninsula in Russia. It has also been reported in mines in Ontario (Canada) and as a component of meteorites found in Northwest Africa (NWA).
Rarity
Very rare
Collector aspects
## Quality criteria As a mineral almost never encountered in the collector's market, hibbingite is valued primarily in scientific and institutional collections. The value of a specimen is determined primarily by its "freshness" – that is, the lowest possible degree of oxidation, which translates into the preservation of its original, greenish color. Precise documentation of the locality and the presence of associated minerals confirming the paragenesis are also important. ## Popular localities The most scientifically valuable specimens come from the type locality in the USA (Duluth Complex) and from Russian intrusions in the Norilsk region. However, these materials are almost exclusively available in institutional and research circulation.
Care and storage
## Cleaning The mineral is extremely sensitive to moisture and oxygen, so it must not be cleaned with water or any liquid chemicals. Mechanical cleaning, e.g., with a very delicate brush, is theoretically possible, but due to the microscopic size of the crystals and their fragility, it is usually not practiced or recommended. ## What to avoid Contact of the specimen with air and moisture, which cause its irreversible decomposition (oxidation), must be strictly avoided. Exposure to sunlight and elevated temperatures, which can accelerate degradation processes, is also not advisable. ## Storage Hibbingite specimens require special storage conditions. They should be kept in sealed, airtight containers, preferably in an inert gas atmosphere (e.g., argon) or at least with the use of moisture absorbers (silica gel). The container should be stored in a dark and cool place.