Plumbogummite

Chemical formula: Pb²⁺Al₃(PO₄)(PO₃OH)(OH)₆

Plumbogummite is a rare lead aluminum phosphate, forming characteristic reniform or spherical aggregates of pale blue, yellowish, or gray color.

## Characteristics Plumbogummite is a hydrated lead aluminum phosphate belonging to the plumbogummite group. It rarely forms distinct, small crystals with a tabular or rhombohedral habit. Most often, it occurs as dense, compact masses, spherical, reniform, or botryoidal aggregates with a radial internal structure. It frequently forms pseudomorphs after other minerals, especially pyromorphite and galena, retaining their external crystalline form. ## Physical Properties This mineral is characterized by a hardness of 4-5 on the Mohs scale. Its luster is typically resinous to dull or earthy, and on fresh fractures, it can be vitreous. It is a translucent mineral. The density is approximately 4.01 g/cm³. ## Colors and Varieties Plumbogummite exhibits various colors, most commonly light blue, grayish-white, yellowish-gray, yellow, yellowish-brown, and less frequently greenish, bluish, or reddish-brown. Its color is often non-uniform, forming bands or spots. No named varieties are distinguished, and specimens are classified mainly based on color and mode of occurrence. ## History and Name The name plumbogummite comes from the Latin words *plumbum* (lead) and *gummi* (gum), referring to its chemical composition and its appearance resembling dried gum. The mineral was first described in 1819 based on specimens found in the Huelgoat mines in Brittany (France), which is its type locality. ## Uses Plumbogummite has no industrial significance. However, it is a valued and sought-after collector's mineral, especially due to its attractive forms and colors, as well as pseudomorphs after other minerals.

Properties

Mohs hardness: 4-5
Luster: Resinous, Dull
Streak: Colourless to white
Density: 4.014
Fracture: Irregular/Uneven,Sub-Conchoidal
Transparency: Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Plumbogummite is most easily recognized by its characteristic forms of occurrence – spherical, reniform, or botryoidal aggregates. Its typical colors (pale blue, yellowish, gray) and resinous or dull luster are also characteristic. The formation of pseudomorphs after hexagonal pyromorphite crystals is a very distinctive feature. ## Distinguishing from Similar Minerals Plumbogummite can be confused with other secondary minerals of similar appearance, such as smithsonite, hemimorphite, or variscite. Smithsonite and hemimorphite are usually harder and react with hydrochloric acid. Variscite has a lower density and usually a more greenish hue. From similar-looking hinsdalite and crandallite, it can only be distinguished using advanced analytical methods. ## Crystal Forms Well-formed crystals are rare and microscopic, taking the form of rhombohedra or tabular crystals. Radiating-fibrous, spherical, botryoidal, reniform, stalactitic aggregates, and dense, compact masses dominate. Pseudomorphs after pyromorphite, galena, and cerussite are very common.

Geological environment

## Genesis Plumbogummite is a secondary mineral, forming in the oxidation (weathering) zones of lead ore deposits, especially galena. It forms as a result of the reaction of phosphate-rich solutions (originating, for example, from the weathering of apatite or organic matter) with lead minerals in the presence of aluminum derived from clay rocks or weathering feldspars. ## Mineral Associations It often co-occurs with minerals from which it forms or which accompany it in the oxidation zone. Typical associated minerals include pyromorphite, cerussite, galena, anglesite, mimetite, hinsdalite, crandallite, quartz, goethite, and clay minerals. ## Localities The most important historical locality is Huelgoat in Brittany (France). Beautiful blue specimens come from the Yangshuo mine in the Guangxi region of China. Other known localities include the Roughton Gill mine in Cumbria (England), some mines in Idaho and Pennsylvania (USA), as well as Tsumeb (Namibia) and Broken Hill (Australia).

Rarity

Rare

For collectors

## Quality Criteria The most highly valued plumbogummite specimens are those with intense blue color, especially those from China. Well-formed pseudomorphs, particularly complete "barrels" after pyromorphite, with preserved luster and color, also have high value. Contrast with the host mineral (e.g., quartz) and the absence of damage to delicate, spherical forms enhance attractiveness. ## Popular Localities Collectors most seek blue, botryoidal specimens from mines in the Yangshuo region (China), which have dominated the market in recent decades. Classic, though rarer, are specimens from historical localities in England (Cumbria) and France (Huelgoat). Pseudomorphs after pyromorphite are known from many places, including mines in Idaho (USA).

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, avoiding prolonged soaking. Ultrasonic cleaners should not be used, as they can damage delicate aggregates. ## What to Avoid Plumbogummite is sensitive to acids and strong bases, which can chemically damage it. Contact with household and laboratory chemicals should be avoided. It is a relatively soft mineral, susceptible to scratching, so it must be protected from contact with harder minerals. ## Storage It is recommended to store specimens in separate, padded boxes or on stands in closed display cases to protect them from dust and mechanical damage. It does not require special lighting or humidity conditions, but extreme conditions should be avoided.