Plagionite
Chemical formula: Pb<sup>2+</sup><sub>5</sub>Sb<sup>3+</sup><sub>8</sub>S<sup>2-</sup><sub>17</sub>
Plagionite is a rare lead-antimony sulfosalt, forming characteristic, thick, tabular crystals with a metallic luster and dark gray color.
Properties
- Mohs hardness
- 2.5
- Luster
- Metallic
- Streak
- Black, reddish
- Density
- 5.5-5.7
- Cleavage
- Poor/Indistinct on {112}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Plagionite can be identified by its dark gray color, metallic luster, and characteristic thick, tabular crystals, which are often curved. Its relatively low hardness (2.5) and high density are also important diagnostic features. The streak is black with a reddish tint. ## Distinguishing from Similar Minerals Plagionite is sometimes confused with other lead-antimony sulfosalts, such as semseyite, zinkenite, or boulangerite. It is distinguished from semseyite by its thicker, more tabular crystal habit (semseyite forms thinner tablets, often in rosette aggregates). From zinkenite and boulangerite, which often form acicular or fibrous aggregates, it is distinguished by its crystal form. Reliable differentiation often requires advanced analytical methods, such as XRD. ## Crystal Forms Plagionite crystals are monoclinic, thickly tabular, often with a rhombic outline. Crystal surfaces can be striated. Crystals are usually small, rarely exceeding one centimeter. It also occurs in granular and massive aggregates.
Geological environment
## Genesis Plagionite is a hydrothermal mineral. It forms in ore veins that develop at low to medium temperatures, rich in antimony and lead. ## Mineral Associations It most commonly co-occurs with other lead-antimony sulfosalts, such as zinkenite, semseyite, boulangerite, as well as with galena, sphalerite, pyrite, chalcopyrite, quartz, and stibnite. ## Localities The most important historical and classic locality, from which well-formed crystals originate, is Wolfsberg in the Harz Mountains, Germany. Other known localities include Baia Sprie and Herja in Romania, the Příbram mine in the Czech Republic, and some localities in Bolivia (e.g., San José mine in Oruro) and Nevada (USA).
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp, undamaged crystals with a strong metallic luster. Crystals with a distinct tabular habit, set on a contrasting rock matrix, are particularly sought after. Crystal size is of great importance – specimens with crystals exceeding 1 cm are considered exceptional. Aesthetic crystal groups are much more highly prized than single, isolated crystals or massive aggregates. ## Popular Localities Classic, most prized plagionite specimens come from Wolfsberg, Germany. They serve as a historical standard for this species. High-quality specimens, often in the form of rich aggregates, also come from Romanian mines in the Maramureș region (e.g., Herja).
Care and storage
## Cleaning Plagionite specimens should be cleaned very carefully, using a soft brush to remove dust. Distilled water can be used, but prolonged soaking should be avoided, and the specimen should be thoroughly dried immediately. Ultrasonic cleaners are not recommended. ## What to Avoid Avoid contact with acids and other chemicals that can damage the mineral's surface. Plagionite is brittle, so protect it from impacts and falls. Prolonged exposure to moisture can lead to the formation of tarnish on its surface. ## Storage Plagionite specimens are best stored in stable conditions, in closed boxes or display cases, to protect them from dust, moisture, and mechanical damage. It is advisable to separate it from harder minerals that could scratch it.