Yuanjiangite
Chemical formula: AuSn
Yuanjiangite is a very rare mineral, a natural gold-tin alloy, occurring as microscopic, metallic grains.
Description
## Characteristics Yuanjiangite is a natural alloy (intermetallic phase) of gold and tin. It occurs as very small, irregular or rounded grains, rarely exceeding 0.5 mm in size. Due to its microscopic size, its visual characteristics are difficult to observe without specialized equipment. It is an opaque mineral with a strong metallic luster. ## Physical Properties The Mohs hardness of yuanjiangite is approximately 4. The mineral is characterized by a very high density, theoretically 13.97 g/cm³, which is a direct result of the presence of gold in its composition. It is opaque, and its streak is black. ## Colors and Varieties Its color is described as tin-white to steel-gray. Due to its extreme rarity and microscopic occurrence, no color varieties or commercial forms have been distinguished. ## History and Name Yuanjiangite was first described in 1994 by Zuxiang Yu and colleagues. Its name comes from its discovery locality – it was found in placer deposits of the Yuan River (Yuanjiang) in Hunan Province, China. ## Applications Due to its extreme rarity, yuanjiangite has no industrial applications. Its significance is purely scientific and collectible, being an object of desire for specialized micromineral collectors.
Diagnostic features
## Identification Identification of yuanjiangite with the naked eye or a magnifying glass is impossible. Definitive identification requires advanced analytical techniques, such as chemical composition analysis using an electron microprobe (EDS/WDS), which confirms the stoichiometric ratio of gold to tin (AuSn). Under a reflected light microscope, it appears as a small, bright, metallic point. ## Distinguishing from Similar Minerals Visually, it is practically indistinguishable from many other rare platinum-group minerals (PGM), native gold, platinum, and other natural tin alloys (e.g., stistaite - SnSb). It can also be confused with fine industrial contaminants. The only certain method of distinction is chemical analysis. ## Crystal Forms It most often forms anhedral (irregular), rounded, or irregular grains. Very rarely, poorly developed crystals with hexagonal outlines are observed.
Geological environment
## Genesis Yuanjiangite is a mineral of magmatic origin, associated with ultramafic rocks. It primarily crystallizes in platinum-group element deposits, and then, as a result of weathering and erosion processes, it is released from the host rock and accumulates in detrital (placer) deposits in river sediments. ## Mineral Associations It co-occurs with platinum-group minerals (osmium, iridium, ruthenium), native gold, platinum, chromite, magnetite, and other rare alloys such as stistaite (SnSb) and zhanghengite (CuZn). ## Localities The most important confirmed occurrences of this mineral worldwide include: - Yuan River (Yuanjiang) in Hunan Province, China (type locality). - Placer deposits in the Miass River basin, Ural Mountains, Russia. - Ultramafic rock complex near the Tulameen River, British Columbia, Canada.
Rarity
Extremely rare
Collector aspects
## Quality Criteria For such a rare micromineral, the primary criterion for value is the certainty of its identification, confirmed by chemical analysis. Specimens with relatively large grains (for this mineral), exceeding 0.1-0.2 mm, or those showing outlines of crystallographic forms, are exceptionally prized. The presence of other rare associated minerals on the same fragment also increases its value. ## Popular Localities Specimens from the type locality in China are practically unavailable on the collector's market. The few samples that end up in specialized collections most often come from placer deposits in the Urals, Russia, or from Canada.
Care and storage
## Cleaning Yuanjiangite specimens are typically microscopic grains embedded in host rock or mounted in resin (so-called polished sections). Cleaning such specimens is inadvisable and practically impossible without the risk of damage. Any procedures should only be performed under laboratory conditions. Loose grains should be protected from dust. ## What to Avoid Avoid contact with chemicals, especially strong acids (e.g., aqua regia dissolves gold). Ultrasonic cleaning is absolutely forbidden, as it can cause delicate grains to dislodge from the rock matrix. High temperatures and mechanical abrasion should also be avoided. ## Storage Specimens containing yuanjiangite, most often in the form of micromounts, should be stored in stable, dry conditions, in special boxes that protect against dust and mechanical damage. Polished sections should be kept in containers that prevent surface scratching.