Metahaiweeite

Chemical formula: Ca(U⁶⁺O₂)₂Si₆O₁₅·nH₂O

Metahaiweeite is a rare, secondary uranyl mineral, forming tiny, yellow crystals and coatings in the oxidation zones of uranium deposits.

## Characteristics Metahaiweeite is a hydrated uranyl calcium silicate, typically occurring as very small, acicular or bladed crystals. It forms radial, rosette-like aggregates, as well as dense, felted aggregates and coatings. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. This mineral exhibits weak to moderate fluorescence under ultraviolet light. ## Physical Properties Metahaiweeite crystals are brittle. Hardness on the Mohs scale is undetermined, as is density, which has not been precisely measured due to the small size of the crystals and possible inclusions. Luster is described as vitreous to silky, especially for fibrous aggregates. It is transparent to translucent. ## Colors and Varieties This mineral occurs in various shades of yellow – from pale yellow, through canary yellow, to yellowish-green. No distinct color varieties or commercial varieties are recognized. ## History and Name The name "metahaiweeite" refers to its relationship with the mineral haiweeite through dehydration (a reversible process). It was described as a new mineral in 1959 by T.C. McBurney and J. Murdoch. The name "haiweeite" comes from the Haiwee Reservoir in California, near which the type locality (Coso Mine, formerly Haiwee) is located. The prefix "meta-" indicates a lower state of hydration relative to haiweeite.

Properties

Mohs hardness: 3.5
Color: pale yellow to greenish yellow
Luster: Vitreous
Streak: Pale yellow
Density: 3.35
Cleavage: {100}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Metahaiweeite is primarily identified by its form – tiny, acicular crystals forming radial aggregates or felted coatings of a characteristic yellow color. It occurs in a typical environment for secondary uranium minerals. A key feature is its weak to moderate, greenish fluorescence under UV light. However, final identification requires advanced methods such as X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with many other secondary uranyl minerals, such as uranophane, beta-uranophane, boltwoodite, or haiweeite. Distinguishing it from haiweeite is impossible without laboratory analysis, as they differ only in their degree of hydration. It differs from other minerals in this group by subtle color shades, crystal form, and specific mineral associations, but reliable differentiation in the field is very difficult. ## Crystal Forms Crystals are very small, acicular or bladed, often flattened. They typically form radial or stellate aggregates, as well as dense, felted or fibrous masses and coatings on host rocks.

Geological environment

## Genesis Metahaiweeite is a secondary mineral, forming in the oxidation zones of uranium deposits. It is created by the weathering and alteration of primary uranium minerals, such as uraninite, in the presence of silica-rich waters. It is a product of haiweeite dehydration under low humidity conditions. ## Mineral Associations It co-occurs with other secondary uranium minerals, such as haiweeite (with which it is directly related), uranophane, boltwoodite, as well as with opal (hyalite) and chalcedony. The primary mineral in its environment is usually uraninite. ## Localities The most important and best-documented occurrences of metahaiweeite are in the USA, including the type locality in the Coso Mine (Haiwee) area in Inyo County, California. It is also known from other localities in California, Nevada, and Utah. Outside the USA, its occurrence has been reported in small quantities in Argentina (Mendoza province) and Mexico (Peña Blanca area, Chihuahua).

Rarity

Rare

For collectors

## Quality Criteria For collectors, the most desirable specimens are those with clearly formed, radial aggregates of tiny, acicular crystals of an intense, canary-yellow color. Contrast with dark host rock enhances visual appeal. Due to the microscopic size of the crystals, specimens suitable for viewing under magnification (so-called microminerals) are the standard, and the quality of these micro-aggregates determines their value. ## Popular Localities The most classic and prized specimens come from the type locality in California (Coso / Haiwee Mine). Specimens from this region are considered reference material for this mineral species.

Care and storage

## Cleaning Metahaiweeite specimens are extremely delicate and brittle. Mechanical cleaning is not recommended. If absolutely necessary, compressed air (from a safe distance) can be used to remove dust. Avoid contact with water, which can alter the mineral's hydration state. ## What to Avoid As a secondary mineral, metahaiweeite is sensitive to changes in humidity and temperature. It should be stored away from humid places or areas with fluctuating temperatures, which could cause further dehydration or rehydration. It should be protected from chemicals and acids. As a uranium mineral, it is radioactive and should be handled with appropriate precautions (avoid inhaling dust, wash hands after contact). ## Storage It is recommended to store specimens in closed, stable containers (e.g., "perky box" type) to protect them from mechanical damage, dust, and sudden changes in humidity. It should be stored away from radiation-sensitive minerals.

External references

https://www.mindat.org/min-2663.html