Wilancookite

Chemical formula: (Ba₅Li₂&#₉₇₂₃;)Ba₆Be₂₄P₂₄O₉₆·₂₆H₂O

Wilancookite is an extremely rare mineral from the beryllophosphate group, forming colorless, tabular crystals with a vitreous luster.

## Characteristics Wilancookite is a hydrated barium lithium beryllophosphate belonging to the pahasapaite structural group. This mineral forms very small, colorless, tabular crystals, reaching up to 0.2 mm in size. The crystals are transparent and exhibit a vitreous luster. Due to its extreme rarity and small crystal size, it is a mineral of purely scientific and collector interest. ## Physical Properties Wilancookite crystals exhibit a hardness of approximately 4 on the Mohs scale. They are brittle, and their density, calculated based on the chemical formula and unit cell parameters, is 2.91 g/cm³. The mineral does not show fluorescence under ultraviolet light. ## History and Name The mineral is named in honor of William "Bil" Anson Cook Jr. (born 1947), an American mineral collector from Newry, Maine, who specializes in pegmatite minerals. Wilancookite was approved as a new mineral species by the International Mineralogical Association (IMA) in 2023 (IMA2023-092). The discovery was made in material from the only known locality worldwide.

Properties

Mohs hardness: 4-5
Color: Colourless
Luster: Vitreous
Streak: White
Density: 3.05
Cleavage: Perfect on {0001}
Fracture: Brittle
Transparency: Transparent
Crystal system: Isometric

Diagnostic features

## Identification Identification of Wilancookite is impossible without advanced analytical techniques, such as Raman spectroscopy or chemical analysis using an electron microprobe (EDS/WDS). Visually, it is indistinguishable from many other colorless secondary minerals found in granitic pegmatites. ## Differentiation from Similar Minerals It can be confused with other microscopic beryllophosphates, such as pahasapaite, to which it is structurally related. Definitive differentiation requires chemical analysis, especially to confirm the presence of barium and lithium in specific proportions. ## Crystal Forms Wilancookite forms thin, tabular crystals with a hexagonal outline, flattened parallel to {001}. It occurs as single, isolated crystals on the surface of other minerals.

Geological environment

## Genesis Wilancookite is a secondary mineral that crystallizes in the late stage of hydrothermal processes in complex granitic pegmatites rich in phosphorus, beryllium, and lithium. It forms in rock cavities and fissures as a result of the alteration of earlier minerals. ## Mineral Associations This mineral coexists with quartz, microcline, eosphorite, zanazziite, wardite, whitmoreite, jahnsite-(CaMnFe), and other rare phosphates. ## Localities The only confirmed occurrence of Wilancookite in the world (type locality) is the Palermo No. 1 quarry in Groton, Grafton County, New Hampshire, USA.

Rarity

Extremely rare

For collectors

## Quality Criteria As a "micromount" mineral, the quality of a specimen is primarily determined by the degree of crystal development and sharpness of edges, their transparency, and aesthetic placement on the rock matrix. Due to its extreme rarity, every analytically confirmed specimen is extremely valuable to specialized collectors. ## Popular Localities The only source of specimens is historical material from the Palermo No. 1 quarry in New Hampshire, USA. New acquisition of material is practically impossible, making this mineral one of the rarest on the collector's market.

Care and storage

## Cleaning Due to its extreme rarity, brittleness, and small crystal size, mechanical cleaning is not recommended. Specimens should be protected from dust in specialized containers. ## What to Avoid Avoid contact with chemicals, ultrasonics, and extreme temperature changes. The mineral is brittle, so it should be handled with utmost care, avoiding shocks and pressure. ## Storage Wilancookite specimens should be stored under stable conditions, preferably in sealed, padded "micromount" boxes, to protect them from mechanical damage and dust. Avoid exposure to moisture and direct sunlight, although there is no data on its light sensitivity.