Gold
Chemical formula: Au
Gold is a noble metal and a native element, valued for its characteristic yellow color, luster, malleability, and high value.
Properties
- Mohs hardness
- 2.5-3
- Luster
- Metallic
- Streak
- Golden yellow
- Density
- 19.3
- Cleavage
- None
- Fracture
- Hackly
- Transparency
- Opaque
- Crystal system
- Cubic
Diagnostic features
## Identification Gold is relatively easy to identify based on key features: intense yellow color, strong metallic luster, high density (the specimen is surprisingly heavy for its size), and high softness and malleability. Unlike many sulfides, its streak is golden-yellow and shiny. It is not magnetic. ## Distinguishing from Similar Minerals It is most often confused with pyrite ("fool's gold") and chalcopyrite. Pyrite is much harder (6-6.5 on the Mohs scale), brittle, and has a greenish-black streak. Chalcopyrite is also harder (3.5-4), more brittle, and its color is often brassy yellow with a tendency to tarnish. Gold is malleable – when struck, it flattens, while pyrite and chalcopyrite crumble into powder. Golden mica (vermiculite, phlogopite) is light and easily separates into flakes. ## Crystal Forms Gold crystals are rare and highly prized. They most often take the form of octahedra, less commonly cubes or rhombic dodecahedra. They are often distorted, rounded, or form skeletal and dendritic (fern-like) aggregates. The most common forms are irregular nuggets, flakes, and grains.
Geological environment
## Genesis Gold primarily crystallizes from hot hydrothermal solutions in quartz veins, often associated with magmatic intrusions. It occurs in igneous rocks (granitoids, porphyries) and metamorphic rocks (greenschists). As a result of weathering and erosion of parent rocks, chemically resistant and heavy gold grains are transported by water and concentrate in river sediments, forming secondary, detrital or alluvial deposits (placers). ## Mineral Associations In hydrothermal veins, gold most often co-occurs with quartz, pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, and also with tellurides (sylvanite, calaverite). In detrital deposits, it is accompanied by other heavy and resistant minerals, such as magnetite, ilmenite, zircon, garnets, platinum, and diamond. ## Localities Historically and currently, the most important gold deposits are found in South Africa (Witwatersrand), Australia (Kalgoorlie, Bendigo), USA (Nevada, California, Alaska), Russia (Urals, Siberia), Canada (Ontario, Quebec), China, and Peru. In Europe, significant specimens come from Romania (Roșia Montană), where beautiful crystalline forms were found. In Poland, trace amounts of gold occur in the Sudetes (areas around Złotoryja, Głuchołazy) and in the river sands of Lower Silesia.
Rarity
Not very common
For collectors
## Quality Criteria Well-formed, sharp gold crystals are most highly valued by collectors, which is a great rarity. Skeletal forms (octahedra) and distinct dendritic aggregates ("ferns") are particularly sought after. The attractiveness of a specimen is enhanced by its association with a white, crystalline quartz matrix, which creates a contrast for the metal's yellow color. Large, aesthetic nuggets of interesting shape also fetch high prices, often exceeding the value of the contained metal. ## Popular Localities Classic, highly prized specimens of crystalline gold come from historical localities in California (Mariposa, Nevada, Placer counties), especially from the Red Ledge, Eagle's Nest, or Colorado Quartz mines. Equally famous are dendritic and crystalline golds from Romania (Roșia Montană, formerly Verespatak). In recent years, the market has been dominated by spectacular, crystalline specimens from Venezuela (e.g., from the Ikabaru region) and Brazil.
Care and storage
## Cleaning Gold specimens are chemically inert and can be safely cleaned. For removing loose dirt, warm soapy water and a soft brush are sufficient. For specimens intergrown with quartz, a solution of oxalic acid can be used to remove iron oxide stains, but caution should be exercised. Always rinse the specimen thoroughly in distilled water after using any chemicals. ## What to Avoid Gold is very soft and prone to scratching, so contact with harder minerals and tools should be avoided. Despite its chemical resistance, it reacts with aqua regia (a mixture of hydrochloric and nitric acids) and with mercury, forming an amalgam. Contact with these substances should be avoided. ## Storage Due to its softness, gold specimens, especially delicate crystalline and dendritic forms, should be stored separately in padded boxes or on soft stands. Protect from dust and mechanical damage. It does not require special protection from light or humidity.