Grandreefite
Chemical formula: Pb<sup>2+</sup><sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)F<sub>2</sub>
Grandreefit is a very rare lead fluoride and sulfate, forming colorless, tabular crystals, known primarily from the Grand Reef Mine in Arizona.
Description
## Characteristics Grandreefit is a lead fluoride and sulfate. It occurs as very small, thin, tabular crystals with a square or rectangular outline. The crystals are usually colorless and transparent, often forming subparallel intergrowths and small aggregates. ## Physical Properties This mineral is characterized by a strong, adamantine luster. It is transparent. Its density is approximately 6.75 g/cm³, which is a high value, typical for lead minerals. ## Colors and Varieties Grandreefit is usually colorless. No colored varieties are distinguished. ## History and Name The mineral's name comes from its discovery locality – the Grand Reef Mine in Graham County, Arizona (USA). It was described as a new mineral species in 1990 by Sidney A. Williams. ## Applications Due to its extreme rarity and occurrence as microscopic crystals, grandreefit has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and microminerals.
Diagnostic features
## Identification Identification of grandreefit is based on its characteristic form of thin, colorless tabular crystals with an adamantine luster. The key is the provenance (Grand Reef Mine) and co-occurrence with other secondary lead minerals. ## Distinguishing from Similar Minerals It can be confused with other colorless, secondary minerals of the oxidation zone, such as anglesite or cerussite. However, it is distinguished by its specific, thin-tabular crystal form and perfect cleavage in one direction. Final identification requires advanced research methods, e.g., X-ray diffraction (XRD). ## Crystal Forms It forms very thin, tabular crystals with a square or rectangular outline. Crystals often show parallel or subparallel intergrowths, forming small, fragile aggregates.
Geological environment
## Genesis Grandreefit is a secondary mineral formed in the oxidation zone of polymetallic, hydrothermal ore deposits. It crystallizes in rock cavities and fissures as a result of the weathering of primary lead sulfides (galena) in the presence of fluoride ions. ## Mineral Associations It most commonly co-occurs with other secondary lead minerals, such as anglesite, linarite, and caledonite, as well as with galena, fluorite, and quartz. ## Localities The main and type locality for grandreefit is the Grand Reef Mine in the Santa Teresa Mountains, Graham County, Arizona, USA. This is the only locality from which well-formed, identifiable specimens originate.
Rarity
Very rare
Collector aspects
## Quality Criteria The collector's value of grandreefit primarily depends on the presence of well-formed, sharp, and undamaged microcrystals. Rich clusters on a small rock matrix and associations with other rare minerals, such as linarite or caledonite, are prized. Due to their microscopic size, clarity and color are not key criteria. ## Popular Localities The only source of collector specimens is the historic Grand Reef Mine in Arizona. Material from this locality is highly sought after by collectors specializing in systematics and rare minerals.
Care and storage
## Cleaning Grandreefit specimens, typically microminerals, are best cleaned with compressed air or very gently with a soft, dry brush. Avoid contact with water and any chemical agents. ## What to Avoid Grandreefit is susceptible to mechanical damage due to its perfect cleavage. As a lead mineral, it is toxic – avoid inhaling dust and wash hands after any contact. It should not be heated or exposed to acids. ## Storage It is recommended to store specimens in sealed, closed "micromount" boxes to protect them from dust, moisture, and mechanical damage. Display should only be done under magnification.