Plumbopharmacosiderite
Chemical formula: Pb²⁺₀.₅Fe³⁺₄(As⁵⁺O₄)₃(OH)₄·5H₂O
Plumbopharmacosiderite is a rare lead iron arsenate, forming small, cubic crystals with a characteristic light green color.
Properties
- Mohs hardness
- 2.5-3
- Luster
- Vitreous
- Streak
- White
- Density
- 2.89
- Cleavage
- None
- Fracture
- Irregular/Uneven
- Transparency
- Transparent
- Crystal system
- Isometric
Diagnostic features
## Identification The key diagnostic feature of plumbopharmacosiderite is its characteristic cubic crystal habit combined with a light green or yellowish-green color. It occurs in paragenesis typical of the oxidation zones of ore deposits, which is also an important clue. Hardness (2.5-3) and a white streak aid in identification. ## Distinguishing from Similar Minerals Plumbopharmacosiderite can be confused with other minerals from the pharmacosiderite group, such as pharmacosiderite (does not contain lead) or natropharmacosiderite (contains sodium instead of lead). Definitive differentiation requires advanced chemical analysis (e.g., EDS). It can also be confused with other secondary arsenates, such as mimetite, which is, however, significantly harder (3.5-4) and crystallizes in the hexagonal system. ## Crystal Forms This mineral forms almost exclusively small, well-formed cubic crystals, often with slightly rounded faces and edges. These crystals occur individually or in small groups, growing on the host rock.
Geological environment
## Genesis Plumbopharmacosiderite is a secondary mineral, forming in the oxidation (weathering) zones of polymetallic ore deposits rich in arsenic and lead. It forms as a result of the reaction of acidic, arsenic-rich waters with primary minerals containing lead and iron, such as galena and pyrite. ## Mineral Associations It often co-occurs with other secondary minerals of the oxidation zone. Its typical associations include pharmacosiderite, segnitite, beudantite, mimetite, anglesite, cerussite, goethite, and quartz. ## Localities The most important and classic locality, which is also the type locality, is Monte Falò in Piedmont, Italy. This mineral has also been found in other locations worldwide, including the Clara mine in the Black Forest (Germany) and Cornwall (United Kingdom), which are known for the occurrence of rare secondary minerals.
Rarity
Rare
For collectors
## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp, and undamaged crystals of an intense, light green color. Rich groups of aesthetically arranged crystals on a small rock matrix are also highly valued. Crystal size is crucial – specimens with crystals exceeding a few millimeters are rare and command significantly higher prices. Transparency and luster also increase the specimen's value. ## Popular Localities Specimens from the type locality, Monte Falò in Italy, hold particular historical value. However, the German locality at the Clara mine in Wolfach (Black Forest) has supplied many high-quality specimens with well-formed crystals to the collector market.
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. After wet cleaning, the specimen must be thoroughly dried immediately, preferably using compressed air or by leaving it in a dry, airy place. ## What to Avoid Contact with all chemicals, especially acids, which can damage the mineral, should be avoided. It is sensitive to high temperatures, which can lead to dehydration and structural changes. It should not be cleaned in ultrasonic cleaners. As a relatively soft mineral, it is susceptible to scratches. ## Storage Plumbopharmacosiderite specimens are best stored in separate, sealed display boxes to avoid abrasion and contact with other minerals. It should be protected from dust, humidity, and direct, prolonged exposure to sunlight.