Magnesiodumortierite
Chemical formula: MgAl<sub>6</sub>BSi<sub>3</sub>O<sub>17</sub>(OH)
Magnesiodumortierite is an extremely rare, magnesium analogue of dumortierite, forming blue, fibrous aggregates in metamorphic rocks.
Properties
- Mohs hardness
- 7
- Luster
- Vitreous
- Streak
- White
- Density
- 3.03
- Cleavage
- Good on {100}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Identification of magnesiodumortierite is based on its characteristic, intensely blue color, fibrous or radial aggregate form, and occurrence environment – aluminum-rich metamorphic rocks. Hardness (approximately 7) is also a helpful indicator. Definitive identification is only possible using advanced analytical methods, such as chemical analysis (EDS/WDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals This mineral is visually almost indistinguishable from dumortierite. However, dumortierite is much more common and has a slightly higher density. It is distinguished from blue corundum by its lower hardness and fibrous structure. Other blue minerals, such as kyanite, typically have a different crystal habit (bladed) and perfect cleavage. ## Crystal Forms Well-formed crystals are microscopic in size and have a prismatic habit. Macroscopically, the mineral forms radial or tangled fibrous aggregates, and also occurs as irregular grains in the host rock.
Geological environment
## Genesis Magnesiodumortierite forms under conditions of high-grade regional metamorphism, in rocks rich in aluminum, boron, and magnesium. It forms in gneisses and mica schists, often in the presence of quartz. ## Mineral Associations It most commonly co-occurs with quartz, muscovite, phlogopite, kyanite, sillimanite, and also with tourmalines (especially dravite). ## Localities This is an extremely rare mineral. It has been identified in several locations worldwide, the most important of which is the type locality in the Alps on the border of Austria and Italy (Zillertal and Val di Fleres). Its occurrence has also been confirmed in Vradal, Norway, and in small quantities in other metamorphic localities around the world.
Rarity
Extremely rare
For collectors
## Quality Criteria The most highly valued specimens are those in which magnesiodumortierite forms rich, intensely blue, radial aggregates clearly visible against the host rock (most often white quartz or gneiss). Color contrast and aggregate aesthetics are key. Specimen size is also important, although even small samples with well-defined aggregates are valuable. ## Popular Localities The most well-known and prized specimens come from the type locality in the Alps (Austria/Italy). Material from these localities is considered reference material and fetches the highest prices in the collector's market.
Care and storage
## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush to remove dust. Due to the extreme rarity and potential fragility of the aggregates, washing with water, and especially using ultrasonics, should be avoided. ## What to Avoid Avoid contact with all chemicals, including acids and bases. The mineral is stable, but should not be heated or exposed to sudden temperature changes. Prolonged exposure to strong sunlight is not recommended. ## Storage Magnesiodumortierite specimens, especially those with visible fibrous aggregates, should be stored in closed display boxes to protect them from dust and mechanical damage. They should be kept away from sources of moisture and direct sunlight.