Penberthycroftite

Chemical formula: [Al₆(As⁵⁺O₄)₃(OH)₉(H₂O)₅]·8H₂O

Penberthycroftite is an extremely rare aluminum arsenate, forming microscopic, colorless crystals in the shape of hexagonal plates.

## Characteristics Penberthycroftite is a hydrated aluminum arsenate with a complex chemical composition. It occurs as extremely small, hexagonal, platy crystals, rarely exceeding 40 micrometers in diameter and 5 micrometers in thickness. The crystals are colorless and transparent, often forming rosette-like or irregular aggregates on the surface of other minerals. ## Physical Properties Due to the microscopic size of the crystals, most physical properties, such as hardness, density, fracture, or cleavage, have not been precisely determined. The luster is described as vitreous. ## Colors and Varieties This mineral is colorless. No colored or commercial varieties are known. ## History and Name Penberthycroftite was discovered in the Penberthy Croft mine, in St Hilary, Cornwall (United Kingdom). Its name is derived directly from its type locality. The mineral was approved by the International Mineralogical Association (IMA) in 1992, and its first description appeared in 1994 in the Mineralogical Magazine. The authors of the description were A.J. Criddle, J.G. Francis, C.J. Stanley, and J.E. Chisholm. ## Uses Penberthycroftite has no industrial or commercial applications. Its significance is purely scientific and collectible, as an extremely rare mineral species.

Properties

Color
White
Luster
Vitreous
Streak
White
Density
2.18
Cleavage
Basal cleavage on {010}
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of penberthycroftite is impossible without advanced laboratory equipment. Preliminary identification under a microscope relies on observing characteristic, very small, colorless, hexagonal plates. Final confirmation requires chemical analysis (EDS) and X-ray diffraction (XRD) structural analysis. ## Distinguishing from Similar Minerals It can be confused with other secondary arsenate or sulfate minerals of similar form, such as pharmacolite or gypsum. Differentiation is based on chemical composition analysis – the presence of aluminum and arsenic and the absence of calcium or sulfur. ## Crystal Forms It forms very thin, platy crystals with a hexagonal outline. These crystals often group into rosette-like or irregular aggregates.

Geological environment

## Genesis Penberthycroftite is a secondary mineral, forming in the oxidation zones (gossans) of polymetallic deposits. It forms as a result of the weathering of primary ore minerals, especially arsenopyrite, in an environment rich in aluminum, derived from the decomposition of clay minerals or feldspars. ## Mineral Associations This mineral coexists with other secondary arsenate and sulfate minerals. At the type locality, it was found in association with pharmacosiderite, scorodite, beudantite, olivenite, cornwallite, as well as quartz and goethite. ## Localities The only confirmed and described occurrence of penberthycroftite in the world is its type locality: Penberthy Croft mine in St Hilary, Cornwall, United Kingdom.

Rarity

Extremely rare

For collectors

## Quality Criteria As a "micromount" type mineral, the collectible value of a specimen depends on the abundance and quality of the microscopic crystals. Specimens with numerous, well-formed, sharply outlined hexagonal plates forming photogenic aggregates are most prized. Contrast with the rock matrix and the presence of rare associated minerals are also important. ## Popular Localities The only source of specimens is the historic Penberthy Croft mine in Cornwall. Material from this locality is extremely difficult to acquire and appears on the collector's market sporadically.

Care and storage

## Cleaning Specimens with penberthycroftite should be handled with utmost care. Due to the microscopic size and delicacy of the crystals, no mechanical or chemical cleaning is recommended. Dust can be removed very carefully with a soft brush or a photographic air blower. ## What to Avoid Avoid contact with water, chemicals, ultrasonic cleaners, and sudden temperature changes. The crystals are extremely fragile and can be easily damaged or abraded from the specimen surface. ## Storage Specimens should be stored under stable conditions, in closed "micromount" boxes, to protect them from dust, moisture, and mechanical damage. They should not be exposed to direct sunlight.

External references

Sources

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