Vonsenite
Chemical formula: Fe<sup>2+</sup><sub>2</sub>Fe<sup>3+</sup>O<sub>2</sub>(BO<sub>3</sub>)
Black, almost opaque iron borate, forming fibrous, radial, or massive aggregates, found in metamorphic rocks.
Properties
- Mohs hardness
- 5
- Luster
- Metallic
- Streak
- Black, greenish-black
- Density
- 4.21-4.27
- Cleavage
- Good on {010}, poor on {100}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Characteristic features of vonsenite include its black color, metallic or submetallic luster, and frequent occurrence in fibrous and radial aggregates. It is relatively hard (5 on the Mohs scale) and dense. In field conditions, its identification without specialized equipment is difficult. ## Distinguishing from Similar Minerals Vonsenite is visually very similar to ludwigite, with which it forms an isomorphic series. Distinguishing these two minerals is practically only possible through chemical analysis (determining the iron to magnesium ratio). It can also be confused with tourmaline (schorl) or ilvaite. Tourmaline is usually harder and has a different crystal habit, while ilvaite often exhibits a more distinct fracture. ## Crystal Forms Vonsenite crystals are rare. They usually have a prismatic, columnar, or acicular habit and are elongated along the crystallographic c-axis. Most often, it forms fibrous, radial aggregates, as well as compact and massive aggregates.
Geological environment
## Genesis Vonsenite is a mineral formed under conditions of contact metamorphism. It forms in magnesian skarns, at the contact of granite or granodiorite intrusions with iron-rich sedimentary rocks, such as dolomites. It forms in a boron-rich environment. ## Mineral Associations This mineral often co-occurs with other minerals typical of skarn zones. Its most common associations include magnetite, pyrrhotite, diopside, as well as other borates like ludwigite, kotoite, and szaibelyite. ## Localities The most important and classic locality for vonsenite, from which type material originates, is the Old City Quarry in Riverside, California (USA). Other known localities include Jayville, New York (USA), Arendal, Norway, the Baita Bihor mine (formerly Rézbánya) in Romania, as well as numerous occurrences in Siberia, Russia (e.g., in Yakutia).
Rarity
Not very common
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, though rare, crystals with distinct luster. Aesthetic, radial fibrous aggregates are also highly prized, especially if they contrast with a light matrix rock. The size of the specimen and the richness of vonsenite aggregates also increase its value. ## Popular Localities Specimens from the type locality in Riverside, California, have historical value. Visually attractive, radial aggregates often come from Jayville, USA, and from some localities in Russia.
Care and storage
## Cleaning Vonsenite specimens can be cleaned with a soft, dry brush to remove dust. For heavier soiling, distilled water may be used, but prolonged soaking should be avoided. After wet cleaning, the specimen should be thoroughly dried. ## What to Avoid Avoid contact with acids and strong chemicals, which can react with the mineral. Vonsenite is brittle, so it must be protected from impacts and falls. It is not sensitive to light, but extreme temperature changes should be avoided. ## Storage Collector's specimens of vonsenite are best stored in individual, padded boxes or display cases to prevent abrasion and mechanical damage. Due to its brittleness, it should not be stored loosely with harder minerals.