Omeiite
Chemical formula: Os³⁺(As₂)³⁻
Omeiite is an extremely rare osmium arsenide, occurring as tiny, metallic grains in ultramafic igneous rocks.
Properties
- Mohs hardness
- 7
- Color
- Dull steel gray
- Luster
- Metallic
- Streak
- Black
- Density
- 0
- Cleavage
- Parallel to elongation
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Amateur identification of omeiite is practically impossible due to its microscopic size and resemblance to other platinum-group minerals (PGM). Definitive identification requires advanced analytical methods, such as electron microscopy with chemical composition analysis (EDS/WDS) or X-ray diffraction (XRD). In a hand sample, it might be suspected as tiny, metallic inclusions in chromitite or ultramafic rocks. ## Distinguishing from Similar Minerals Omeiite is visually indistinguishable from many other arsenides and sulfides of platinum-group metals, such as sperrylite, irarsite, or osarsite, with which it often co-occurs. Key to differentiation is the confirmation of the dominance of osmium and arsenic in its chemical composition. ## Crystal Forms It most often occurs as anhedral (irregular) or subhedral (poorly formed) grains. Well-formed, microscopic crystals with a regular habit, taking the form of cubes and octahedra, are rarely observed.
Geological environment
## Genesis Omeiite is a mineral of magmatic origin. It crystallizes in ultramafic rock intrusions, particularly in dunites and peridotites, where it concentrates in chromite deposits (chromitites). It forms during magmatic segregation processes, crystallizing from residual melts rich in platinum-group elements (PGE). ## Mineral Associations This mineral co-occurs with other platinum-group minerals, as well as sulfides and arsenides. Its typical associated minerals include: chromite, irarsite, laurite, osarsite, iridosmine, rutheniridosmine, sperrylite, as well as native osmium, iridium, and platinum. ## Localities The most important and classic locality, also being the place of discovery, is the chromite deposits in ultramafic rocks in the Mount Emei region of Sichuan Province, China. This mineral has also been identified in the Bushveld Complex in South Africa, in the Onverwacht deposit in former Transvaal, as well as in alluvial sediments in Colombia and in the Urals in Russia.
Rarity
Very rare
For collectors
## Quality Criteria The quality of an omeiite specimen is determined almost exclusively by its scientific value and confirmed identity. Since the mineral occurs as microscopic grains, its "visual appeal" is not a typical criterion. The most valuable specimens are those in which omeiite grains are relatively large (for this mineral, i.e., fractions of a millimeter), well-separated from the matrix, and, most importantly, have a chemical composition confirmed by analysis. A specimen from the type locality (Mount Emei) is more highly valued than those from other localities. ## Popular Localities The only source of specimens that occasionally appear on the collector's market is the type locality in China. Specimens from other places, such as South Africa or Russia, are extremely rare and are mainly found in institutional collections.
Care and storage
## Cleaning Omeiite specimens, due to their microscopic size and occurrence within a rock matrix, generally do not require cleaning. If necessary, compressed air can be used to remove dust. Contact with water and chemicals should be avoided. ## What to Avoid Avoid contact with acids and other aggressive chemicals that may react with arsenides. Despite its high hardness, the mineral is brittle, so it should be protected from impacts and scratches. It should not be subjected to ultrasound. ## Storage Specimens containing omeiite should be stored under stable, dry conditions, preferably in a sealed "micromount" box, to protect them from dust and mechanical damage. Due to its arsenic content, it should be kept out of reach of children, and inhalation of dust that might arise from sample damage should be avoided.