Hydroxylhedyphane

Chemical formula: Ca₂Pb₂+₃(As₅+O₄)₃(OH)

Hydroxylhedyphane is a rare calcium lead arsenate of the apatite supergroup, forming small, hexagonal crystals with high luster.

## Characteristics Hydroxylhedyphane is a mineral belonging to the apatite supergroup, being a calcium lead arsenate. It most commonly occurs as small, well-formed, hexagonal crystals with a prismatic or tabular habit. It can also form granular aggregates and crusts. Its crystals are usually transparent to translucent, with a characteristic, strong luster. ## Physical Properties The mineral's hardness on the Mohs scale is 4.5 to 5, making it relatively soft. It has a high density, approximately 5.89 g/cm³, which is typical for lead-bearing minerals. The luster is described as subadamantine, resinous, or greasy. It does not exhibit distinct cleavage. ## Colors and Varieties Hydroxylhedyphane is most often colorless, white, grayish, or takes on pale yellow hues. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral's name refers to its chemical composition – the dominance of the hydroxyl (OH) group – and to the historical mineral "hedyphane," first described in 1830 by Johann Friedrich August Breithaupt. The name "hedyphane" comes from the Greek words *hēdys* (sweet) and *phainō* (to appear), which likely referred to its strong luster. For many years, hedyphane was considered a single mineral, but modern research has shown it to be a series of minerals with different dominant anions. Hydroxylhedyphane was formally recognized as a distinct mineral species by the International Mineralogical Association (IMA) in 2009. ## Uses Due to its rarity and small crystal size, hydroxylhedyphane has no industrial applications. It is solely an object of scientific and collector interest.

Properties

Mohs hardness: 4.5-5
Luster: Sub-Adamantine, Resinous, Greasy
Streak: White
Density: 5.89
Cleavage: Imperfect on {0001}
Fracture: Conchoidal to uneven
Transparency: Transparent to translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Key diagnostic features of hydroxylhedyphane are its hexagonal crystal form, high density (the specimen is surprisingly heavy for its size), and strong, resinous or subadamantine luster. Its occurrence in association with other secondary lead and arsenic minerals in specific localities is also a strong indicator. ## Differentiation from Similar Minerals Hydroxylhedyphane can be confused with mimetite and pyromorphite. Mimetite often forms barrel-shaped crystals or spherical aggregates and has a different calcium-to-lead ratio. Pyromorphite is a phosphate, not an arsenate, and rarely occurs in paragenesis with hydroxylhedyphane. Final differentiation from other minerals in the hedyphane group (e.g., fluorhedyphane) requires advanced chemical analysis (EDS). ## Crystal Forms Crystals are hexagonal, with habits ranging from short to long columns (prismatic) or thick tablets. Crystal terminations can be flat (pinacoid) or capped with pyramids. It also occurs as granular aggregates and thin crusts on the host rock.

Geological environment

## Genesis It is a secondary mineral, forming in the oxidation zones of ore deposits that have undergone metamorphic processes. Its formation requires a specific geochemical environment, rich in lead, calcium, and arsenic simultaneously. ## Mineral Associations Hydroxylhedyphane co-occurs with other minerals typical of altered deposits, such as berzeliite, caryinite, franklinite, and willemite (in skarn localities like Långban and Franklin). In other environments, it is accompanied by cerussite, mimetite, segnitite, and beudantite. ## Localities The most important and classic occurrences of this mineral are the Långban mine in Värmland, Sweden (type locality), and the ore deposits in Franklin and Sterling Hill in New Jersey, USA. It has also been reported in Tsumeb, Namibia, and several other places worldwide where rare secondary minerals occur.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized hydroxylhedyphane specimens are those with sharp, well-formed, lustrous crystals, set on a contrasting rock matrix. Transparency, purity of color, and crystal size (which rarely exceeds a few millimeters) significantly increase their value. Specimens from classic, historical localities are particularly sought after. ## Popular Localities Specimens from the Långban mines in Sweden and Franklin in the USA are considered among the best in the world. They typically feature excellent crystal development and often occur in association with other rare minerals, which enhances their attractiveness.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Ultrasonic cleaners are not recommended, as they can damage delicate crystals. ## What to Avoid As a lead arsenate, the mineral is toxic. Inhaling dust should be avoided, and hands must be thoroughly washed after any contact with the specimen. It is sensitive to acids. Due to its relatively low hardness, it is susceptible to scratches. ## Storage It is recommended to store specimens in separate, closed display boxes, away from harder minerals, to prevent mechanical damage. The label should include a warning about the mineral's toxicity.

Sources

IMA Mineral List (CNMNC)