Sherwoodite

Chemical formula: Ca₅.₅(AlV₄+V₅+₁₂O₃₉)·₂₈H₂O

Sherwoodite is a rare, hydrated calcium and aluminum vanadate, forming microscopic, dark green to black crystals with a hexagonal outline.

## Characteristics Sherwoodite is a complex, hydrated calcium and aluminum vanadate, occurring as very small, platy crystals. It typically forms rosette-like or spherulitic aggregates, and individual crystals rarely exceed 0.5 mm. Its color ranges from dark, bluish-green to almost black. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties Sherwoodite crystals exhibit a vitreous to dull luster. They are translucent to opaque. The Mohs hardness is approximately 2.5, making it a very soft mineral. Its density is low, approximately 2.33 g/cm³. ## Colors and Varieties This mineral is monocolored, occurring only in shades of dark green, often with a bluish or black tone. No named varieties are distinguished. ## History and Name Sherwoodite was discovered in the Peanut uranium-vanadium mine in Montrose County, Colorado (USA). It was described by Mary E. Thompson, Carl H. Roach, and Robert Meyrowitz in 1958. The mineral is named in honor of Alexander M. Sherwood (1888-1956), a chemist with the United States Geological Survey, for his contributions to the mineralogy of uranium and vanadium ores. ## Uses Sherwoodite has no industrial applications. Its significance is purely scientific and as a collector's item, as a rare and interesting representative of the vanadate group.

Properties

Mohs hardness: 2.5
Luster: Vitreous to Dull
Streak: Greenish gray
Density: 2.33
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Sherwoodite is identified by its characteristic appearance – small, platy crystals with a hexagonal outline, often forming rosette-like aggregates. Key features include its dark green color, occurrence in association with other vanadium minerals on sandstone, and very low hardness. ## Distinguishing from Similar Minerals It can be confused with other dark-colored secondary vanadium minerals, such as montroseite or doloresite. Differentiation usually requires advanced analytical methods (XRD, EDS), although the characteristic form of hexagonal plates is a strong diagnostic indicator. ## Crystal Forms Crystals are thin, platy, with a distinct hexagonal or pseudohexagonal outline. They usually occur as radial, rosette-like, or spherulitic aggregates on the surface of the host rock (sandstone).

Geological environment

## Genesis Sherwoodite is a secondary mineral formed in the oxidation zones of uranium-vanadium deposits located in sandstones. It forms as a result of the weathering of primary vanadium minerals, such as montroseite, under low temperature and pressure conditions. ## Mineral Associations It most often co-occurs with other vanadium minerals, such as doloresite, metadelrioite, duttonite, and with uranium minerals. It is also frequently accompanied by gypsum, quartz, and clay minerals, which constitute the host rock. ## Localities The most important and classic localities for sherwoodite are in the USA, in the Colorado Plateau region, especially in Montrose County (Colorado) and San Juan County (Utah). Outside the USA, its occurrences have been reported in the Mounana uranium mine in Gabon.

Rarity

Very rare

For collectors

## Quality Criteria The most prized sherwoodite specimens are those with well-formed, sharp crystals forming aesthetic, rosette-like aggregates. The contrast of dark green crystals with the lighter host rock (sandstone) significantly enhances the specimen's attractiveness. Due to the microscopic size of the crystals, specimens are primarily evaluated under magnification. The richness and coverage of the matrix by crystals are key factors. ## Popular Localities Specimens from the type locality in the USA, i.e., the Peanut mine in Montrose County, Colorado, are considered the best in the world. High-quality material also comes from other mines in the Uravan Mineral Belt region in Colorado and Utah.

Care and storage

## Cleaning Sherwoodite specimens are extremely delicate and sensitive to water. Wet cleaning should be avoided. Only very careful dust removal using a soft brush or a photographic air blower is permissible. ## What to Avoid Contact with water must be strictly avoided, as it can cause dissolution or damage to the microscopic crystals. The mineral is very soft, susceptible to scratching and mechanical damage. It should be protected from chemicals, high temperatures, and direct sunlight. ## Storage It is recommended to store specimens in closed, stable containers (such as "micromount boxes") to protect them from dust, moisture, and physical damage. Display should be in conditions of controlled, low humidity.

Sources

IMA Mineral List (CNMNC)