Goldfieldite

Chemical formula: (Cu1+4☐2)Cu1+6Te4+4S2-13

Goldfieldite is a rare mineral of the tetrahedrite group, being a tellurium-rich variety containing copper, known for its occurrence in rich gold deposits.

Description

## Characteristics Goldfieldite is a mineral from the tetrahedrite group, representing its tellurium analog. It typically forms massive, granular aggregates embedded in the host rock. Well-formed crystals are extremely rare and have a tetrahedral shape. The mineral has a dark gray to black color and a characteristic metallic luster. Sometimes a bluish or purplish tarnish appears on its surface. ## Physical Properties Its Mohs hardness is 3-3.5, making it a relatively soft mineral. It is brittle, and its fracture is uneven to subconchoidal. It does not exhibit cleavage. The density is relatively high, approximately 5.03 g/cm³. It is completely opaque. ## Colors and Varieties Goldfieldite is a mineral with a consistent, dark gray to black color. It represents the end-member, tellurium-rich member of the isomorphic series with tetrahedrite (antimony-rich) and tennantite (arsenic-rich). No color or commercial varieties are distinguished. ## History and Name The mineral's name comes from its type locality – the town of Goldfield in Esmeralda County, Nevada, USA, where it was discovered. It was described in 1909 by William F. Hillebrand, Charles Palache, and Waldemar T. Schaller. ## Applications Due to its rarity, goldfieldite is primarily a valued collector's mineral. In its occurrences, it can be a minor source of tellurium, as well as copper and gold, with which it often coexists.

Diagnostic features

## Identification Helpful identification features include its black color, metallic luster, relatively high density, and lack of cleavage. However, the key indicator is the environment of occurrence – its presence in gold-telluride veins in association with other tellurides. ## Distinguishing from Similar Minerals Goldfieldite is visually indistinguishable from other minerals of the tetrahedrite group, such as tetrahedrite (antimony-rich) and tennantite (arsenic-rich). They form a continuous isomorphic series, and their differentiation requires advanced chemical analyses (e.g., EDS) to confirm the dominance of tellurium. ## Crystal Forms Crystals are extremely rare and, if present, take the form of small, poorly developed tetrahedra. It is most commonly found as massive, granular aggregates or as irregular inclusions in other minerals.

Geological environment

## Genesis Goldfieldite forms under hydrothermal conditions, in ore veins of low to medium formation temperature. It is a typical mineral for epithermal gold and tellurium deposits. ## Mineral Associations This mineral often coexists with native gold, pyrite, bismuthinite, famatinite, enargite, and other tellurides such as sylvanite, calaverite, krennerite, and petzite. It also occurs in association with other minerals from the tetrahedrite group. ## Localities The most important and historical occurrence is the Goldfield area in Nevada (USA), especially in the Mohawk mine. Significant specimens also come from the Emperor mine on Viti Levu Island (Fiji), the Trixie mine in Utah (USA), and the Kawazu mine in Shizuoka Prefecture (Japan).

Rarity

Rare

Collector aspects

## Quality Criteria The most highly valued goldfieldite specimens are those with well-formed, sharp crystals, which are extremely rare. In the case of massive material, the value is determined by the richness of the aggregate and attractive associations with other minerals, especially with visible native gold or rare tellurides. ## Popular Localities Specimens from the type locality in Goldfield, Nevada, are considered classic and serve as a benchmark for this mineral. Rich associations with tellurides and gold from the Emperor mine in Fiji are also highly prized.

Care and storage

## Cleaning Goldfieldite specimens should be cleaned very carefully, preferably dry, using a soft brush. Contact with water should be kept to a minimum – if necessary, distilled water can be used, and then the specimen should be immediately and thoroughly dried. ## What to Avoid Avoid all chemical agents, acids, and detergents, which can react with the mineral and lead to its oxidation or surface damage. The mineral is sensitive to moisture, so it should not be stored in humid conditions. It should also be protected from sudden temperature changes. ## Storage It is recommended to store specimens in a dry, stable environment. The best solution is a closed "membrane box" or a display case with a desiccant, which prevents oxidation and protects the brittle mineral from mechanical damage.