Ellinaite
Chemical formula: CaCr³⁺₂O₄
Ellinaite is a newly discovered, extremely rare mineral from the spinel group, characterized by the dominance of chromium and calcium.
Properties
- Mohs hardness
- 4.5-5
- Color
- Black
- Luster
- Metallic
- Streak
- Black
- Density
- 5.217
- Cleavage
- None
- Fracture
- Irregular/Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Identification of ellinaite is impossible without advanced analytical techniques. It requires the use of a scanning electron microscope (SEM) with chemical composition analysis (EDS) to confirm the dominance of calcium and chromium. Visually, it appears as small, black, metallic-lustered octahedral crystals. ## Distinguishing from similar minerals Ellinaite is visually indistinguishable from other black, microscopic minerals of the spinel group, such as chromite, magnetite, or hercynite. The only method to differentiate them is chemical analysis, which in the case of ellinaite will show the unique formula CaCr₂O₄. ## Crystal forms This mineral forms exclusively very small, well-formed, regular (octahedral) crystals, whose size does not exceed 20 micrometers. They occur as single crystals grown on other minerals.
Geological environment
## Genesis Ellinaite forms under very specific and rare conditions of pyrometamorphism, i.e., the transformation of rocks under very high temperature but low pressure. Its genesis is associated with the natural combustion of bitumens contained in sedimentary rocks (marls), which leads to the formation of high-temperature mineral parageneses. It occurs in calcite veins cutting pyrometamorphic rocks. ## Mineral associations This mineral co-occurs with other rare minerals formed under similar conditions, such as ye'elimite, larnite, brownmillerite, mayenite, as well as calcite, fluorapatite, fluorellestadite, perovskite, and magnesioferrite. ## Localities The only confirmed occurrence of ellinaite in the world is its type locality – the Hatrurim Formation (the so-called "Mottled Zone") near Mount Sdom in the Judean Desert, Israel.
Rarity
Extremely rare
For collectors
## Quality criteria The quality of an ellinaite specimen is determined almost exclusively by the quality and richness of the material from the type locality. The most desirable samples are those where ellinaite crystals are as large as possible (though still microscopic), numerous, and well-formed as octahedra. The aesthetics of the matrix rock and the presence of contrasting associated minerals are also important. ## Popular localities The only source of specimens is the type locality in the Hatrurim Formation in Israel. Material from this location is extremely difficult to obtain and appears on the collector's market very rarely, mainly through specialized dealers and micromineral collectors.
Care and storage
## Cleaning Specimens containing ellinaite should be cleaned with the utmost care, preferably using compressed air to remove dust. Due to the microscopic nature of the crystals, any mechanical intervention (e.g., a brush) can irreversibly damage them. The use of liquids is not recommended. ## What to avoid Avoid contact with chemicals, acids, and ultrasonic cleaners. Samples should be protected from shocks and vibrations that could cause small crystals to detach from the matrix rock. ## Storage Ellinaite specimens are best stored in specialized micromount boxes, which protect them from dust and mechanical damage. The label should precisely indicate the location of the microscopic crystals on the sample.