Guimarãesite
Chemical formula: Ca₂Zn²⁺₅Be₄(PO₄)₆(OH)₄·6H₂O
Guimarãesite is a very rare, hydrated beryllium, zinc, and calcium phosphate, forming spherical aggregates of tiny, platy crystals.
Properties
- Mohs hardness
- 4.5
- Color
- brown
- Luster
- Silky
- Streak
- White
- Density
- 0
- Cleavage
- perfect cleavage on {100} and good cleavage on {010} (by analogy)
- Fracture
- Irregular/Uneven
- Transparency
- Transparent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Identification of guimarãesite is primarily based on its unique morphology – white, spherical aggregates composed of tiny, platy crystals. Its occurrence environment in association with other rare phosphates in granitic pegmatites is also characteristic. Definitive identification requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS). ## Distinguishing from Similar Minerals It can be confused with other secondary, white minerals forming coatings or spherulitic aggregates, such as wardite or strengite. Visual differentiation is practically impossible without knowledge of the mineral association and location. Guimarãesite is distinguished by its unique chemical composition, containing calcium, zinc, and beryllium simultaneously. ## Crystal Forms It forms spherical or hemispherical radial-fibrous aggregates, composed of very fine, platy or acicular crystals, elongated along the [100] axis.
Geological environment
## Genesis Guimarãesite is a secondary mineral, formed in the late stages of hydrothermal processes in complex granitic pegmatites, rich in phosphorus and beryllium. It forms in rock cavities and fissures as a result of the alteration of earlier phosphate and beryllium minerals. ## Mineral Associations This mineral co-occurs with other rare phosphates. In its type locality (Jocão mine), it was found in association with eosphorite, zanazziite, wardite, strengite, cyrilovite, frondelite, rockbridgeite, quartz, albite, and muscovite. ## Localities The only confirmed occurrence of guimarãesite in the world is its type locality: Jocão mine, Conselheiro Pena municipality, Doce River valley, Minas Gerais state, Brazil.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of a guimarãesite specimen primarily depends on the richness and definition of the spherical aggregates on the rock matrix. Specimens with numerous, well-formed, white spheres contrasting with the substrate are most valued. The absence of damage to these delicate structures is also important. Due to the microscopic nature of the mineral, the size of individual aggregates rarely exceeds 1-2 mm. ## Popular Localities The only source of specimens is the Jocão mine in Brazil, which makes every specimen from this locality extremely valuable for specialized collectors of rare and "micromount" type minerals.
Care and storage
## Cleaning Due to the delicate and fragile nature of the aggregates, mechanical or chemical cleaning is not recommended. Any attempts may lead to irreversible damage to the specimen. It is safest to leave it in its original state. ## What to Avoid Avoid contact with water, chemicals, ultrasound, and sudden temperature changes. Fragile aggregates are very susceptible to mechanical damage, so they should be handled with extreme caution. ## Storage Guimarãesite specimens should be stored in stable conditions, in a sealed "micromount" box, to protect them from dust, moisture, and physical damage. They should not be exposed to direct sunlight.