Indium
Chemical formula: In
Indium is a rare, soft, silvery-white metal, occurring in nature mainly as microscopic inclusions in zinc ores.
Properties
- Mohs hardness
- 1.2
- Luster
- Metallic
- Streak
- Silvery white
- Density
- 7.31
- Cleavage
- None
- Fracture
- Hackly
- Transparency
- Opaque
- Crystal system
- Tetragonal
Diagnostic features
## Identification Identification of native indium is impossible without advanced analytical techniques. Due to the microscopic size of the grains, identification relies on chemical composition analysis using an electron microprobe (EDS/WDS) in conjunction with observations under a scanning electron microscope (SEM). Its silvery-white color, metallic luster, and very high ductility are characteristic features, but only visible under high magnification. ## Distinguishing from similar minerals Under a microscope, indium can be confused with other native metals such as silver, tin, lead, or bismuth, which may occur in similar environments. Certain differentiation is only possible based on chemical analysis. Silver is harder, and tin has a lower specific gravity. ## Crystal forms Indium crystallizes in the tetragonal system. In nature, it forms irregular, rounded grains, flakes, scales, and dendrites of microscopic size. Well-formed crystals are practically nonexistent.
Geological environment
## Genesis Native indium is a mineral of hydrothermal origin. It forms in the final stages of crystallization from metal-rich solutions, usually in polymetallic veins. Most often, it is found as microscopic inclusions within other sulfides, primarily sphalerite (zinc ores), which is the main carrier of indium in the Earth's crust. Less frequently, it occurs in association with cassiterite. ## Mineral associations It most often co-occurs with sphalerite, in which it forms fine inclusions. It is also accompanied by other ore minerals, such as galena, chalcopyrite, cassiterite, native tin, native lead, native bismuth, and various sulfosalts. ## Localities Confirmed occurrences of native indium are very few. It has been identified, among others, in the Orlovskoye tin-tungsten deposit and the Ulan-Sar deposit.
Rarity
Extremely rare
For collectors
## Quality criteria Specimens of native indium are almost exclusively of scientific interest and for highly specialized collectors. The value of a specimen is determined not by aesthetics, but by the confirmation of the presence of this element in its native form. The most valuable samples are those from a well-documented locality, with the largest (though still microscopic) and most numerous indium grains possible, whose presence has been confirmed by chemical analysis. Such a specimen is usually a fragment of rock or another mineral (e.g., sphalerite) with accompanying analytical documentation. ## Popular localities Due to its extreme rarity, there are no "popular" localities in the collector's sense. The research material in which native indium was first described came from the Orlovskoye deposit in Russian Transbaikalia, and this place is recognized as the type locality.
Care and storage
## Cleaning Specimens of native indium are typically microscopic and occur as inclusions in other minerals (e.g., sphalerite). They do not require and should not be subjected to mechanical or chemical cleaning, as this risks their complete destruction. ## What to avoid Avoid all acids and strong chemicals that may react with indium or the surrounding minerals. As a metal, indium is sensitive to oxidizing agents. It should be protected from moisture to prevent slow tarnishing of the surface. ## Storage Storing specimens with native indium does not require special conditions beyond those standard for mineralogical collections. It is best to keep them in a dry place, in closed display boxes, to protect them from dust and contaminants. Due to their microscopic nature, these specimens are often stored as microscopic preparations (thin sections or polished sections).