Mackinawite

Chemical formula: Fe<sup>₂+</sup>S<sup>₂-</sup>

Mackinawite is an iron sulfide with a characteristic bronze-metallic luster, forming small, scaly aggregates in sedimentary and hydrothermal rocks.

## Characteristics Mackinawite is an iron(II) sulfide that typically occurs as very fine, scaly or lamellar aggregates. It rarely forms macroscopic crystals, and its presence is often confirmed by microscopic analysis. This mineral has a characteristic metallic luster and a brownish or grayish hue. Due to its microcrystalline nature, its visual features are best seen on fresh fracture surfaces, which quickly tarnish upon exposure to air. ## Physical Properties Mackinawite is a very soft mineral, with a hardness of only 2.5 on the Mohs scale, meaning it can be scratched with a fingernail. It has a metallic luster and is opaque. Its density is approximately 4.17 g/cm³. It is a brittle mineral, and its aggregates are easily crushed. ## Colors and Varieties The typical color of mackinawite is brownish, bronze, grayish-white, or grayish-yellow. On fresh surfaces, it may exhibit a pinkish tint. This mineral does not form color varieties or commercial varieties; its identification is primarily based on chemical composition and crystal structure. ## History and Name The name mackinawite comes from the Mackinaw mine in Snohomish County, Washington (USA), where the mineral was first described in 1963 by Howard T. Evans Jr. and colleagues. It was identified during studies of copper and nickel ores. ## Uses Mackinawite has no direct industrial application. However, it is of scientific interest, particularly in geochemistry and microbiology, due to its role in anaerobic biogeochemical processes, such as iron corrosion and the formation of sulfide deposits.

Properties

Mohs hardness: 2.5
Color: Bronzy
Luster: Metallic
Streak: Black
Density: 0
Cleavage: Perfect on {001}.
Fracture: Uneven
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Mackinawite can be identified by its low hardness (2.5 on the Mohs scale), characteristic brownish metallic luster, and tendency to occur as fine, scaly aggregates. Its rapid tarnishing in air is also a diagnostic feature. However, definitive identification requires advanced methods such as X-ray diffraction (XRD) or reflected light microscopy. ## Distinguishing from Similar Minerals Mackinawite is sometimes confused with other sulfides, such as pyrrhotite, chalcopyrite, or pentlandite. It is distinguished from them by its significantly lower hardness, tetragonal crystal structure, and characteristic mode of occurrence. Unlike pyrrhotite, mackinawite is not magnetic. ## Crystal Forms Mackinawite crystallizes in the tetragonal system. It most often forms microscopic, scaly or lamellar crystals, which arrange into irregular aggregates or coatings. It is rarely observed as tabular crystals with a square outline, reaching sizes of up to several millimeters.

Geological environment

## Genesis Mackinawite is a low-temperature mineral. It forms under reducing (anaerobic) conditions, often with the involvement of microorganisms (sulfate-reducing bacteria). It forms in bottom sediments of lakes and seas, in clays, and as a product of early diagenesis. It can also crystallize in low-temperature hydrothermal systems, especially in copper, nickel, and iron sulfide deposits. ## Mineral Associations Mackinawite often co-occurs with other sulfides, such as greigite, pyrite, marcasite, chalcopyrite, cubanite, pentlandite, and pyrrhotite. In sedimentary environments, it is associated with clay minerals and carbonates. ## Localities Key mackinawite localities include the Mackinaw mine in Washington (USA), where it was discovered. It also occurs in nickel deposits in Sudbury (Canada), in the Bushveld Complex (South Africa), in Outokumpu deposits (Finland), and in many modern and ancient marine and lacustrine sediments worldwide.

Rarity

Rare

For collectors

## Quality Criteria The quality of mackinawite specimens is difficult to assess in the traditional sense, as it rarely forms well-developed crystals visible to the naked eye. Most desirable are specimens showing distinct, even if small, square crystals or rich aggregates of lamellar aggregates with a strong metallic luster. Due to its instability, it is crucial that the specimen is fresh and unaltered. ## Popular Localities Specimens, though rarely available on the collector's market, mainly come from classic localities such as the nickel-copper deposits in Sudbury (Ontario, Canada) or the Bushveld Complex (South Africa). Microscopic crystals are also found in clay geodes in the Crimean Peninsula region.

Care and storage

## Cleaning Mackinawite specimens are extremely delicate and prone to oxidation. Mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used to remove loose dust. Avoid contact with water and chemicals. ## What to Avoid Mackinawite is very sensitive to oxygen and moisture, under the influence of which it quickly tarnishes and transforms into other minerals, such as pyrite, marcasite, or iron oxides. It should be protected from contact with air, water, acids, and other chemicals. It should not be heated. ## Storage The best method for storing mackinawite is to keep it in airtight, oxygen-free containers, such as "perky boxes" with a seal or in an inert gas atmosphere (e.g., argon). Exposure should be minimized to prevent its degradation.