Magnesio-riebeckite
Chemical formula: ☐Na<sub>2</sub>(Mg<sub>3</sub>Fe<sup>3+</sup><sub>2</sub>)Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>
A magnesium-iron amphibole of the sodium group, forming fibrous, blue or black crystals, an important indicator of metamorphic rocks.
Properties
- Mohs hardness
- 5-6
- Luster
- Vitreous, Silky
- Streak
- Grayish blue
- Density
- 3.06-3.22
- Cleavage
- Perfect on {110}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Magnesioriebeckite can be identified by its characteristic dark blue to black color, elongated, often fibrous crystal habit, and typical amphibole cleavage. The luster can be vitreous on crystal faces and silky in fibrous aggregates. ## Distinguishing from Similar Minerals It can be confused with riebeckite, from which it is practically impossible to distinguish macroscopically without advanced chemical analyses (EDS, WDS). It differs from riebeckite by its higher magnesium content. From other dark amphiboles, such as hornblende, it is usually distinguished by a more intense, bluish color. Tourmaline (schorl) has a similar color but is harder and has a different, usually triangular crystal cross-section. ## Crystal Forms Crystals are prismatic, columnar, acicular, or fibrous. They often form felted, radial, or chaotically arranged aggregates. Well-formed, free-standing crystals are rare.
Geological environment
## Genesis Magnesioriebeckite is a mineral characteristic of low-grade metamorphic rocks that have undergone transformations under high-pressure and relatively low-temperature conditions (glaucophane schist facies metamorphism). It also forms in alkaline igneous rocks, such as syenites and granites, as well as in some banded iron formations (BIF). ## Mineral Associations It most commonly co-occurs with other sodium amphiboles (riebeckite, glaucophane, crossite), as well as with albite, aegirine, quartz, microcline, magnetite, hematite, and stilpnomelane. ## Localities Significant occurrences of magnesioriebeckite have been reported in many places worldwide. It is found in iron deposits in the Mesabi Range in Minnesota (USA), in the Ilímaussaq igneous complex in Greenland, in the Khibiny Massif on the Kola Peninsula (Russia), and also in the vicinity of Broken Hill in New South Wales (Australia). It is also found in the Alps, including Switzerland and Italy.
Rarity
Not very common
For collectors
## Quality Criteria Specimens with well-formed, large crystals of intense blue color are most valued by collectors. Color contrasts, such as blue magnesioriebeckite needles on a light, quartz or albite matrix, are also attractive. Aesthetic, radial aggregates are also sought after. The purity of the specimen and the absence of mechanical damage significantly increase its value. ## Popular Localities Good quality specimens come, among others, from the mountainous regions of Bolivia, where they form visually attractive aggregates. Classic localities, such as Greenland or the Kola Peninsula, provide scientifically important material that also finds its way into specialized collections.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. For heavier soiling, compressed air can be used. Avoid washing in water, especially for delicate, fibrous aggregates, which can be mechanically damaged. ## What to Avoid Avoid contact with acids and strong chemical agents that can react with the mineral. Brittle and fibrous forms are prone to breakage and therefore require delicate handling. Do not expose specimens to sudden temperature changes. ## Storage Specimens, especially those with a fibrous structure, are best stored in closed collector's boxes that protect against dust and mechanical damage. Avoid placing heavier minerals on fragile magnesioriebeckite aggregates.