Magnesiohulsite
Chemical formula: Mg₂Fe³⁺O₂(BO₃)
Magnesiohulsite is a rare, black, magnesium-rich borate from the hulsit group, forming prismatic crystals and radial aggregates.
Properties
- Mohs hardness
- 6
- Color
- Black, bluish-grey, greyish white in reflect light.
- Luster
- Metallic
- Streak
- dark brown
- Density
- 4.18
- Cleavage
- Good on {010}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Magnesiohulsite is identified by its black color, metallic luster, prismatic crystal habit, and radial aggregates. Its occurrence in contact-metasomatic rocks (skarns) is also characteristic. However, final identification requires chemical analysis (e.g., EDS) to confirm the dominance of magnesium over iron. ## Distinguishing from Similar Minerals Magnesiohulsite is visually indistinguishable from paigeite and hulsite, with which it forms isomorphic series. Paigeite is an iron-dominant (Fe²⁺) member of the series, and hulsite contains tin. Identification requires advanced analytical methods. It can also be confused with tourmaline (schorl), which, however, has a typical triangular crystal cross-section and is harder (7-7.5 on the Mohs scale). ## Crystal Forms Crystals are elongated, prismatic, often with longitudinal striations on the faces. They usually form radial, stellate, or haphazardly tangled aggregates. Less commonly, they occur as fibrous masses or granular aggregates.
Geological environment
## Genesis Magnesiohulsite is a mineral formed under contact metasomatism conditions. It forms in magnesian skarns, which developed at the contact of granitoid intrusions with carbonate rocks, such as dolomites. It is a product of reactions between boron-rich solutions and magnesium-rich rocks. ## Mineral Associations This mineral coexists with other borates and skarn minerals. It is most commonly associated with kotoite, ludwigite, szaibelyite, magnetite, chondrodite, clinohumite, spinel, as well as calcite and dolomite. ## Localities The most important and classic localities for magnesiohulsite are in Russia, Yakutia (Sakha Republic), especially in the boron deposits in the Talgin Mountains and on the Tas-Khayakhtakh plateau. It is also known from Brooks Mountain on the Seward Peninsula in Alaska (USA) and from the Hol Kol mine in North Korea.
Rarity
Rare
For collectors
## Quality Criteria For collectors, the most valuable specimens are those with well-formed, sharp crystals forming aesthetic, radial aggregates. Contrast with a light background (e.g., white calcite) significantly enhances the visual appeal of the specimen. Crystal size is also an important factor, although they rarely exceed a few millimeters in length. ## Popular Localities The most prized specimens, considered classic for this mineral, come from Yakutia, Russia. Specimens from Alaska are also sought after by collectors specializing in skarn minerals and rare borates.
Care and storage
## Cleaning It is recommended to clean specimens only with a soft brush to remove dust. The use of water, especially with detergents, is inadvisable due to the risk of reaction with associated minerals and potential damage to delicate aggregates. ## What to Avoid Avoid contact with acids and other chemicals that may damage the mineral. Do not subject it to ultrasonic or steam cleaning. Prolonged exposure to moisture can be harmful. ## Storage Magnesiohulsite specimens, especially those with delicate, acicular crystals, should be stored in closed display boxes to protect them from dust and mechanical damage. It is not sensitive to light.