Rasvumite
Chemical formula: KFe<sup>2+</sup>Fe<sup>3+</sup>S<sup>2-</sup><sub>3</sub>
A rare potassium iron sulfide with a metallic, brownish-bronze luster, found in nepheline syenite pegmatites.
Properties
- Mohs hardness
- 4
- Luster
- Metallic
- Streak
- Black
- Density
- 2.96
- Cleavage
- Good on {010} and {110}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Key diagnostic features of rasvumite are its brownish-yellow color, strong metallic luster, black streak, and relatively low hardness. However, the most important indicator is its paragenesis – its occurrence in ultra-alkaline rocks (nepheline syenites) in association with other rare minerals. ## Distinguishing from similar minerals Rasvumite can be confused with other brown sulfides with a metallic luster, such as pyrrhotite, pentlandite, or chalcopyrite. Distinguishing based on visual characteristics alone is difficult. The decisive factor is the environment of occurrence – rasvumite forms in a unique environment of alkaline pegmatites, while the aforementioned minerals are typical of magmatic deposits associated with mafic and ultramafic rocks or hydrothermal formations. ## Crystal forms It most often occurs as irregular, anhedral grains intergrown in massive aggregates. Very rarely, it forms small, columnar or platy crystals with a rhombic outline.
Geological environment
## Genesis Rasvumite is a mineral of magmatic origin, crystallizing in the late stage from strongly alkalized, silica-undersaturated melts. It forms in hyperagpaitic nepheline syenite pegmatites and associated hydrothermal veins. ## Mineral associations This mineral occurs in a characteristic assemblage of alkaline minerals. It is most often accompanied by djerfisherite, villiaumite, nepheline, aegirine, sodalite, ussingite, lomonosovite, and eudialyte. ## Localities The most important and classic localities of rasvumite worldwide are the Khibiny Massif on the Kola Peninsula in Russia (type locality), the Ilimaussaq complex in Greenland, and Mont Saint-Hilaire in Quebec, Canada. These are the only places where well-formed specimens of this mineral have been found.
Rarity
Very rare
For collectors
## Quality criteria The collector's appeal of rasvumite primarily depends on its rarity. Specimens rich in this mineral, forming distinct aggregates on a rock matrix, are most valued. Samples where rasvumite is accompanied by well-formed crystals of other rare minerals, such as villiaumite or eudialyte, have exceptional value. Finding specimens with visible, even small, rasvumite crystals is extremely rare and significantly increases the specimen's value. ## Popular localities The most well-known collector specimens come from two localities: the Khibiny Massif in Russia and the Poudrette quarry at Mont Saint-Hilaire in Canada. Specimens from Greenland are also known but are less commonly found on the collector's market.
Care and storage
## Cleaning Rasvumite should only be dry-cleaned, using a soft brush to remove dust. As a sulfide, it is potentially susceptible to oxidation, so all contact with water and chemical agents should be avoided. ## What to avoid Contact with water, moisture, acids, and other chemicals that could lead to its permanent damage or chemical alteration must be strictly avoided. It is not sensitive to light, but prolonged storage in a humid environment can cause its slow weathering. ## Storage Rasvumite specimens are best stored in a dry and stable environment. Airtight containers or display cases with a desiccant are recommended to minimize the risk of oxidation. It should be isolated from minerals that may react with sulfur.