Ferri-winchite
Chemical formula: ☐NaCa(Mg<sub>4</sub>Fe<sup>3+</sup>)Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>
A rare amphibole from the winchite group, characterized by the presence of trivalent iron, found in metamorphic rocks.
Properties
- Mohs hardness
- 5-6
- Luster
- Vitreous, Silky
- Streak
- Pale green
- Density
- 3.16
- Cleavage
- Good on {110}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Identifying ferri-winchite in the field is very difficult and usually impossible without advanced analytical techniques. In a collection, it can be tentatively recognized by its prismatic or fibrous appearance, green to blue-green color, vitreous luster, and association with other metamorphic minerals. Characteristic is the angle between the prism faces, approximately 120 degrees, typical for amphiboles. ## Distinguishing from Similar Minerals Ferri-winchite is visually indistinguishable from other dark amphiboles, such as hornblende, actinolite, riebeckite, or other minerals from the winchite group (e.g., winchite, ferro-winchite). Definitive differentiation requires chemical analysis (e.g., electron microprobe) to determine the exact composition, especially the Mg to Fe²⁺ and Fe³⁺ ratio, and the content of sodium and calcium. ## Crystal Forms It most often forms elongated prismatic crystals, often gathered into fan-shaped or radial aggregates. It also occurs as fibrous, acicular aggregates or as anhedral grains within the host rock.
Geological environment
## Genesis Ferri-winchite is a mineral typical of rocks that have undergone metamorphism under greenschist facies, glaucophane schist facies, or amphibolite facies conditions. It forms in environments with relatively low temperature and variable pressure, rich in sodium and iron in a higher oxidation state. It is a product of regional or contact metamorphism. ## Mineral Associations It often co-occurs with other metamorphic minerals such as albite, epidote, chlorite, quartz, calcite, talc, hematite, magnetite, and other amphiboles (e.g., actinolite, glaucophane). ## Localities As a rare mineral, ferri-winchite is known from only a few confirmed localities worldwide. The type locality, from which it was first described, is the Kajlidongri mine in the Jhabua district, Madhya Pradesh state, India. Other known occurrences include the St. Marcel area in Italy and some iron ore deposits in Sweden.
Rarity
Rare
For collectors
## Quality Criteria The collector's appeal of ferri-winchite primarily depends on well-formed crystals and their size. Most desirable are specimens with distinct, sharply terminated prisms of intense green or blue-green color. Aesthetic radial or fan-shaped aggregates are also valuable, especially when set against a contrasting matrix rock. The absence of mechanical damage is important. ## Popular Localities The most classic and highly prized specimens by collectors come from the type locality at the Kajlidongri mine in India. Most of the reference material and specimens of the best crystallographic quality originate from there.
Care and storage
## Cleaning Specimens should be cleaned carefully, using a soft, dry brush to remove dust. For heavier soiling, a damp cloth or a brush dipped in distilled water can be used, followed by gentle drying of the specimen. Ultrasonic cleaners should be avoided, as they can damage fragile crystals or fibrous aggregates. ## What to Avoid Avoid contact with strong acids and bases, which can react with the mineral. Do not subject it to sudden temperature changes. Although not particularly light-sensitive, prolonged exposure to direct sunlight is not recommended for any collector's specimens. ## Storage Ferri-winchite specimens, especially those with a delicate, fibrous structure, should be stored in separate boxes or on a soft surface to prevent mechanical damage and abrasion. Protect from dust and moisture. It is best to store them under stable room conditions.