Mejillonesite

Chemical formula: NaMg<sub>2</sub>(PO<sub>3</sub>OH)(PO<sub>4</sub>)(OH)<sub>4</sub>·H<sub>2</sub>O

Mejillonesite is a very rare, hydrated sodium magnesium phosphate, forming tiny, colorless, bladed crystals.

## Characteristics Mejillonesite is a hydrated sodium magnesium phosphate, occurring as very small, colorless, bladed or acicular crystals. It typically forms small, radial aggregates or rosettes, whose individual crystals rarely exceed 1 mm in length. Due to its size and form of occurrence, it is a mineral of interest mainly to specialized collectors (micromounts). ## Physical Properties Mejillonesite crystals exhibit a vitreous luster and are transparent. The Mohs hardness is approximately 3. This mineral is brittle, and its density, calculated from the chemical formula and unit cell parameters, is 2.52 g/cm³. ## History and Name The mineral was discovered in 2004 on Mejillones Hill, on the peninsula of the same name in the Antofagasta region of Chile. Its name is derived directly from the type locality where it was first identified. It was approved by the International Mineralogical Association (IMA) under number 2004-035.

Properties

Mohs hardness
3
Luster
Vitreous
Streak
White
Density
2.52
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of mejillonesite is possible almost exclusively by advanced analytical methods, such as X-ray diffraction (XRD) and chemical spectroscopy (EDS/WDS). Visually, at the occurrence site, it can be suspected based on characteristic, tiny, colorless, radial aggregates in association with other rare phosphates. ## Differentiation from Similar Minerals It can be confused with many other secondary, colorless acicular minerals, such as gypsum or other rare phosphates. Certain differentiation without specialized equipment is impossible. ## Crystal Forms It forms elongated, bladed or acicular crystals, which combine into radial or stellate aggregates and rosettes.

Geological environment

## Genesis Mejillonesite is a secondary mineral that formed as a result of the interaction of solutions derived from guano (seabird droppings) with igneous rocks (diorite). It forms under extremely dry, desert climatic conditions. ## Mineral Associations At the type locality, it co-occurs with other rare phosphates, such as bobdownsite, clinovariscite, phosphoribosite, juangodoyite, newberyite, struvite, and sal ammoniac (ammonium chloride). ## Localities The only confirmed locality of mejillonesite in the world is its type locality - Mejillones Hill on the Mejillones Peninsula, in the Antofagasta Province of northern Chile.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a specimen depends on the richness and aesthetics of the microcrystalline aggregates on the rock matrix. Samples with clearly formed, radial rosettes, well visible under a microscope, are most valued. Association with other rare minerals from this locality is also important.

Care and storage

## Cleaning Mejillonesite specimens, typically micromounts, do not require cleaning. If absolutely necessary, compressed air (from a safe distance) can be used to remove dust. Any contact with water or other liquids should be avoided. ## What to Avoid As a mineral formed in a desert climate, it is potentially sensitive to moisture. Contact with water, acids, and other chemicals must be strictly avoided. It should not be heated or subjected to ultrasonic cleaning. ## Storage It is recommended to store specimens exclusively in specialized micromount boxes, in a dry place, away from sources of moisture and sudden temperature changes.

Sources

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