Hydroxylpyromorphite

Chemical formula: Pb₂+₅(PO₄)₃(OH)

Hydroxylpyromorphite is a lead phosphate of the apatite group, forming characteristic barrel-shaped crystals with colors ranging from green to yellowish-brown.

## Characteristics Hydroxylpyromorphite is a mineral of the phosphate class, belonging to the apatite group and the pyromorphite series. It is the hydroxyl (OH) analogue of pyromorphite, in which the dominant anion is chlorine (chloropyromorphite) or fluorine (fluoropyromorphite). It forms crystals with a hexagonal prismatic habit, often with a barrel-shaped outline due to curved faces. It also occurs in radial, spherical, reniform aggregates, and as crusts and coatings. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3.5-4. It is characterized by a very high density, approximately 7.04 g/cm³, which is typical for lead-bearing minerals. The luster is resinous, transitioning to adamantine on crystal faces. It is a brittle, translucent mineral, rarely transparent or opaque. ## Colors and Varieties Hydroxylpyromorphite exhibits various colors, most commonly green, yellow, brown, and gray, rarely colorless. The coloration is often a result of impurities of other elements. It forms continuous solid solutions with mimetite (in which arsenic replaces phosphorus) and with other minerals of the pyromorphite series, which affects its properties and color. No commercial varieties are distinguished. ## History and Name The mineral's name refers to its chemical composition – the presence of the hydroxyl (OH) group – and its relation to pyromorphite. It was formally described as a distinct mineral in 1951. The type locality (locus typicus) is the Kurfürst Friedrich mine in Bad Ems, Rhineland-Palatinate, Germany. ## Uses Due to its attractive appearance, crystal form, and color, hydroxylpyromorphite is a prized collector's stone. It has no significant industrial application, although locally it can be a minor lead ore.

Properties

Mohs hardness: 3.5-4
Luster: Resinous to Adamantine
Streak: White
Density: 7.04
Cleavage: Indistinct on {1011}
Fracture: Conchoidal to uneven
Transparency: Transparent to Translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Features facilitating the identification of hydroxylpyromorphite include its high density (the specimen is surprisingly heavy for its size), hexagonal, often barrel-shaped crystals, resinous or adamantine luster, and typical green, yellow, or brown coloration. Its occurrence in association with other secondary lead minerals is also an important clue. ## Distinguishing from Similar Minerals Distinguishing hydroxylpyromorphite from other minerals of the pyromorphite series (chloropyromorphite, fluoropyromorphite) and from mimetite is impossible without advanced chemical analysis (e.g., EDS analysis). Mimetite more often forms more rounded crystals with yellow and orange colors, but this is not a rule. From wulfenite, with which it is sometimes confused, it is distinguished by its crystal form – wulfenite forms thin, tabular crystals, not prismatic ones. ## Crystal Forms Most commonly found are hexagonal prismatic crystals, ranging from short and stout to slender, acicular. Characteristic are barrel-shaped forms, with rounded and transversely striated faces. Crystals can form parallel or radial aggregates, as well as druses, crusts, and spherical and botryoidal aggregates.

Geological environment

## Genesis Hydroxylpyromorphite is a secondary mineral, forming in the oxidation (weathering) zones of lead ore deposits. It forms as a result of reactions of phosphorus-rich solutions (originating, for example, from the decomposition of organic matter or the weathering of apatite in surrounding rocks) with primary lead minerals, such as galena, or with other secondary minerals, e.g., cerussite. ## Mineral Associations It co-occurs with other minerals of the oxidation zone. Most commonly these are: cerussite, anglesite, mimetite, wulfenite, as well as iron and manganese oxides (goethite, limonite) and remnants of primary ore (galena). ## Localities Important localities worldwide include historical mines in the Bad Ems region in Germany (type locality). Beautiful specimens also come from the Daoping mine in the Guangxi autonomous region of China, known for its intensely green crystals. Other known localities include the Bunker Hill mine in Idaho (USA) and the Broken Hill region in New South Wales (Australia).

Rarity

Not very common

For collectors

## Quality Criteria The most highly prized hydroxylpyromorphite specimens are distinguished by intense, saturated color (bright green is particularly sought after), large, well-formed, and undamaged crystals with a distinct barrel shape. High luster, transparency, and aesthetic placement of crystals on a contrasting rock matrix significantly increase the collectible value of the specimen. ## Popular Localities Specimens from the Daoping mine in China are considered the best in the world in terms of quality and color. Classic, historical examples come from German mines in Bad Ems. Localities in Idaho (USA) provide specimens of varied forms and colors.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush and distilled water. Due to its softness, strong rubbing should be avoided. Ultrasonic cleaners are not recommended, as they can cause fractures. ## What to Avoid The mineral is sensitive to acids, which cause its decomposition. As a lead mineral, it is toxic – avoid inhaling dust and wash hands after any contact with the specimen. Protect it from scratching by harder minerals. ## Storage It is recommended to store specimens in separate, padded boxes or display cases to prevent mechanical damage and dust accumulation. It is not sensitive to light or temperature changes under household conditions.

Sources

IMA Mineral List (CNMNC)