Starkeyite

Chemical formula: MgS⁶⁺O₄·4H₂O

Starkeyite is a hydrated magnesium sulfate, occurring as white, earthy coatings and crusts in arid climates.

## Characteristics Starkeyite is a mineral from the sulfate group; chemically, it is magnesium sulfate tetrahydrate. It most commonly occurs as dusty or earthy coatings, as well as crusts and efflorescences on rock or soil surfaces. It rarely forms crystals visible to the naked eye, which appear as tiny, acicular or fibrous forms. It is a brittle mineral, and its aggregates are usually soft and easily crumbled. ## Physical Properties Starkeyite crystals, though rare, exhibit a vitreous to silky luster. The mineral is transparent to translucent. Its hardness on the Mohs scale is low, making it very delicate. Its density is slightly greater than gypsum, approximately 2.03 g/cm³. ## Colors and Varieties Starkeyite is usually white, colorless, or grayish. Due to its porous nature and occurrence as coatings, it can be easily stained other colors by admixtures and impurities, e.g., iron oxides. ## History and Name The mineral was named after John Starkey (1897-1964), a chemist at The New Mexico Potash Company, who first synthesized its equivalent in the laboratory. It was described as a new mineral in 1956 by Oliver R. Grawe. The type locality is a potash mine in Eddy County, New Mexico, USA. ## Uses Starkeyite has no direct industrial application. It is of scientific interest as a product of evaporation and weathering in magnesium sulfate-rich environments. It is also sought after by collectors specializing in rare minerals or minerals from specific localities.

Properties

Mohs hardness
2-3
Color
White to very pale yellow or pale greenish white.
Luster
Dull
Streak
White
Density
2
Cleavage
Perfect on {110}
Fracture
Irregular/Uneven
Transparency
Translucent,Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Starkeyite most often forms white, dusty or earthy coatings and crusts. A key diagnostic feature is its solubility in water. In field conditions, it can be identified based on its occurrence (dry, desert climates, mines) and appearance. It tastes bitter and salty, but taste testing is not a recommended method for mineral identification. ## Distinguishing from Similar Minerals Starkeyite can be confused with other white, soluble sulfates such as epsomite (MgSO₄·7H₂O), hexahydrite (MgSO₄·6H₂O), or leonhardite (a dehydrated form of laumontite). Distinguishing them without specialized tests (e.g., X-ray diffraction) is very difficult, as they often co-occur and transform into one another depending on ambient humidity. They differ mainly in their degree of hydration. ## Crystal Forms Well-formed crystals are extremely rare. If they occur, they take the form of tiny, acicular or fibrous crystals, often forming radial or tangled aggregates. It is usually found as earthy, dusty aggregates and crusts.

Geological environment

## Genesis Starkeyite is a secondary mineral, formed by the evaporation of magnesium sulfate-rich waters. It forms in arid and desert climates as efflorescences on soil and rock surfaces. It also forms as a product of dehydration of more hydrated magnesium sulfates, such as epsomite, under low humidity conditions. It is found in drying salt lakes, playa deposits, and as a weathering product of magnesium-rich igneous and sedimentary rocks. ## Mineral Associations This mineral often co-occurs with other soluble sulfates and chlorides. Typical associated minerals include epsomite, hexahydrite, mirabilite, thenardite, gypsum, halite, and sylvite. ## Localities The most important starkeyite localities are in arid regions of the world. It was identified at its type locality in Eddy County, New Mexico (USA). It also occurs in other arid regions of the United States (e.g., Nevada), the Atacama Desert in Chile, Bolivia, Egypt, and in volcanic lake deposits, e.g., in Iceland.

Rarity

Rare

For collectors

## Quality Criteria The quality of collector specimens of starkeyite is primarily assessed based on the presence of visible crystals, which is very rare. Specimens with well-formed, acicular crystals forming visually attractive aggregates are most valued. The purity of the specimen and the absence of undesirable impurities are also important. Due to its instability, it is crucial that the specimen is well-preserved and shows no signs of dissolution or recrystallization. ## Popular Localities Specimens from the type locality in New Mexico (USA) have historical value. Good quality crystalline aggregates also come from some arid regions of Chile and Bolivia.

Care and storage

## Cleaning Starkeyite specimens are very sensitive to water and should only be dry-cleaned. A soft brush or compressed air from a safe distance can be used to remove dust. ## What to Avoid Contact with water and other liquids must be strictly avoided, as the mineral is soluble and will be destroyed. It must be protected from high humidity, which can cause deliquescence or transformation into other, more hydrated magnesium sulfates. Ultrasonic and chemical cleaning should be avoided. ## Storage Starkeyite requires storage in dry conditions. It is best kept in a sealed container (e.g., a "membrane box") with a desiccant (e.g., silica gel). It should be protected from mechanical damage due to its low hardness and brittleness.

External references

Sources

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