Ferri-kaersutite
Chemical formula: NaCa<sub>2</sub>(Mg<sub>3</sub>Fe<sup>3+</sup>Ti<sup>4+</sup>)(Si<sub>6</sub>Al<sub>2</sub>)O<sub>22</sub>O<sub>2</sub>
Ferri-kaersutite is a rare, dark brown or black sodium-calcium amphibole, distinguished by the presence of titanium and trivalent iron.
Properties
- Mohs hardness
- 5-6
- Luster
- Vitreous
- Streak
- Light brown
- Density
- 3.23
- Cleavage
- Perfect on {110}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Ferri-kaersutite is identified by its dark color (brown to black), vitreous luster, and columnar crystal habit. A key diagnostic feature, typical of amphiboles, is the approximately 120° angle between cleavage planes, visible in cross-sections of crystals. However, definitive identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS), to confirm the dominance of Fe³⁺ over Fe²⁺ and the appropriate content of titanium, sodium, and calcium. ## Differentiation from Similar Minerals It can be confused with other dark amphiboles, such as hornblende, arfvedsonite, or riebeckite, as well as with pyroxenes (e.g., augite). It is distinguished from pyroxenes by its cleavage angle (for pyroxenes, it is approximately 90°). Differentiation from other amphiboles based on visual characteristics is practically impossible and requires specialized chemical analysis. ## Crystal Forms It most commonly forms well-developed, prismatic (columnar) crystals. It also occurs as granular, fibrous aggregates or as embedded grains in the host rock.
Geological environment
## Genesis Ferri-kaersutite is an igneous mineral, crystallizing under conditions of high temperature and low pressure. Its formation is associated with alkaline extrusive rocks, such as trachytes, phonolites, and basalts. The presence of titanium and a high oxidation state of iron indicate specific chemical conditions in the magma. ## Mineral Associations It co-occurs with minerals typical of alkaline rocks, such as sanidine, nepheline, leucite, augite, magnetite, ilmenite, and other sodium amphiboles and pyroxenes. ## Localities Confirmed occurrences of ferri-kaersutite are rare. It has been identified, among others, in the Laacher See volcanic complex in the Eifel mountains (Germany), in the Mont-Dore massif in Auvergne (France), in the Canary Islands (Spain), and in some localities in Italy (e.g., Vesuvius).
Rarity
Very rare
For collectors
## Quality Criteria For collectors, the most valuable specimens are those with well-formed, sharply terminated crystals with a distinct luster. Crystals embedded in a contrasting rock matrix, such as light trachyte, are particularly prized. Crystal size is also an important factor – larger, undamaged prisms are much rarer and more desirable. Twinned crystals and aggregates also enhance the specimen's appeal.
Care and storage
## Cleaning Ferri-kaersutite specimens can be cleaned with a soft, dry brush to remove dust. If necessary, 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 strong acids and chemicals, which can react with the mineral and damage its surface. It should not be subjected to sudden temperature changes or cleaned with ultrasonic cleaners, as internal stresses may cause fracturing along cleavage planes. ## Storage Store in stable conditions, away from moisture and direct sunlight, which, however, does not significantly affect its color. Due to its hardness (5-6), it is relatively scratch-resistant, but should be stored separately from harder minerals (e.g., quartz, corundum) to avoid damage.