Bixbyite-(Mn)

Chemical formula: Mn₃+₂O₃

Bixbyite-(Mn) is a rare manganese oxide, forming characteristic, metallic-lustered, black crystals with a regular, cubic habit.

## Characteristics Bixbyite-(Mn), formerly known simply as bixbyite, is a manganese and iron oxide. Its most recognizable features are perfectly formed, razor-sharp cubic crystals, often modified at the corners. They are usually opaque and have an intense, black color with a strong, metallic or submetallic luster. Crystals can occur individually, embedded in the host rock (most often rhyolite), or form penetration twins, where several cubes interpenetrate each other. Crystal surfaces are sometimes smooth and mirror-like, though occasionally covered with fine growth striations. ## Physical Properties This mineral is relatively hard, which allows it to maintain sharp crystal edges. Its Mohs hardness ranges from 6 to 6.5. It is brittle, and its density is significant, approximately 4.95 g/cm³, which is noticeable even when holding a small specimen. Luster is one of its most characteristic features – strongly metallic or submetallic, resembling polished metal. ## Colors and Varieties Bixbyite-(Mn) is a mineral of constant color – always black. It does not form color varieties. The name "bixbyite" is sometimes confused with "bixbite," which is a commercial name for red beryl (emerald), which is an error – these are two completely different minerals. The official name "Bixbyite-(Mn)" was introduced to distinguish it from potential, hypothetical minerals of the same structural group in which an element other than manganese would dominate. ## History and Name The mineral was first described in 1897 by S. L. Penfield and H. W. Foote. They named it after Maynard Bixby (1853-1935), an American prospector and mineral dealer, who first provided samples for study. The specimens came from Thomas Range in Juab County, Utah, USA. In 2021, the name was modified to Bixbyite-(Mn) by the International Mineralogical Association (IMA) to precisely indicate manganese as the dominant element in its structure. ## Uses Bixbyite-(Mn) has no industrial applications. It is valued solely as a collector's stone due to its rarity, aesthetics, and perfectly formed crystals.

Properties

Mohs hardness: 6-6.5
Luster: Metallic, Sub-Metallic
Streak: Black
Density: 4.95
Cleavage: Poor/Indistinct
Fracture: Uneven
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification The key diagnostic feature of Bixbyite-(Mn) is its crystal habit – almost perfect cubes with sharp edges. Other important features include its black color, strong metallic or submetallic luster, and high hardness. It often occurs in characteristic, light-colored rhyolite. The streak is black or brownish-black. ## Distinguishing from Similar Minerals Bixbyite-(Mn) can be confused with other black, metallic minerals forming cubic crystals. - **Magnetite**: It is strongly magnetic, whereas Bixbyite-(Mn) shows no magnetism. - **Galena**: It has a much lower hardness (about 2.5 on the Mohs scale) and is much heavier (higher density). It also has perfect cleavage in three directions, forming cubic fragments upon impact. - **Pyrite**: Usually has a brass-yellow color, not black, although it can be iridescent. Pyrite also has a greenish-black streak. ## Crystal Forms Bixbyite-(Mn) crystallizes in the isometric system, primarily forming cubes. These crystals are often modified by small faces of the rhombic dodecahedron or octahedron at the corners. It often forms penetration twins, where two or more cubic crystals interpenetrate each other, creating aesthetic, complex groups.

Geological environment

## Genesis Bixbyite-(Mn) is a high-temperature mineral. It forms under pneumatolytic conditions, crystallizing from manganese-rich vapors and gases in vugs and fissures of acidic volcanic rocks, such as rhyolites. It forms at temperatures around 450-500°C in a low-pressure environment. ## Mineral Associations It most commonly co-occurs with minerals typical of its formation environment. In rhyolites from Thomas Range, Utah, its typical associated minerals include topaz (especially sherry-colored), red beryl (bixbite), pseudobrookite, hematite, and quartz (in amethyst or smoky varieties). ## Localities The most important and world-renowned locality, providing the best specimens, is Thomas Range in Juab County, Utah, USA. This is the type locality for this mineral. Other significant occurrences include Black Range in New Mexico (USA) and San Luis Potosí state in Mexico. It has also been reported in Patagonia, Argentina.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized Bixbyite-(Mn) specimens are characterized by sharp, well-defined crystals with a mirror-like, metallic luster. An ideal specimen is a single, large cube or an aesthetic penetration twin, contrastingly embedded in a light rhyolitic matrix. Crystal size is a key factor – specimens with crystals exceeding 1 cm are already considered exceptional. The absence of damage, high luster, and aesthetic composition with other minerals (e.g., topaz) significantly increase collectible value. ## Popular Localities By far the most sought-after specimens by collectors come from the classic locality in Thomas Range, Utah, USA. This locality, particularly the Maynard's Claim area (named after the discoverer), has yielded the best known specimens of this mineral and is considered the global standard for Bixbyite-(Mn).

Care and storage

## Cleaning Bixbyite-(Mn) specimens are best cleaned dry using a soft brush or compressed air to remove dust. If wet cleaning is necessary, use only distilled water and a very soft brush, then dry the specimen thoroughly. Caution should be exercised if the crystals are embedded in a fragile, rhyolitic matrix. ## What to Avoid Avoid contact with acids and strong chemical agents, which can damage the crystal surfaces. The mineral is brittle, so protect it from impacts and falls that could cause chipping or fractures. It does not require special protection from light or temperature, but extreme thermal shocks should be avoided. ## Storage Bixbyite-(Mn) specimens are best stored in individual, padded boxes or display cases to prevent abrasions and mechanical damage. Due to the fragility of the rock matrix in which it often occurs, avoid placing heavy objects on the specimen.

Sources

IMA Mineral List (CNMNC)