Mundrabillaite

Chemical formula: (N³⁻H₄)₂Ca(PO₃OH)₂·H₂O

Mundrabillite is a very rare, hydrated ammonium calcium phosphate, found in caves with bat guano.

## Characteristics Mundrabillite is an extremely rare mineral from the phosphate group. It forms colorless to white, bladed or tabular crystals, which usually occur as radial aggregates or rosettes. Individual crystals are very small, rarely exceeding 1 mm in length. Due to its origin and composition, it is an unstable mineral sensitive to environmental conditions. ## Physical Properties Mundrabillite crystals are characterized by a vitreous luster and are transparent. This mineral is very soft, with a Mohs hardness of only 1. It is also light, with a density close to 1.98 g/cm³. It exhibits perfect cleavage in one direction. ## Colors and Varieties This mineral is usually colorless or white. No colored or commercial varieties have been identified. ## History and Name The name mundrabillite comes from its discovery location – Petrogale Cave on the Nullarbor Plain, near the settlement of Mundrabilla in Western Australia. It was described and approved as a new mineral by the International Mineralogical Association (IMA) in 1977. Its co-discoverers were Peter J. Bridge and A. W. G. Lind. ## Applications Mundrabillite has no industrial applications. It is solely an object of scientific interest and a valuable and rare specimen for advanced systematic mineral collections and those from rare localities.

Properties

Mohs hardness
1-2
Color
Colourless
Luster
Vitreous
Streak
white
Density
2.05
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Characteristic features of mundrabillite include its exceptional softness (Mohs hardness of 1), bladed crystal habit forming radial aggregates, and specific occurrence environment (caves with guano). It reacts with water, which is a key diagnostic feature, but also destructive to the sample. ## Distinguishing from Similar Minerals It can be confused with other secondary phosphates found in caves, such as archerite or biphosphammite. Differentiation requires advanced research methods, such as X-ray diffraction (XRD), due to visual similarity and occurrence environment. However, its habit in the form of radial bladed aggregates is quite characteristic. ## Crystal Forms Mundrabillite forms flattened, bladed or thin-tabular crystals, elongated in one direction. Most often, they occur as fan-shaped or radial aggregates and rosettes, as well as fibrous masses.

Geological environment

## Genesis Mundrabillite is a secondary mineral, formed under very specific conditions. It forms as a result of the reaction of solutions derived from the decomposition of bat guano with calcium carbonate, which constitutes the cave rocks (limestones). It is a product of biomineralogical activity in an environment rich in phosphorus and ammonia. ## Mineral Associations This mineral co-occurs with other phosphates typical of cave environments, such as archerite, biphosphammite, aphthitalite, halite, syngenite, stercorite, weddellite, and gypsum. ## Localities Apart from the type locality in Petrogale Cave in Western Australia, its occurrence has been confirmed in only a few places worldwide. These include caves in Namibia (e.g., Arnhem Cave) and on the Arabian Peninsula, where it also occurs in guano deposits.

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued mundrabillite specimens are those with well-formed, radial aggregates with a distinct luster, set on a small rock matrix. Due to the microscopic size of the crystals, the size and richness of the aggregate are crucial. Contrast with a darker host rock enhances the visual appeal of the specimen. ## Popular Localities The most famous and classic specimens come from the discovery site in Petrogale Cave in Australia. Specimens from Namibia are also found on the collector's market, though less frequently.

Care and storage

## Cleaning Mundrabillite specimens should not be wet cleaned. Due to its water solubility, contact with water will destroy the mineral. Only very careful dust removal with a soft brush or a stream of compressed air from a distance is permissible. ## What to Avoid Contact with water, acids, detergents, and all other chemicals must be strictly avoided. The mineral is very sensitive to changes in humidity – it can disintegrate under its influence. It should be protected from high temperatures and direct sunlight. ## Storage Mundrabillite specimens require storage in conditions of controlled, low humidity. The best solution is a sealed container (a 'perky box') with a desiccant (e.g., silica gel). It should be stored at a stable temperature, away from light sources and vibrations.

External references

Sources

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