Bobfergusonite
Chemical formula: ☐Na<sub>2</sub>Mn<sup>2+</sup><sub>5</sub>Fe<sup>3+</sup>Al(PO<sub>4</sub>)<sub>6</sub>
Bobfergusonite is a rare phosphate mineral, forming dark green to black, prismatic crystals with a vitreous luster.
Properties
- Mohs hardness
- 4
- Luster
- Vitreous to resinous
- Streak
- Greenish-brown
- Density
- 3.65
- Cleavage
- Good on {010}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Bobfergusonite can be identified by its characteristic habit of long, prismatic crystals, often forming radial aggregates. Its dark green to black color, vitreous luster, and occurrence in granitic pegmatites are key features. It reacts with acids. ## Distinguishing from Similar Minerals It can be confused with other dark pegmatite minerals, such as arrojadite, wagnerite, or some tourmalines (schorl). Differentiation often requires analysis of crystal habit – the prismatic, elongated forms of bobfergusonite are quite typical. Final confirmation of identity often requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Crystals are elongated along the c-axis, forming prismatic shapes. They often occur as radial or fan-shaped aggregates, intergrown with surrounding minerals such as quartz or feldspars.
Geological environment
## Genesis Bobfergusonite is an igneous mineral, crystallizing in the late stages of the evolution of phosphorus-rich granitic pegmatites. It forms in complex zones of these pegmatites, where elements such as sodium, manganese, and phosphorus are concentrated. ## Mineral Associations This mineral co-occurs with other pegmatite-typical phosphates, such as fillowite, arrojadite-(KFe), triphylite, as well as with quartz, microcline, albite, and muscovite. ## Localities The most important and classic localities for this mineral are in Canada, in Manitoba province, in the Cross Lake area, where it was discovered. It is also known from pegmatites in North Carolina, USA (e.g., Foote mine), and from rare occurrences in other phosphate pegmatites worldwide.
Rarity
Rare
For collectors
## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp, and undamaged crystals with a distinct luster. Radial aggregates contrasting with a light rock matrix (e.g., with quartz or albite) enhance their attractiveness. Crystal size is also an important factor – specimens with crystals exceeding one centimeter in length are rare and sought after. ## Popular Localities Specimens from the type locality in Cross Lake, Manitoba (Canada) are historically the most significant. Specimens from the Foote mine in North Carolina (USA), which have yielded many high-quality examples of this mineral, are also found on the collector's market.
Care and storage
## Cleaning Bobfergusonite specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Ultrasonic cleaners should be avoided. ## What to Avoid The mineral is sensitive to acids and strong chemicals. It should be protected from sudden temperature changes and prolonged exposure to direct sunlight, which can affect its color. Due to its brittleness, impacts and falls should be avoided. ## Storage Collector specimens of bobfergusonite are best stored in individual, padded boxes or display cases, away from harder minerals that could scratch it. Stable humidity and temperature conditions are recommended.