Mountkeithite
Chemical formula: (Mg₁-xFe³⁺x)(S⁶⁺O₄)x/₂(OH)₂·nH₂O (x 3x/2)
Mountkeithite is a rare, hydrated magnesium and iron sulfate, forming flexible, fibrous aggregates with a pearly luster.
Properties
- Mohs hardness
- 2
- Color
- Pale pink, white
- Luster
- Pearly
- Streak
- White
- Density
- 2.12
- Cleavage
- {0001}
- Fracture
- Fibrous
- Transparency
- Translucent
- Crystal system
- Hexagonal
Diagnostic features
## Identification Mountkeithite is relatively easy to identify based on its unique form. It forms characteristic, asbestos-like aggregates of long, flexible, and pliable fibers. Its pearly luster, low hardness, and reaction to water (solubility) are key diagnostic features. ## Distinguishing from Similar Minerals At first glance, it may be confused with some fibrous minerals, such as chrysotile (serpentine asbestos) or tremolite (amphibole asbestos). Mountkeithite differs from them by its significantly lower hardness (it is very soft to the touch), the flexibility of its fibers, and its solubility in water, which asbestos minerals do not exhibit. ## Crystal Forms This mineral exclusively forms fibrous aggregates (asbestos-like habit). The fibers are elongated along the crystallographic c-axis and gathered into dense, felted, or parallel-arranged aggregates.
Geological environment
## Genesis Mountkeithite is a secondary mineral, forming in the weathering zones of nickel sulfide deposits. It forms as a result of the alteration of magnesium-rich ultramafic rocks (e.g., dunites) under the influence of sulfate-bearing waters. It is a product of the alteration of other magnesium minerals under specific chemical conditions. ## Mineral Associations It most commonly co-occurs with minerals typical of the oxidation zones of nickel deposits. These include hexahydrite, epsomite, morenosite, goethite, as well as relics of primary sulfides like pentlandite and pyrite. ## Localities The most important and classic occurrence is the Mount Keith nickel mine in Western Australia, where the mineral was discovered. This is the primary and practically sole locality from which well-formed collector specimens originate.
Rarity
Very rare
For collectors
## Quality Criteria The most prized mountkeithite specimens are those consisting of long, clean, undamaged fibers with a distinct pearly or silky luster. The size and richness of the aggregate, as well as the absence of impurities, are important. Specimens with a well-preserved, "fluffy" fibrous structure are more highly valued than those that are compact or broken. ## Popular Localities The only source of valuable collector specimens is the type locality – the Mount Keith mine in Western Australia. Practically all specimens available on the market originate from this single location.
Care and storage
## Cleaning Mountkeithite specimens are extremely delicate and sensitive to water. They should not be washed in water, as they may dissolve or be damaged. To remove dust, a very soft, dry brush can be used, or dust can be carefully blown off with compressed air from a distance. ## What to Avoid Contact with water and all chemicals must be absolutely avoided. The mineral is sensitive to moisture, so it should be stored in a dry environment. It should also be protected from high temperatures and mechanical damage due to its low hardness and delicate structure. ## Storage It is recommended to store specimens in sealed, airtight containers (so-called "membrane boxes" or lidded boxes) away from sources of moisture. It is best to place a desiccant, such as silica gel, in the container. Display should be in a closed display case, under stable temperature and humidity conditions.