Nybøite
Chemical formula: NaNa₂(Mg₃Al₂)(Si₇Al)O₂₂(OH)₂
Nybøite is a rare, prismatic sodium amphibole, found in eclogites, distinguished by its dark green to black color.
Properties
- Mohs hardness
- 5-6
- Color
- Grey-green; Blue-grey
- Luster
- Vitreous
- Streak
- Light gray
- Density
- 3.12
- Cleavage
- Perfect on {110}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Nybøite is identified by its dark green to black color, prismatic crystal habit, and the characteristic amphibole cleavage angle (approximately 120°). However, its specific occurrence environment – high-pressure metamorphic rocks such as eclogites – is crucial for identification. Final confirmation requires advanced chemical analyses (EDS/WDS) or X-ray diffraction (XRD). ## Distinguishing from similar minerals It can be confused with other dark amphiboles, such as hornblende, glaucophane, or arfvedsonite, as well as with pyroxenes (e.g., omphacite, aegirine), with which it often coexists. From glaucophane, it usually differs by a darker, more green than blue color. From hornblende and pyroxenes, it is mainly distinguished by its chemical composition and specific formation conditions. Precise differentiation in the field is very difficult and often impossible without laboratory tests. ## Crystal forms It forms columnar, prismatic crystals, often terminated by flat faces. It also occurs as granular, fibrous, or radial aggregates in the host rock.
Geological environment
## Genesis Nybøite is a metamorphic mineral, typical of rocks subjected to very high pressure and moderate to high temperatures. It forms in the eclogite facies and blueschist facies. It is a product of sodium metamorphism (metasomatism) of ultramafic and mafic rocks. ## Mineral associations It most commonly coexists with minerals typical of eclogites and related rocks, such as omphacite, garnet (pyrope, almandine), glaucophane, paragonite, kyanite, talc, dolomite, quartz, rutile, and titanite. ## Localities The most important and classic locality is the Nordfjord region in Norway (including the Nybø and Årsheim farms). It is also found in other places worldwide with similar geology, such as the Western Alps (Italy, France), the Dora Maira massif in Italy, Guatemala, New Caledonia, and China (Jiangsu province).
Rarity
Very rare
For collectors
## Quality criteria The most prized specimens by collectors are those with well-formed, sharp, and lustrous nybøite crystals, clearly contrasting with the host rock. Specimens where dark green or black prisms contrast with a lighter background, such as white quartz or pink garnet, are desirable. Crystal size is a key factor – specimens with crystals exceeding one centimeter in length are exceptionally rare and sought after. ## Popular localities By far the most classic and valued specimens come from the type locality in Norway (Nybø, Nordfjord). Specimens from the Alps, especially from Italy, are also sometimes available on the collector's market, although they often appear as microscopic aggregates.
Care and storage
## Cleaning Nybøite specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. If necessary, compressed air can be used. Due to its brittleness, ultrasonic cleaners should be avoided. ## What to avoid Contact with acids and strong chemicals, which can damage the mineral's surface, should be avoided. It should not be heated or exposed to rapid temperature changes. Although not particularly light-sensitive, prolonged exposure to strong sunlight is not recommended. ## Storage Nybøite specimens, especially those with well-formed crystals, should be stored in separate boxes or on a soft surface to prevent mechanical damage and abrasions. Due to its brittleness, it should be protected from impacts and falls.