Keilite
Chemical formula: Fe<sup>2+</sup>S<sup>2-</sup>
Keilite is an iron sulfide (FeS) found in meteorites, being the iron analogue of niningerite.
Properties
- Mohs hardness
- 3.5-4
- Luster
- Metallic
- Streak
- Black
- Density
- 4.62
- Cleavage
- Perfect on {100}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Cubic
Diagnostic features
## Identification Identification of keilite is possible only through advanced laboratory techniques. This requires analysis in reflected light on a polished section and confirmation of chemical composition using an electron microprobe (EDS/WDS). Its characteristic feature is a chemical composition close to FeS with minor admixtures of magnesium, manganese, and calcium. ## Distinguishing from Similar Minerals Keilite is part of an isomorphic series with niningerite ((Mg,Fe)S) and alabandite ((Mn,Fe)S). Distinguishing it from these minerals requires precise chemical analysis to determine the dominant element. Visually in reflected light, it is almost identical. It can also be confused with troilite (FeS), which crystallizes in the hexagonal system, whereas keilite crystallizes in the isometric system. This difference can be determined using X-ray diffraction (XRD) or electron backscatter diffraction (EBSD). ## Crystal Forms Keilite forms microscopic, most often anhedral (irregular) or subhedral (partially developed) grains. Forms resembling cubes or octahedra, typical of the isometric system, are observed less frequently.
Geological environment
## Genesis Keilite is a mineral formed under highly reducing conditions, with very low oxygen fugacity. Its formation is characteristic of the early stages of the Solar System's formation. It occurs almost exclusively in meteorites from the enstatite chondrite group (EH and EL types), which formed in the inner part of the solar nebula. ## Mineral Associations This mineral co-occurs with other minerals typical of enstatite chondrites, such as enstatite, kamacite, taenite, troilite, niningerite, oldhamite (CaS), daubréelite (FeCr₂S₄), schreibersite ((Fe,Ni)₃P), and perryite ((Ni,Fe)₅(Si,P)₂). ## Localities As a meteoritic mineral, its localities are the fall sites of specific meteorites. It has been identified, among others, in enstatite chondrites such as: Zakłodzie (Poland), Qingzhen (China), Indarch (Azerbaijan), Saint-Sauveur (France), and in several meteorites found in Antarctica (e.g., Yamato 691).
Rarity
Very rare
For collectors
## Quality Criteria Keilite is not a collector's mineral in the traditional sense. It does not form macroscopic crystals or visually attractive specimens. Its value is purely scientific. For research institutions and specialized meteorite collectors, the value lies in the meteorite itself, in which the presence of keilite has been identified. The quality of such a specimen is assessed based on its type, mass, degree of weathering, and history (e.g., whether it was an observed fall). ## Popular Localities The most "known" localities are the sites where meteorites containing this mineral were first identified or are intensely studied. An example is the Zakłodzie meteorite found in Poland, a unique enstatite achondrite in which the presence of keilite has been documented.
Care and storage
## Cleaning Due to their microscopic nature and occurrence within the rock matrix of meteorites, individual keilite grains are not cleaned. Microscopic preparations (polished sections) are cleaned using standard optical methods, employing isopropanol and a soft cloth. ## What to Avoid Contact with strong acids, which can react with sulfides, should be avoided. Meteorite samples containing keilite should be protected from moisture and high humidity to prevent oxidation and the formation of secondary sulfates. ## Storage Meteorite fragments containing keilite should be stored in dry conditions, preferably in sealed containers with a desiccant (e.g., silica gel). Microscopic polished sections should be stored in special boxes, protecting them from dust and mechanical damage.