Pseudograndreefite

Chemical formula: Pb²⁺₆(S⁶⁺O₄)F₁₀

Pseudograndreefite is a rare lead fluoride and sulfate, forming colorless to white, tabular crystals with a pearly luster.

## Characteristics Pseudograndreefite is a mineral from the sulfate and fluoride group. It occurs as very small, tabular or bladed crystals, rarely exceeding 1 mm, which often form rosette-like or radial aggregates. Crystals are typically colorless to white, sometimes with a pale yellow tint. ## Physical Properties The mineral is characterized by a hardness of approximately 2.5 on the Mohs scale. It has perfect cleavage in one direction, which, combined with a pearly luster on cleavage surfaces, is its characteristic feature. The luster on other faces is vitreous. It is a rather dense mineral, due to its high lead content. ## History and Name The name "pseudograndreefite" refers to its visual similarity to grandreefite and the fact that it is its dimorph (having the same chemical composition but a different crystal structure). It was described as a new mineral in 1990 by S. A. Williams based on material from the Grand Reef mine in Arizona, USA. ## Uses Due to its rarity and microscopic crystal sizes, pseudograndreefite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or minerals from specific localities (so-called micromounts).

Properties

Mohs hardness
2.5
Color
Colourless
Luster
Sub-adamantine
Streak
White
Density
7.0
Cleavage
Perfect on {001}
Fracture
Conchoidal
Transparency
Transparent
Crystal system
Orthorhombic

Diagnostic features

## Identification Identification of pseudograndreefite is based on its characteristic form – small, colorless, tabular crystals, often in radial aggregates. A strong, pearly luster on cleavage planes is key. Due to its microscopic size, final confirmation requires advanced analytical methods, such as X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with grandreefite, to which it is closely related and often co-occurs. Differentiation under amateur conditions is practically impossible without crystallographic analysis. It may also be confused with other colorless lead sulfates or fluorides, but its specific habit and pearly luster are helpful in preliminary identification. ## Crystal Forms It forms thin, tabular crystals with a hexagonal outline, elongated in one direction. These crystals often combine into radial or rosette-like aggregates, as well as loose, randomly oriented clusters on the surface of the host rock.

Geological environment

## Genesis It is a secondary mineral, forming in the oxidation zones of polymetallic ore deposits rich in lead and sulfides. It forms as a result of the reaction of sulfate-containing solutions with lead minerals (e.g., galena) in the presence of fluoride ions, likely originating from the decomposition of fluorite. ## Mineral Associations It co-occurs with other secondary lead minerals. It is most commonly found in association with grandreefite, anglesite, leadhillite, fluorite, galena, sphalerite, linarite, and quartz. ## Localities The most important and type locality, from which the best specimens originate, is the Grand Reef mine in Graham County, Arizona, USA. It has also been identified in the Redmond mine in Haywood County, North Carolina, USA.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals forming aesthetic, radial aggregates are most valued by collectors. The size of these aggregates and their contrast with the surrounding host rock (matrix) are also important. Purity and lack of damage to delicate crystals significantly increase the value of the specimen. Due to the microscopic nature of the mineral, specimens are evaluated under magnification. ## Popular Localities The only source of specimens of collector significance is the type locality – the Grand Reef mine in Arizona, USA.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using only compressed air to remove dust. Due to the small size and delicacy of the crystals, contact with water and brushes should be avoided. ## What to Avoid All chemicals should be avoided, especially acids, which can react violently with the mineral. The crystals are soft and brittle, susceptible to mechanical damage. They should not be subjected to ultrasonic cleaning. ## Storage It is recommended to store specimens in specialized micromount boxes that protect them from dust, shocks, and direct contact. Exposure to moisture and extreme temperatures should be avoided.

External references

Sources

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