Polydymite
Chemical formula: Ni<sup>2+</sup>Ni<sup>3+</sup><sub>2</sub>S<sup>2-</sup><sub>4</sub>
Polydymite is a rare nickel sulfide from the thiospinel group, forming small, metallic-lustered, octahedral crystals with a brownish-black color.
Properties
- Mohs hardness
- 4.5-5.5
- Luster
- Metallic
- Streak
- Black
- Density
- 4.54-4.8
- Cleavage
- Imperfect on {111}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Cubic
Diagnostic features
## Identification Key diagnostic features of polydymite include its crystal habit (octahedra), metallic luster, black streak, and high density. Its occurrence in association with other nickel and cobalt sulfides is also characteristic. ## Distinguishing from Similar Minerals Polydymite is visually very similar to other minerals of the linnaeite group, especially linnaeite (Co₃S₄) itself and violarite (FeNi₂S₄). Distinguishing them without advanced chemical analysis (EDS) is practically impossible. It is distinguished from magnetite by its lack of magnetism. ## Crystal Forms Polydymite crystals almost always take the form of regular octahedra. They often occur as twins according to the so-called spinel law. It also forms granular aggregates and massive inclusions within other sulfides.
Geological environment
## Genesis Polydymite is a hydrothermal mineral. It forms at low to medium temperatures in ore veins, often associated with ultramafic rocks (e.g., serpentinites) or in sediments of volcanic origin. It can also crystallize in lacustrine sediments and bituminous shales. ## Mineral Associations This mineral often co-occurs with other sulfides, such as millerite, pentlandite, chalcopyrite, pyrite, marcasite, as well as other members of the linnaeite group (linnaeite, violarite, siegenite). Secondary nickel minerals, such as morenosite, may also accompany it. ## Localities The most important and classic locality, from which well-formed crystals originate, is the Grüneau mine in Rhineland-Palatinate, Germany. Other known localities include Sudbury in Ontario (Canada), the Ural regions (Russia), and some deposits in Zimbabwe and the USA (e.g., in Missouri and Nevada).
Rarity
Rare
For collectors
## Quality Criteria Specimens with sharp, well-formed, lustrous crystals in the form of ideal octahedra are most valued by collectors. Crystals set on a contrasting rock matrix are highly prized. Crystal size is a key factor – specimens with crystals exceeding 2-3 mm are already considered exceptional. Aesthetically pleasing twinning is also desirable. ## Popular Localities By far the most sought-after and classic polydymite specimens come from the historical locality of Grüneau in Germany. These are the benchmark for this mineral and command the highest prices in the collector's market.
Care and storage
## Cleaning Polydymite specimens should be cleaned very carefully, preferably dry, using a soft brush to remove dust. If necessary, a damp cloth or cotton swab with distilled water can be used, immediately drying the specimen. Ultrasonic cleaners should be avoided. ## What to Avoid Polydymite is a sulfide that can undergo oxidation, especially in humid environments. It should be protected from prolonged contact with water and moisture, which can lead to the formation of greenish, secondary nickel sulfates on its surface. Contact with acids and other chemicals should be avoided. ## Storage It is recommended to store specimens in a dry place, preferably in closed boxes or display cases away from sources of moisture. To prevent oxidation, desiccants (e.g., silica gel) can be used.