Fengruiite
Chemical formula: [Ag¹⁺₆Sb³⁺₂S²⁻₇][Ag¹⁺₉Cu¹⁺S²⁻₂Te²⁻₂]
Fengruite is a newly discovered, extremely rare sulfosalt mineral, composed of two structurally distinct modules containing silver, antimony, copper, sulfur, and tellurium.
Properties
- Mohs hardness
- 3-4
- Color
- black gray
- Luster
- Metallic
- Streak
- steel-gray to iron-black
- Density
- 6.62
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Identification of fengruite is impossible without advanced laboratory techniques. It requires analysis by electron microprobe (EDS/WDS) to determine its chemical composition and X-ray diffraction (XRD) to confirm its unique crystal structure. Under a microscope in reflected light, it appears gray with a metallic luster. ## Distinguishing from similar minerals It can be confused with other black, metallic sulfides and sulfosalts, such as acanthite, chalcocite, stephanite, or polybasite, with which it often coexists. Differentiation relies solely on chemical analysis, which will reveal the simultaneous presence of silver, antimony, copper, sulfur, and tellurium in characteristic proportions. ## Crystal forms Fengruite occurs as anhedral to subhedral grains, usually of irregular shape, intergrown with other minerals. Well-formed, idiomorphic crystals have not been observed.
Geological environment
## Genesis Fengruite forms through hydrothermal processes in quartz-calcite veins within gold and silver deposits. It crystallizes in the late stages of mineralization, filling spaces between previously formed minerals. ## Mineral associations This mineral coexists with acanthite, chalcopyrite, sphalerite, galena, pyrite, native gold, quartz, and calcite. At its discovery site, it was found in direct contact with acanthite and chalcopyrite. ## Localities The only confirmed locality for fengruite worldwide is its type locality: the Pai-Lao-Tung gold and silver deposit, in Tongbai County, Henan Province, China.
Rarity
Extremely rare
For collectors
## Quality criteria As a microscopic mineral, fengruite is not evaluated according to typical criteria such as size or crystal form. Its collector value lies in its extreme rarity and its status as a new, recently described species. The most valuable specimens are those where the presence of fengruite has been analytically confirmed, and the grains are relatively "rich" and visible in intergrowths with other minerals. The entire rock fragment (matrix) from the type locality has value.
Care and storage
## Cleaning Due to its extreme rarity and small grain size, fengruite specimens (inclusions in the matrix) should not be cleaned mechanically or chemically. Any attempts at cleaning may damage or remove the tiny mineral grains. If necessary, dust can be removed with a gentle stream of compressed air. ## What to avoid Avoid contact with any chemicals, acids, and cleaning agents. As a sulfide and telluride, the mineral is susceptible to oxidation and chemical reactions. It should be protected from moisture and sudden temperature changes. ## Storage Specimens containing fengruite should be stored under stable conditions, preferably in a sealed, dry "micromount" container, away from light and contaminants. Handling should be minimized to avoid mechanical damage.