Arnhemite

Chemical formula: K₄Mg₂(P₂O₇)₂·₅H₂O

Arnhemite is a historical, poorly described, and currently unrecognized mineral species, likely synonymous with a magnesium-potassium phosphate.

## Characteristics Arnhemite was described as a white, crystalline substance forming aggregates resembling cotton wool or felt. It consisted of microscopic, acicular crystals. It was found in bat guano. Modern mineralogy does not recognize arnhemite as a distinct mineral species, and its original description is insufficient for full characterization. ## History and Name The mineral's name comes from the city of Arnhem in the Netherlands. The substance was found in the Guano Caves (Guanogrotten) near the village of Hattam. It was first described in 1886 by H.J.H. Struve. Arnhemite's status as a mineral was disqualified by the International Mineralogical Association (IMA), likely due to its identity with another, previously described mineral or a lack of sufficient data for unambiguous identification.

Properties

rruff id: 80e778f976b165f58c2573324d93
ima status: approved

Diagnostic features

## Identification Identification of arnhemite is practically impossible and is purely historical. It relies solely on comparison with the original, very general description and location. Modern analytical techniques, such as X-ray diffraction (XRD), would be required to determine its true nature. ## Distinguishing from Similar Minerals Due to its lack of mineralogical status, comparing it with other minerals has no practical application. Historically, it might have been confused with other secondary phosphates found in guano.

Geological environment

## Genesis Arnhemite formed as a product of reactions between solutions derived from bat guano and surrounding components. It is a secondary mineral, crystallizing under the specific conditions found in caves. ## Mineral Associations It co-occurred with other guano minerals, such as struvite. ## Localities The only known occurrence is the historical locality in the Guano Caves near Hattam, close to Arnhem in the Netherlands.

Rarity

Extremely rare

For collectors

## Quality Criteria Arnhemite is not an item of collector's trade. The only value lies in historical specimens from the original locality, which may be found in institutional or museum collections. This value is purely scientific and historical.

Care and storage

## Cleaning Due to the historical and uncertain nature of the material, any conservation is not recommended. Historical specimens should be preserved in their original state. ## What to Avoid Avoid contact with water, chemicals, and any attempts at mechanical cleaning, which could destroy the delicate structure. ## Storage Store in a dry, stable environment, in a sealed container, to protect from dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)