Gudmundite

Chemical formula: Fe<sup>₃+</sup>(SbS)<sup>₃-</sup>

Gudmundite is an antimony iron sulfide with a metallic luster, found in hydrothermal ore deposits.

## Characteristics Gudmundite is a relatively rare mineral from the sulfosalt group. It most often forms granular or massive aggregates; well-formed crystals are less common. It has a silvery-white to steel-gray color and a strong, metallic luster. A characteristic feature is its tendency to develop a yellowish or brownish tarnish over time, which is a result of oxidation. Crystals, if present, are usually small, prismatic or tabular in habit, often with visible striations on their faces. ## Physical Properties This mineral is opaque and quite hard – its Mohs hardness is 5.5 to 6. It has a high density, ranging from 6.72 to 6.85 g/cm³, which makes specimens noticeably heavy for their size. It has an uneven fracture and indistinct cleavage. ## Colors and Varieties Gudmundite is a mineral of consistent color, ranging from silvery-white to steel-gray. It does not form colored varieties, and the only changes in its appearance result from the formation of yellowish, brownish, or iridescent tarnishes on its surface. ## History and Name The mineral's name comes from its first discovery location – the Gudmundstorp farm (now Gudmundtorp) near the historic Sala silver mine, in the Västmanland region of Sweden. It was described and named in 1928 by the Swedish mineralogist Per Geijer. ## Applications Gudmundite has no significant industrial application. It is primarily of interest to mineral collectors, especially well-formed crystals and specimens co-occurring with other rare minerals.

Properties

Mohs hardness: 5.5-6
Luster: Metallic
Streak: Black
Density: 6.72-6.85
Cleavage: Poor
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Key diagnostic features of gudmundite include its high density, metallic luster, silvery-white color, and tendency to form a yellowish tarnish. Its occurrence with other antimony minerals, such as stibnite or tetrahedrite, is an important clue. ## Distinguishing from Similar Minerals Gudmundite is sometimes confused with arsenopyrite, from which it is distinguished by the lack of a characteristic garlic odor when struck. It is significantly harder than stibnite. It is very similar in appearance to löllingite and safflorite – for massive specimens, definitive differentiation often requires advanced studies, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Crystal Forms Gudmundite crystals are rare. They usually take the form of short prisms, often tabularly flattened. Longitudinal striations are often observed on their faces. Most often, however, this mineral forms massive, compact, or granular aggregates within the rock mass.

Geological environment

## Genesis Gudmundite forms under hydrothermal conditions, in ore veins forming at low and medium temperatures. It is also found in some metamorphic rocks, such as skarns, and in metamorphosed sulfide deposits. ## Mineral Associations This mineral often co-occurs with other sulfides and sulfosalts. Its most common associated minerals include: pyrrhotite, arsenopyrite, stibnite, tetrahedrite, galena, chalcostibite, ullmannite, and native antimony. ## Localities Important global localities known for good quality gudmundite specimens include: the historic Sala mine in Sweden (type locality), the Isérables deposit in Valais canton, Switzerland, Pezinok in Slovakia, the Consols mine in Broken Hill, Australia, Weiden in Bavaria (Germany), and the Yellow Pine area in Idaho (USA).

Rarity

Not very common

For collectors

## Quality Criteria The most highly valued by collectors are specimens with well-formed, sharp, and lustrous crystals, which is a great rarity for this mineral. The value of a specimen is also enhanced by its association with other rare or aesthetic minerals. For massive aggregates, the purity of the material and lack of damage are important. ## Popular Localities Specimens from the type locality in Sala, Sweden, are considered classic and most desirable. Well-formed crystals from the Isérables deposit in Switzerland and from Pezinok, Slovakia, are also highly prized.

Care and storage

## Cleaning It is recommended to clean specimens dry, using a soft brush or compressed air to remove dust. Contact with water should be kept to a minimum. If water is necessary, distilled water should be used, and the specimen should be thoroughly dried immediately. ## What to Avoid Avoid contact with acids and other chemicals that can permanently damage the mineral's surface. Gudmundite is sensitive to moisture, which accelerates the oxidation process and tarnish formation. It should also be protected from sudden temperature changes and direct, prolonged exposure to sunlight. ## Storage Gudmundite specimens are best stored in a dry and stable environment. Sealed collector boxes are recommended, preferably with a desiccant (e.g., silica gel) to slow down the oxidation process. Due to its brittleness, the mineral should be protected from impacts and falls.

Sources

IMA Mineral List (CNMNC)