Cassidyite

Chemical formula: Ca₂Ni²⁺(PO₄)₂·2H₂O

Cassidyite is a rare, hydrated calcium nickel phosphate, forming tiny, spherulitic aggregates with a characteristic apple-green color.

## Characteristics Cassidyite is a hydrated calcium nickel phosphate belonging to the fairfieldite group. It is extremely rare, most often forming very small, radially fibrous aggregates and spherulites (spherical clusters) not exceeding 0.5 mm in size. Due to the size of its crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties This mineral is characterized by a Mohs hardness of approximately 3.5. It has a vitreous luster and is transparent to translucent. Its calculated density is 3.25 g/cm³. It is a relatively brittle mineral. ## Colors and Varieties Cassidyite occurs in a uniform, characteristic shade from light green to apple-green. No color or commercial varieties have been distinguished. ## History and Name The mineral's name honors William A. Cassidy (born 1928), an American meteoriticist from the University of Pittsburgh, who collected samples containing this mineral. Cassidyite was described and approved as a new mineral by the International Mineralogical Association (IMA) in 1966. The discovery was made in the Wolf Creek meteorite found in Western Australia. ## Uses Cassidyite has no industrial application. Its significance is purely scientific and collectible; however, due to its extreme rarity and microscopic size, it is a coveted object for specialized collectors of rare minerals and micromounts.

Properties

Mohs hardness
3.5
Color
Pale green to bright green.
Luster
Vitreous
Streak
White
Density
0
Cleavage
Perfect on {010}, Good on {110}
Fracture
Uneven
Transparency
Translucent
Crystal system
Triclinic

Diagnostic features

## Identification Identification of cassidyite under amateur conditions is practically impossible. It requires advanced analytical methods such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS). A preliminary indication may be its apple-green color, spherulitic aggregate form, and specific occurrence environment (weathered iron meteorites). ## Distinguishing from Similar Minerals It can be confused with other secondary, green nickel phosphates, such as annabergite or lipscombite. Annabergite, however, usually forms acicular crystals and has a different shade of green. Lipscombite is typically darker, blue-green to black. Definitive differentiation is only possible through laboratory analysis. ## Crystal Forms Cassidyite forms spherical aggregates (spherulites) composed of very fine, radially arranged crystals with a platy or bladed habit. Individual, well-formed crystals are extremely rare and microscopic in size.

Geological environment

## Genesis Cassidyite is a secondary mineral that forms as a result of the weathering of iron meteorites (octahedrites). It forms in desert conditions through the reaction of phosphorus (derived from the mineral schreibersite, (Fe,Ni)₃P) with nickel (from the metallic part of the meteorite) and calcium (from surrounding rocks or dust). ## Mineral Associations This mineral co-occurs with other meteorite weathering products. It is most often associated with goethite, maghemite, akaganeite, jarosite, reevesite, as well as relics of primary meteoritic minerals such as schreibersite and troilite. ## Localities Its occurrence has been confirmed in only a few locations worldwide, always in weathered fragments of iron meteorites. The main localities are: - Wolf Creek Crater, Western Australia (type locality) - Mundrabilla meteorite, Western Australia - Campo del Cielo crater, Argentina - Henbury crater, Northern Territory, Australia

Rarity

Extremely rare

For collectors

## Quality Criteria Due to the microscopic nature of the mineral, the main quality criterion is the abundance and size of spherulitic aggregates on the matrix. Specimens where the green cassidyite spheres are numerous, well-formed, and contrast with a darker matrix (e.g., goethite) are most valued. Aggregate sizes exceeding 0.5 mm are considered exceptional. ## Popular Localities The most classic and sought-after specimens by collectors come from the type locality - Wolf Creek Crater in Australia. Material from Campo del Cielo in Argentina has also yielded collector-quality specimens.

Care and storage

## Cleaning Cassidyite specimens, due to their microscopic size and brittleness, generally should not be mechanically cleaned. If absolutely necessary, compressed air (from a safe distance) can be used to remove dust. Contact with water and any chemicals should be avoided. ## What to Avoid Contact with acids, which can dissolve it, must be strictly avoided. The mineral is sensitive to high temperatures, which can lead to its dehydration and structural damage. It should be protected from moisture and sudden temperature changes. ## Storage Cassidyite specimens, typically in the form of micromounts, should be stored under stable conditions in closed, padded "micromount boxes." Protect from dust, vibrations, and direct sunlight.

External references

Sources

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