Barstowite
Chemical formula: Pb<sup>2+</sup><sub>4</sub>CO<sub>3</sub>Cl<sub>6</sub>·H<sub>2</sub>O
Barstowite is a rare, secondary lead mineral, forming colorless or white, silky needles and aggregates, found in the oxidation zones of ore deposits.
Properties
- Mohs hardness
- 3
- Luster
- Silky
- Streak
- White
- Density
- 5.91
- Cleavage
- Perfect on {100}
- Fracture
- Splintery
- Transparency
- Translucent to Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Barstowite is recognized by its characteristic form – fine, acicular crystals forming silky aggregates. Its occurrence in association with other secondary lead minerals, such as cerussite and anglesite, is a key indicator. Its white color, silky luster, and high density (perceptible even in small specimens) also aid in identification. ## Distinguishing from Similar Minerals It can be confused with other acicular, white secondary minerals. It differs from cerussite in crystal habit (cerussite more often forms thicker, tabular crystals and twinning). From hydrocerussite, it can be distinguished by precise analysis (e.g., XRD), although hydrocerussite more often forms coatings and crusts. Final identification requires advanced analytical methods. ## Crystal Forms Crystals are strongly elongated, acicular (needle-like habit), often flattened. They typically form radial, stellate, or haphazardly tangled aggregates. They rarely occur as single, isolated needles.
Geological environment
## Genesis Barstowite is a secondary mineral, forming in the oxidation (weathering) zones of lead ore deposits. It forms as a result of reactions between chloride-rich solutions (often of marine origin, as in the case of Cornwall) and other lead minerals, such as galena or cerussite. It is one of the last minerals to crystallize during the weathering process. ## Mineral Associations It most commonly co-occurs with cerussite, anglesite, sphalerite, galena, pyromorphite, phosgenite, and laurionite. In its type locality (Cornwall), it was found in association with cerussite and sphalerite. ## Localities This is an extremely rare mineral, known from only a few locations worldwide. The most important ones include: - Type locality: Bounds Cliff mine and the nearby Penberthy Croft mine in Perranzabuloe, Cornwall, United Kingdom. - Megala Pefka mine, Laurion, Attica, Greece – known for the occurrence of many rare secondary lead minerals. - Malacalzetta mine, Iglesias, Sardinia, Italy.
Rarity
Very rare
For collectors
## Quality Criteria The quality of a barstowite specimen primarily depends on the richness and aesthetics of the crystal aggregates. Specimens with clearly visible, well-formed, radially arranged groups of acicular crystals are most prized. Contrast with the matrix mineral also enhances visual appeal. Due to the microscopic size of the crystals, specimens are mainly evaluated under magnification. ## Popular Localities Specimens from the type locality in Cornwall (United Kingdom) and from Laurion (Greece) are most sought after by collectors due to their historical and mineralogical significance.
Care and storage
## Cleaning Barstowite specimens are extremely delicate and fragile. Mechanical cleaning is highly inadvisable. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust. Contact with water, which can damage the mineral, should be avoided. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is sensitive to shocks and impacts – acicular crystals are easily damaged. It should not be heated or exposed to prolonged moisture. ## Storage Specimens should be stored in stable, sealed "micromount" type containers to protect them from dust, moisture, and mechanical damage. Due to their fragility, they should not be stored loosely with other, harder minerals.