Lombardoite
Chemical formula: Ba₂Mn³⁺(As⁵⁺O₄)₂(OH)
Lombardoite is a very rare barium and manganese arsenate, distinguished by its dark reddish-brown color and high luster.
Properties
- Mohs hardness
- 6-6.5
- Luster
- Adamantine
- Streak
- Yellow–orange
- Density
- 5.124
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Transparent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Lombardoite can be identified by its characteristic dark reddish-brown color, high luster, specific form of radial aggregates, and occurrence in association with other manganese minerals. A yellow-orange streak is also a diagnostic feature. However, definitive identification requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS). ## Distinguishing from Similar Minerals It can be confused with other rare manganese arsenates of similar color, such as gamagarite or wallkilldellite. Distinguishing them is possible almost exclusively by X-ray diffraction (XRD) or chemical analysis, as visual features are very similar. ## Crystal Forms It forms very small, prismatic or tabular crystals that combine into radial or spherulitic aggregates. Single, well-formed crystals are extremely rare.
Geological environment
## Genesis Lombardoite is a hydrothermal mineral. It forms under low-temperature conditions in veins cutting through manganese- and barium-rich rocks. Its presence is associated with the metamorphism of ophiolites. ## Mineral Associations At the type locality (Valletta Mine), it co-occurs with minerals such as braunite, quartz, hematite, gamagarite, saneroite, piemontite, and barite. ## Localities The only confirmed and described locality of lombardoite in the world is its type locality – the Valletta mine, in the Valletta valley, in the province of Cuneo, Piedmont, Italy.
Rarity
Extremely rare
For collectors
## Quality Criteria As a micromineral, lombardoite is primarily valued for the richness and aesthetics of its aggregates on the rock matrix. Specimens with distinct, radial clusters of intense color, contrasting with the light host rock, are most highly prized. The size of the aggregates, although still microscopic, also affects the value. ## Popular Localities The only source of lombardoite specimens is the Valletta mine in Italy. Material from this locality is extremely rare and sought after by specialized collectors.
Care and storage
## Cleaning Specimens should only be cleaned with compressed air to remove dust. Due to the microscopic size of the crystals and their fragility, any form of mechanical or chemical cleaning is discouraged. ## What to Avoid Avoid contact with chemicals, especially acids, which can damage the mineral. The crystals are brittle and sensitive to ultrasound and shocks. Do not expose specimens to sudden temperature changes. ## Storage Lombardoite specimens, as microminerals, should be stored in specialized "micromount" boxes that protect them from dust, moisture, and mechanical damage. Avoid exposure to direct sunlight.