Nickelhexahydrite

Chemical formula: Ni²⁺S⁶⁺O₄·6H₂O

Nickelhexahydrite is a hydrated nickel sulfate, forming characteristic blue-green efflorescences and crusts in the oxidation zones of sulfide deposits.

## Characteristics Nickelhexahydrite is a mineral from the hexahydrite group, a hydrated nickel sulfate. It most commonly occurs as efflorescences, crusts, coatings, and earthy or fibrous aggregates. It rarely forms small tabular or prismatic crystals. Its appearance is often inconspicuous, resembling other secondary sulfates. ## Physical Properties It is a very soft mineral, with a hardness of 2 on the Mohs scale, meaning it can be scratched with a fingernail. It has a relatively low density, approximately 2.04 g/cm³. It is characterized by a vitreous luster and is translucent. It is soluble in water. ## Colors and Varieties Nickelhexahydrite ranges in color from blue-green to very light blue, which is typical for hydrated nickel compounds. Its color is one of the key diagnostic features. No named varieties are distinguished. ## History and Name The mineral's name refers to its chemical composition – the presence of nickel (element) and six water molecules in its structure (Greek *hexa* - six, *hydor* - water). It was first described in 1965 based on samples from the Severniy mine in the Norilsk region of Russia, which is its type locality. ## Uses Nickelhexahydrite has no industrial significance. However, it is an interesting mineral for collectors specializing in secondary minerals, mineralogical systematics, and nickel minerals.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
White
Density
2.036
Cleavage
on {010} less perfect on {100}
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Nickelhexahydrite is most easily identified by its characteristic blue-green color, low hardness (2 on the Mohs scale), and occurrence as coatings and efflorescences on rocks rich in nickel sulfides. Its solubility in water is also a diagnostic feature. ## Distinguishing from Similar Minerals It can be confused with other secondary nickel sulfates, such as morenosite (NiSO₄·7H₂O) or retgersite (NiSO₄·6H₂O, tetragonal). Morenosite contains more water and is unstable in dry air, transforming into nickelhexahydrite. Retgersite has the same chemical composition but crystallizes in a different system and is much rarer. It is distinguished from copper sulfates (e.g., chalcanthite) by its more greenish hue. ## Crystal Forms It most often forms earthy, fibrous aggregates, crusts, and efflorescences. Small, tabular or short-prismatic crystals, often gathered in radial aggregates, are rare.

Geological environment

## Genesis It is a typical secondary mineral, formed as a result of weathering (oxidation) of primary nickel sulfides, mainly millerite and pentlandite. It forms under low temperature and humidity conditions in the oxidation zones (iron caps) of ore deposits. It can also form as an evaporation product of mine waters or as a dehydration product of more hydrated morenosite. ## Mineral Associations It co-occurs with other secondary sulfates, such as morenosite, epsomite, melanterite, gypsum, as well as with primary nickel and iron sulfides, such as millerite, pentlandite, and pyrite. ## Localities Important localities worldwide include the type locality in Russia (Severniy mine, Norilsk region), as well as nickel deposits in Sudbury (Ontario, Canada), South Africa (Bushveld region), Australia, and various locations in the USA (e.g., Pennsylvania and Missouri).

Rarity

Not very common

For collectors

## Quality Criteria For collectors, specimens with well-formed, even if small, crystals are the most valuable. Rich, intensely colored coatings and crusts on a contrasting rock matrix are also prized. A precisely defined locality is important, especially if it is a rare occurrence for this mineral. ## Popular Localities Specimens from classic localities, such as the Norilsk region in Russia or Sudbury in Canada, are sought after by collectors specializing in ore minerals and their weathering zones.

Care and storage

## Cleaning Nickelhexahydrite specimens are very delicate and should be handled with care. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove dust. Contact with water, which can dissolve the mineral, should be avoided. ## What to Avoid The mineral is water-soluble and very sensitive to moisture. It is absolutely essential to avoid storing it in humid conditions, as it may disintegrate or transform into the more hydrated morenosite. Protect it from high temperatures, which can cause dehydration. Avoid all chemicals. ## Storage Specimens should be stored in a dry place, preferably in airtight containers or display cases with a desiccant (e.g., silica gel). Display should be in enclosed cabinets, away from direct sunlight and heat sources.

External references

Sources

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