Protojoséite

Chemical formula: Bi<sub>3</sub>TeS

Protojoseite is a rare, unstable bismuth telluride sulfide that transforms into joseite-B at room temperature.

## Characteristics Protojoseite is a mineral known primarily from laboratory studies, as it is stable only at temperatures above 421°C. At room temperature, it undergoes inversion to the mineral joseite-B, while retaining its external crystalline form. For this reason, natural specimens are actually pseudomorphs of joseite-B after protojoseite. Crystals observed under laboratory conditions take the form of thin, hexagonal plates. ## Physical Properties Due to its instability under standard conditions, most physical properties are determined for its transformation product, joseite-B. Protojoseite itself, when studied at high temperatures, exhibits a metallic luster. Its hardness and density have not been precisely measured for the high-temperature phase. ## Colors and Varieties Observed at high temperatures, protojoseite has a steel-gray to tin-white color with a metallic luster. It has no known varieties. ## History and Name The name "protojoseite" was introduced by Charles Palache, Harry Berman, and Clifford Frondel in the 7th edition of "Dana's System of Mineralogy" in 1944. The prefix "proto-" refers to the fact that it is a high-temperature polymorph (prototype) that transforms into joseite. The name of the parent mineral, joseite, comes from its discovery locality in the São José mine (now the city of Ouro Preto) in Brazil. ## Uses Protojoseite has no industrial applications. It is solely an object of scientific and collecting interest as an example of polymorphism and pseudomorphism.

Properties

Mohs hardness
2
Luster
Metallic
Streak
Greyish black
Density
8.15
Cleavage
Perfect on {0001}
Fracture
Micaceous
Transparency
Opaque
Crystal system
Hexagonal

Diagnostic features

## Identification Identifying protojoseite in practice means identifying its pseudomorph – joseite-B. Characteristic is the hexagonal crystal form, which is a relict of protojoseite's high-temperature structure. The mineral has a metallic luster, steel-gray color, and is very soft. Its perfect cleavage causes it to easily separate into thin lamellae. ## Distinguishing from Similar Minerals It can be confused with other bismuth tellurides, such as tetradymite, tellurobismuthite, or joseite-A itself. Final differentiation requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS), which allow identification of the specific phase and the Te to S ratio. ## Crystal Forms Protojoseite crystallizes in the form of thin, hexagonal, tabular crystals. It also occurs as lamellar aggregates, foliated forms, or massive aggregates, often in association with other bismuth minerals.

Geological environment

## Genesis Protojoseite forms under hydrothermal conditions, in medium- to high-temperature quartz veins associated with intrusions of igneous rocks, such as granites or granodiorites. It forms at temperatures above 421°C and undergoes transformation to joseite-B upon cooling. ## Mineral Associations It most commonly co-occurs with native bismuth, bismuthinite, gold, pyrite, chalcopyrite, arsenopyrite, quartz, and other tellurides, such as joseite-A, tetradymite, and tellurobismuthite. ## Localities As the mineral is unstable, the reported localities refer to its pseudomorphs (joseite-B). The most important occurrences include the São José mine (now Ouro Preto) in Minas Gerais, Brazil, from which the type material originates. Other known localities include the Tasna region in Bolivia, Glacier Gulch in British Columbia (Canada), and the Bolyuba mine in the Stara Planina region in Bulgaria.

Rarity

Very rare

For collectors

## Quality Criteria The most valuable specimens for collectors are those with well-formed, sharp, hexagonal crystals, which are actually pseudomorphs of joseite-B after protojoseite. Rich aggregations of lamellar aggregates on a contrasting quartz matrix are also prized. An important factor is the presence of associated minerals, such as native gold. ## Popular Localities Classic and most valued specimens come from the historical type locality in Ouro Preto, Brazil. Specimens from Bolivia and Canada also provide good quality material, often with distinct crystals.

Care and storage

## Cleaning Specimens, which are pseudomorphs of joseite-B after protojoseite, are very delicate. Mechanical cleaning should be avoided. If absolutely necessary, compressed air can be used to remove loose dust particles. Water or any chemicals should not be used. ## What to Avoid The mineral is very soft and easily scratched or deformed. Contact with harder minerals should be avoided. It is sensitive to sudden temperature changes. It should be protected from moisture and chemicals that can cause its oxidation. ## Storage Specimens should be stored in stable conditions, in a closed display box, away from direct sunlight, moisture, and contaminants. It is best to store it separately to avoid mechanical damage.

Sources

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