Ferrohexahydrite

Chemical formula: Fe₂+S₆+O₄·₆H₂O

Ferrohexahydrite is a hydrated iron(II) sulfate that forms as a product of weathering of sulfide minerals or volcanic activity.

## Characteristics Ferrohexahydrite is a sulfate mineral, specifically iron(II) sulfate hexahydrate. It typically occurs as efflorescences, coatings, or fibrous and botryoidal aggregates. Rarely, it forms small, poorly developed crystals with a tabular or prismatic habit. It is a secondary mineral, formed by the oxidation of sulfide minerals, such as pyrite or marcasite, under conditions of low humidity. It can also be found as a product of volcanic exhalations (sublimate). ## Physical Properties This mineral is very soft and light. It is characterized by a vitreous or dull luster. It is transparent to translucent. Due to its chemical nature, it is unstable under normal atmospheric conditions, readily absorbing water and transforming into melanterite (FeSO₄·7H₂O) or losing water and decomposing into other, less hydrated iron sulfates. ## Colors and Varieties Ferrohexahydrite ranges in color from light green, greenish-white to blue-green. When exposed to air, its surface may oxidize, leading to the formation of yellowish or brownish coatings of iron(III) hydroxides. ## History and Name The mineral's name refers to its chemical composition – the presence of iron (Latin: *ferrum*) and six molecules of water in its structure (Greek: *hexa* - six, *hydor* - water). It was first described in 1922 by Edgar Theodore Wherry based on material from New Idria, California, USA, although a substance of this composition was known earlier. ## Uses Ferrohexahydrite has no direct industrial application. It is of scientific interest as an indicator of geochemical processes occurring in the weathering zones of sulfide deposits and in mining environments. It is also sought after by collectors specializing in secondary and unstable minerals.

Properties

Mohs hardness: 2
Luster: Vitreous
Streak: White
Density: 2.02
Cleavage: Perfect on {001}
Fracture: Conchoidal
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Ferrohexahydrite can be identified by its characteristic light green or blue-green color, its occurrence as efflorescences and coatings in association with sulfide minerals, and its instability. A key feature is its water solubility and bitter, metallic taste (taste testing is not recommended and is dangerous). It reacts to changes in humidity, which can be observed with the naked eye. ## Distinguishing from Similar Minerals It is most often confused with melanterite (FeSO₄·7H₂O), which contains more water. Melanterite usually has a more intense, blue-green color and often forms larger, better-developed crystals. Differentiation is difficult without chemical or crystallographic analysis. It can also be confused with other secondary sulfates, such as gypsum or epsomite, from which it differs in chemical composition and often color (epsomite is usually white or colorless). ## Crystal Forms Crystals are rare, most often tabular or short-prismatic, grouped into radial or botryoidal aggregates. It typically forms fibrous, crusty, stalactitic aggregates, as well as earthy or powdery coatings.

Geological environment

## Genesis Ferrohexahydrite is a secondary mineral. It forms in the oxidation zones of ore deposits containing iron sulfides (pyrite, marcasite, pyrrhotite), especially in dry climates or in dry parts of mines where evaporation exceeds water supply. It can also crystallize as a sublimate from volcanic gases in fumaroles. ## Mineral Associations It often co-occurs with other sulfates formed under similar conditions, such as melanterite, epsomite, gypsum, halotrichite, copiapite, and also native sulfur. It is found in direct proximity to oxidizing sulfides, mainly pyrite and marcasite. ## Localities Significant occurrences have been reported in many places around the world. Classic localities include: New Idria mine in California (USA), where it was first described; Rammelsberg in the Harz Mountains (Germany); Falun (Sweden); Rio Tinto (Spain); Vulcano island (Italy) as a product of fumarolic activity. In Poland, it is sometimes found in old workings of metal ore mines (e.g., in Lower Silesia) and in hard coal mines.

Rarity

Not very common

For collectors

## Quality Criteria Specimens with well-formed, even if small, crystals are most valued by collectors. Intense, vibrant, blue-green color and aesthetic placement on the matrix enhance its appeal. As it is an unstable mineral, it is crucial that the specimen is "fresh," without signs of decomposition or yellow or brown oxidation. Specimens from classic, historical localities are also highly prized. ## Popular Localities Specimens from historical mines in Germany (Rammelsberg), Sweden (Falun), and Spain (Rio Tinto) are among the most sought after by collectors. Specimens from the USA, especially from the discovery site in California, also have historical value. Fibrous and stalactitic forms from various metal ore mines are also popular.

Care and storage

## Cleaning Ferrohexahydrite is water-soluble, so it must absolutely not be cleaned wet. Any contact with water or even humid air will cause it to dissolve or transform into another mineral (melanterite). Specimens can only be cleaned mechanically, with a very soft-bristled brush, to remove loose dust. ## What to Avoid Above all, moisture and water should be avoided. The mineral is very hygroscopic and readily absorbs moisture from the surroundings, leading to its deliquescence (dissolving in absorbed water) or transformation. It should also be protected from high temperatures, which can cause dehydration. Changes in humidity and temperature are destructive to it. ## Storage Ferrohexahydrite specimens must be stored under controlled, low-humidity conditions. The best solution is an airtight container (e.g., a "perky box") with a moisture-absorbing agent (silica gel) placed inside. The container should be kept at a stable temperature, away from direct sunlight.

Sources

IMA Mineral List (CNMNC)