Wurtzite

Chemical formula: Zn²⁺S²⁺

Wurtzite is a rarer, hexagonal polymorph of zinc sulfide (ZnS), distinguished by its resinous luster and dark, reddish-brown color.

## Characteristics Wurtzite is a zinc sulfide and represents a rarer, hexagonal polymorphic variety of sphalerite. It forms pyramidal, tabular, or prismatic crystals, often with visible striations on their faces. It typically occurs in radial or fibrous aggregates, as well as crusts. Its appearance is characterized by a dark, reddish-brown to almost black color and a strong, resinous luster, which can transition to submetallic on crystal faces. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3.5-4. It is translucent, rarely transparent. The density of wurtzite is approximately 4 g/cm³. It exhibits very good cleavage in one direction. ## Colors and Varieties The dominant color is dark, ranging from reddish-brown through orange-brown to black. Color variations usually result from iron impurities. Some varieties, like erythrozincite, contain manganese. No commercial names are distinguished for wurtzite; it is primarily a mineral of scientific and collector interest. ## History and Name The mineral was named in honor of the French chemist Charles Adolphe Wurtz (1817-1884), known for his work in organic chemistry. Wurtzite was first described in 1861 based on specimens found in the San José mine in Oruro, Bolivia, which is its type locality. ## Uses Due to its rarity, wurtzite is not significant as a zinc ore – its more common polymorph, sphalerite, dominates in this role. However, it is a prized collector's mineral, sought after for its well-formed crystals and interesting aggregate forms.

Properties

Mohs hardness: 3.5-4
Luster: resinous, bright submetallic on xl faces
Streak: Brown
Density: 4
Cleavage: Very good and easy on <mi>{11_20)</mi>, more difficult on {0001}.
Fracture: Irregular/Uneven
Transparency: Translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Key diagnostic features of wurtzite include its hexagonal crystal habit (often pyramidal), dark reddish-brown color, resinous luster, and brown streak. Good cleavage in one direction is also helpful for identification. ## Distinguishing from Similar Minerals It is most commonly confused with sphalerite, which is its isometric polymorphic variety. However, sphalerite has perfect cleavage in six directions, resulting in a characteristic stepped fracture, whereas wurtzite has cleavage in one direction. From other dark minerals, such as some garnets or cassiterite, it is distinguished by its lower hardness and brown streak. ## Crystal Forms Wurtzite crystals most often have a pyramidal or prismatic habit, often terminated by a basal pinacoid. It frequently forms radial aggregates, rosette-like clusters, and fibrous forms and crusts.

Geological environment

## Genesis Wurtzite crystallizes under hydrothermal conditions, usually at lower temperatures than its polymorph – sphalerite, although both minerals can occur together. It forms in ore veins, often in association with other sulfides. It can also form in volcanic sediments and geodes. ## Mineral Associations It most commonly co-occurs with sphalerite, galena, pyrite, marcasite, chalcopyrite, as well as with quartz, barite, siderite, and calcite. ## Localities The most important localities worldwide include the historical type locality in Oruro, Bolivia. Beautiful specimens also come from the Příbram mine in the Czech Republic, from mines in the Butte area, Montana (USA), and from some localities in Germany (e.g., in the Harz Mountains).

Rarity

Not very common

For collectors

## Quality Criteria The most valued specimens by collectors are those with sharp, well-formed pyramidal crystals. Aesthetic, radial aggregates are also highly prized. Luster, color intensity, and absence of mechanical damage significantly increase the value of a specimen. Combinations of wurtzite with other minerals, such as quartz or sphalerite, are also attractive. ## Popular Localities Classic, prized specimens come from the type locality in Oruro, Bolivia, and from Příbram, Czech Republic. Specimens from Butte, Montana (USA) are also sought after by collectors.

Care and storage

## Cleaning Wurtzite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. After wet cleaning, the specimen must be thoroughly dried. ## What to Avoid Contact with acids and strong chemicals, which can damage the mineral's surface, should be avoided. Wurtzite is relatively soft and brittle, so it must be protected from impacts and scratches. It should not be heated or exposed to sudden temperature changes. ## Storage Collector specimens of wurtzite are best stored in separate, padded boxes or display cases to avoid contact with harder minerals. It does not require special conditions regarding light or humidity, but stable room conditions are recommended.