Andrémeyerite
Chemical formula: BaFe<sup>2+</sup><sub>2</sub>Si<sub>2</sub>O<sub>7</sub>
Andrémeyerite is a rare barium iron silicate, forming elongated, prismatic crystals of dark green to black color.
Properties
- Mohs hardness
- 5-5.5
- Luster
- Vitreous
- Streak
- Pale green
- Density
- 4.12
- Cleavage
- Good on {100}
- Fracture
- Uneven
- Transparency
- Translucent to Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Andrémeyerite can be identified by its characteristic habit – elongated, prismatic crystals, often forming radial aggregates. Key features also include its dark green to black color, vitreous luster, and high density. Its occurrence in a specific geological environment (volcanic rocks) is also an important clue. ## Distinguishing from Similar Minerals This mineral can be confused with other dark silicates, such as aegirine, hornblende, or pyroxenes. Distinguishing it often requires advanced research methods, such as chemical analysis (EDS) to confirm the presence of barium, which is absent in the aforementioned minerals. Crystal habit and mineral paragenesis are helpful in visual identification. ## Crystal Forms Andrémeyerite crystals are elongated along the crystallographic b-axis, forming prismatic and acicular habits. They often occur as radial or fan-shaped aggregates, embedded in the host rock.
Geological environment
## Genesis Andrémeyerite is a mineral of volcanic origin. It forms in vesicles and cavities in melilite lavas, which are silica-undersaturated volcanic rocks. It crystallizes in the late stages of volcanic activity from barium- and iron-rich hydrothermal solutions or volcanic gases. ## Mineral Associations This mineral co-occurs with other rare minerals formed under similar conditions. The most common associations include: melilite, kirschsteinite, nepheline, kalsilite, götzenite, clinopyroxene, magnetite, and volcanic glass. ## Localities The most important and type locality for andrémeyerite is the Nyiragongo volcano in the Democratic Republic of Congo. This is the only locality from which well-formed, collectible specimens of this mineral originate. It has also been identified in the Ettringer Bellerberg volcanic complex in the Eifel Mountains, Germany, but specimens from there are usually microscopic in size.
Rarity
Very rare
For collectors
## Quality Criteria The most prized andrémeyerite specimens are those with well-formed, sharply terminated crystals of intense, dark green color. Radial aggregates, where individual crystals are clearly visible and undamaged, are highly valued. Contrast with a light host rock (matrix) also significantly enhances the aesthetic and collector's value of the specimen. Crystal size is a key factor – specimens with crystals exceeding a few millimeters are already considered exceptional. ## Popular Localities The only source of valuable collectible specimens is the Nyiragongo volcano in the Democratic Republic of Congo. Specimens from this locality are considered classic and most sought after by collectors of rare minerals.
Care and storage
## Cleaning Andrémeyerite 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 cleaning, the specimen should be thoroughly dried. ## What to Avoid Avoid contact with acids and strong chemical agents, which can damage the mineral's surface. Andrémeyerite is sensitive to sudden temperature changes. It should not be exposed to prolonged direct sunlight, which can cause it to fade. ## Storage It is recommended to store specimens in separate, padded boxes or display cases to prevent scratches and mechanical damage. Protect it from moisture and dust. The best conditions are stable temperature and low air humidity.