Cerchiaraite-(Fe)
Chemical formula: Ba₄Fe³⁺₄(Si₄O₁₂)O₂(OH)₄Cl₂[Si₂O₃(OH)₄]
A rare barium and iron silicate, forming small, transparent crystals with a vitreous luster and characteristic blue or brown color.
Properties
- Mohs hardness
- 4.5
- Luster
- Vitreous
- Streak
- colorless to tan (Cerchiara), pale green-blue (California)
- Density
- 3.710
- Cleavage
- None
- Fracture
- Irregular/Uneven
- Transparency
- Transparent
- Crystal system
- Tetragonal
Diagnostic features
## Identification Cerchiaraite-(Fe) can be identified by its characteristic crystal form – elongated, prismatic columns, often terminated by a pyramid. Key features also include vitreous luster, transparency, and a specific color (blue or brown) depending on the locality. It is a relatively soft mineral (4.5 on the Mohs scale). ## Distinguishing from Similar Minerals Blue varieties from California may be confused with other rare minerals from that locality, such as benitoite. However, Cerchiaraite-(Fe) differs in crystal form (prismatic, not tabular like benitoite) and hardness (benitoite is significantly harder, 6-6.5). Brown specimens from Italy require chemical analysis for certain identification. ## Crystal Forms Crystals are tetragonal, prismatic in habit, elongated along the crystallographic c-axis. They are usually terminated by pyramid faces. They occur as single, attached crystals or form small, radial aggregates.
Geological environment
## Genesis Cerchiaraite-(Fe) is a hydrothermal mineral. It forms in fissures and cavities in metamorphic rocks. In California, it occurs in sanbornite-quartz veins cutting gneisses. In Italy, it is found in siliceous-hematite breccias within ophiolites. ## Mineral Associations This mineral co-occurs with various, often rare, minerals. In California (USA), its associated minerals include sanbornite, quartz, celsian, walstromite, titanite, fresnoite, and benitoite. In the Italian locality (Cerchiara), it occurs with quartz, hematite, braunite, wallisite, and other rare sulfates and arsenates. ## Localities The most important and also type localities for this mineral are the Cerchiara mine in Liguria, Italy, and the Esquire No. 7 and No. 8 deposits in Fresno County, California, USA. These are the only confirmed occurrences of specimens of collector significance.
Rarity
Very rare
For collectors
## Quality Criteria Specimens from California are most valued by collectors due to their intense blue color. Well-formed, sharply terminated, transparent crystals, especially those embedded in a contrasting rock matrix, are highly prized. Crystal size is also important – specimens exceeding a few millimeters are rare. For specimens from Italy, the quality and form of the brown crystals are primarily valued. ## Popular Localities Specimens from the Esquire deposits (No. 7 and No. 8) in Fresno County, California, are by far the most sought after on the collector's market. This locality provides the world's best blue crystals of this mineral. Specimens from the Cerchiara mine in Italy are also valued, especially in Europe, as they come from the type locality.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Distilled water may be used, but prolonged soaking should be avoided. Due to the small size of the crystals, mechanical cleaning is risky. ## What to Avoid Avoid contact with acids and strong chemicals, which can damage the mineral. It should not be heated or subjected to sudden temperature changes. Although there is no specific data on light sensitivity, it is recommended to avoid prolonged exposure to direct sunlight to prevent potential fading. ## Storage It is recommended to store specimens in stable conditions, in sealed display boxes, to protect them from dust and mechanical damage. Due to the small and fragile crystals, avoid placing them in contact with harder minerals.