Magnesiostaurolite
Chemical formula: Mg(Mg,Li)₃(Al,Mg)₁₈Si₈O₄₄(OH)₄
Magnesiostaurolite is the magnesium analog of staurolite, a rare silicate mineral that forms prismatic crystals ranging in color from reddish-brown to almost black.
Properties
- Mohs hardness
- 7-7.5
- Color
- Colorless, light cloudy gray, tan
- Luster
- Vitreous to resinous
- Streak
- White
- Density
- 0
- Cleavage
- {010}
- Fracture
- Irregular/Uneven
- Transparency
- Transparent,Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Identification of magnesiostaurolite is primarily based on its crystal habit – prismatic, often forming penetration twins. Its high hardness (7 on the Mohs scale) and reddish-brown color are also characteristic. However, definitive identification requires advanced chemical analyses (e.g., EDS) to confirm the dominance of magnesium over iron. ## Distinguishing from Similar Minerals The most difficult task is distinguishing it from staurolite, which is visually impossible. Staurolite has practically identical physical properties and appearance. The difference lies in the chemical composition – staurolite is iron-dominant, while magnesiostaurolite is magnesium-dominant. It can also be confused with andalusite, which can be similar in form and color, but usually has a slightly lower density. ## Crystal Forms It forms columnar or prismatic crystals with a nearly hexagonal cross-section. The most characteristic are twins, forming crosses at 60° (St. Andrew's cross) or 90° (Greek cross), inherited from the related staurolite.
Geological environment
## Genesis Magnesiostaurolite is a mineral typical of metamorphic rocks that have undergone transformations under medium to high pressure and temperature conditions. It forms in magnesium- and aluminum-rich crystalline schists, gneisses, and amphibolites. ## Mineral Associations It coexists with minerals typical of metamorphic rocks, such as kyanite, garnet (especially pyrope), phlogopite, talc, chlorite, amphiboles (e.g., anthophyllite, gedrite), and quartz. ## Localities The most important and well-documented occurrences of magnesiostaurolite worldwide include the Asas region in the Mantiqit Mountains, Oman; the area around Pizzo Forno in the canton of Ticino, Switzerland; and some localities in Russia (Southern Urals) and the USA (Georgia and North Carolina states).
Rarity
Very rare
For collectors
## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp crystals, especially those that form perfect cross-shaped penetration twins. The size of the crystals and their setting on a contrasting rock matrix (e.g., on light schist) are also important. A dark, saturated color is desirable, as is the absence of mechanical damage. ## Popular Localities Specimens from Alpine localities in Switzerland (e.g., Pizzo Forno) and Oman are highly valued due to the quality of their crystals and their rarity.
Care and storage
## Cleaning Magnesiostaurolite specimens can be cleaned using a soft brush and distilled water. Ultrasonic cleaners should be avoided, as they can damage fragile crystals or specimens with inclusions. ## What to Avoid The mineral is sensitive to strong acids. Sudden temperature changes and prolonged exposure to direct sunlight should be avoided, although sunlight is not considered particularly harmful. ## Storage Specimens should be stored in separate boxes or on a soft surface to prevent scratching by harder minerals. Due to its brittleness, it should be protected from impacts and falls.