Bixbyite-(Fe)
Chemical formula: Fe<sup>3+</sup><sub>2</sub>O<sub>3</sub>
Bixbyite is a manganese and iron oxide, forming characteristic black, metallic-lustered cubic crystals.
Description
## Characteristics Bixbyite is a manganese and iron oxide, known for forming exceptionally well-developed, razor-sharp crystals. They most often take the form of perfect cubes with a strong, metallic luster and an intense black color. Crystal faces can be smooth and shiny, less often dull or slightly corroded. Specimens often consist of single cubes grown on the matrix rock or their groupings. The name Bixbyite-(Fe) is a historical synonym, and modern mineralogy uses the name Bixbyite, indicating the dominant role of manganese in its chemical composition. ## Physical Properties This mineral is relatively hard and heavy. Its Mohs hardness is 6 to 6.5, allowing it to scratch glass. It is completely opaque, and its luster is described as metallic or submetallic. The density ranges from 4.9-5.0 g/cm³, which makes specimens noticeably heavy for their size. ## Colors and Varieties Bixbyite is monochromatic – it occurs exclusively in black. It does not form color varieties or commercial varieties. Differences in the appearance of specimens result mainly from the crystal habit, size, and luster. ## History and Name The mineral was described in 1897 by Samuel L. Penfield and Horace W. Foote. They named it after the American mineral collector and dealer, Maynard Bixby (1853-1935), who discovered the first specimens in the Thomas Range, Utah, USA. ## Uses Bixbyite has no industrial application. However, it is a highly valued and sought-after collector's mineral, especially due to its perfect, cubic crystals.
Diagnostic features
## Identification The key diagnostic feature of bixbyite is its crystal habit – almost perfect, black, lustrous cubes. Identification is aided by its high hardness (6-6.5 on the Mohs scale), black streak, and characteristic occurrence environment in rhyolites, often in association with topaz. ## Distinguishing from Similar Minerals Bixbyite is sometimes confused with several other black minerals: - **Magnetite**: Easiest to distinguish with a magnet – magnetite is strongly magnetic, while bixbyite shows no magnetism. - **Pyrite**: Pyrite has a lighter, greenish-black streak and often striated cube faces, whereas bixbyite has a black streak and usually smooth faces. - **Galena**: Galena is much softer (hardness approx. 2.5), much heavier (density ~7.6 g/cm³), and has perfect cleavage in three directions, breaking into cubic fragments. ## Crystal Habits Bixbyite crystallizes in the isometric system, mainly forming simple cubes. Crystals can be modified by rhombic dodecahedron or octahedron faces, leading to forms with truncated corners and edges. It occurs as single crystals, inclusions in rock, or in irregular granular aggregates.
Geological environment
## Genesis Bixbyite forms under high-temperature and low-pressure conditions. Its most famous localities are associated with vugs (lithophysae) in volcanic siliceous rocks, such as rhyolite. It crystallizes there from hot gases rich in manganese, iron, and fluorine in a process called pneumatolysis. It is less commonly found in metamorphosed manganese deposits. ## Mineral Associations Minerals typically associated with bixbyite in rhyolites include topaz (colorless, sherry), red beryl (bixbite), pseudobrookite, hematite, spessartine, smoky quartz, and sanidine. ## Localities The most important and well-known bixbyite localities in the world are in the USA, in Utah. These include: - **Thomas Range, Juab County, Utah, USA** - the type locality, where the mineral was discovered. It occurs there in rhyolite with topaz. - **Wah Wah Mountains, Beaver County, Utah, USA** - this is where the largest and most perfect bixbyite crystals in the world come from, often in paragenesis with the extremely rare red beryl. Other known occurrences include Black Range in New Mexico (USA), the vicinity of San Luis Potosí in Mexico, and Patagonia in Argentina.
Rarity
Rare
Collector aspects
## Quality Criteria The most highly valued bixbyite specimens are those with sharp, undamaged crystals with a strong, metallic luster. Key factors influencing value are: - **Crystal size**: The larger the cube, the more valuable the specimen. - **Luster and habit**: Intensely lustrous, perfectly formed cubes are the most desirable. - **Matrix**: Specimens where black crystals contrast with light rhyolite rock are very visually appealing. - **Associations**: The combination of bixbyite with other minerals, especially topaz, and above all with red beryl, drastically increases the value and prestige of the specimen. ## Popular Localities Collectors worldwide most desire specimens from the Wah Wah Mountains in Utah, which are considered the source of the best bixbyites. Specimens from the classic locality in the Thomas Range are also valued, especially those from historical collections.
Care and storage
## Cleaning Bixbyite specimens are best cleaned mechanically using a soft brush and lukewarm water. A small amount of neutral soap can be used, followed by thorough rinsing of the specimen with distilled water. Ultrasonic cleaners are generally safe for isolated crystals but can damage delicate matrix rock. ## What to Avoid Avoid contact with strong acids, which can damage the crystal surface. Despite significant hardness, the sharp edges and corners of the cubes are susceptible to chipping and bruising, so specimens should be protected from impacts and friction. ## Storage Bixbyite is a stable mineral and does not require special conditions. It should be stored in a way that protects it from dust and mechanical damage, preferably in separate boxes or display cases, away from harder minerals that could scratch it.