Sekaninaite
Chemical formula: Fe<sup>2+</sup><sub>2</sub>Al<sub>3</sub>(AlSi<sub>5</sub>)O<sub>18</sub>
A rare, iron-rich mineral from the beryl group, forming dark blue to black, tabular or prismatic crystals, being the iron analog of cordierite.
Properties
- Mohs hardness
- 7-7.5
- Luster
- Vitreous to greasy
- Streak
- Gray to grayish blue
- Density
- 2.71-2.77
- Cleavage
- Indistinct on {010}
- Fracture
- Uneven to conchoidal
- Transparency
- Translucent to opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Sekaninaite is most easily identified by its characteristic dark blue to black color, vitreous luster, and crystal habit – thick, tabular or short prisms with a pseudohexagonal outline. Hardness (7-7.5) is also an important diagnostic feature. ## Distinguishing from Similar Minerals Sekaninaite is visually almost impossible to distinguish from dark, iron-rich varieties of cordierite without advanced chemical analysis (determining the dominance of Fe over Mg). It can also be confused with tourmaline (schorl), from which it differs in crystal form (schorl has a typical triangular cross-section) and the absence of distinct striations on prism faces. It is distinguished from smoky quartz by its bluish tint and crystal habit. ## Crystal Forms Crystals are typically short, prismatic or thick tabular, often with a pseudohexagonal outline. They occur as single, well-formed crystals embedded in rock, in vugs, and also in granular aggregates.
Geological environment
## Genesis Sekaninaite forms under conditions of contact and regional metamorphism in aluminum- and iron-rich, alkali-poor rocks. It forms at high temperatures and relatively low pressures. It is most commonly found in hornfels, gneisses, granulites, and in pegmatites and aplites cutting these rocks. ## Mineral Associations This mineral often co-occurs with andalusite, sillimanite, garnets (especially almandine), hercynite, magnetite, ilmenite, plagioclase, and quartz. ## Localities The most important and classic localities, from which the best-formed crystals originate, are in the Czech Republic, near the town of Dolní Bory (where it was discovered). Other known localities include Asama in Japan (in volcanic products), pegmatites in Sweden (e.g., in the Tunaberg area), and some localities in Germany and Italy.
Rarity
Rare
For collectors
## Quality Criteria Specimens with well-formed, sharp-edged crystals of intense, dark blue or violet-blue color are most valued by collectors. Crystals with a distinct luster, free from mechanical damage, are highly prized. Specimens where sekaninaite crystals are aesthetically set on a contrasting rock matrix are particularly sought after. ## Popular Localities By far the most classic and desirable specimens come from the type locality in Dolní Bory, Czech Republic. Crystals from this location are considered exemplary for this mineral.
Care and storage
## Cleaning Sekaninaite specimens can be cleaned with lukewarm water, mild soap, and a soft brush. Ultrasonic cleaners should be avoided, as they can damage crystals, especially those with inclusions or internal fractures. ## What to Avoid The mineral is sensitive to strong acids, especially hydrofluoric acid. Avoid sudden temperature changes, which can lead to cracking. Despite its relatively high hardness, it is brittle and susceptible to mechanical damage upon impact. ## Storage Collector's specimens of sekaninaite are best stored in separate, padded boxes or display cases to prevent scratching by harder minerals (e.g., corundum, diamond) and damage to softer minerals. It is not sensitive to sunlight.