Oxycalcioroméite

Chemical formula: Ca₂Sb₅+₂O₇

Oxycalcioromeite is a rare mineral from the pyrochlore group, an oxide of calcium and antimony, found as small, well-formed crystals.

## Characteristics Oxycalcioromeite is a mineral belonging to the pyrochlore supergroup and the romeite group. It occurs as small, octahedral crystals, rarely exceeding 1 mm in size. The crystals are usually well-formed, with sharp edges and smooth faces. This mineral is brittle. ## Physical Properties Oxycalcioromeite crystals are characterized by a hardness of 5.5-6 on the Mohs scale. They have a luster ranging from vitreous to resinous. They are transparent to translucent. The mineral's density is relatively high, approximately 5.28 g/cm³. ## Colors and Varieties Oxycalcioromeite occurs in various colors, from yellow and orange, through reddish-orange, to brown. Some crystals may be colorless. No distinct varieties are recognized. ## History and Name The mineral's name, approved by the IMA in 2012, reflects its chemical composition. The prefix "oxy" indicates the dominance of oxygen at the anionic Y site, "calcio" indicates the dominance of calcium at the A site, and "romeite" refers to its belonging to the romeite group. It was redefined as part of the revision of the pyrochlore supergroup nomenclature. ## Uses Due to its rarity and small crystal size, oxycalcioromeite has no industrial application. It is solely an object of interest for collectors and scientists.

Properties

Mohs hardness: 5.5-6
Luster: Vitreous to Resinous
Streak: White to yellowish
Density: 5.28
Cleavage: None
Fracture: Conchoidal
Transparency: Transparent to translucent
Crystal system: Cubic

Diagnostic features

## Identification The key diagnostic feature is the crystal habit – they are almost always perfect, sharp octahedra. The high luster (vitreous to resinous) and color in shades of yellow, orange, and brown are also characteristic. Due to their small size, a loupe or microscope is often necessary for identification. ## Distinguishing from Similar Minerals It can be confused with other minerals from the pyrochlore supergroup, such as microlite or other romeites. Differentiation requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS/WDS), as their physical properties and crystal forms are very similar. ## Crystal Forms Crystals almost exclusively exhibit the form of regular, sharp octahedra {111}. Less commonly, modifications of the cube may occur. They usually appear as single, overgrown crystals on the surface of the host rock.

Geological environment

## Genesis Oxycalcioromeite is a secondary mineral, forming in the oxidation zones of ore deposits, especially those containing antimony minerals. It forms as a result of low-temperature hydrothermal processes or metamorphic processes in rocks rich in calcium and antimony. ## Mineral Associations It often co-occurs with other minerals from the romeite group, as well as with calcite, quartz, hematite, bindheimite, and various manganese oxides. ## Localities The most known and prized specimens come from the Långban mine in Sweden and the Buca della Vena mine in Tuscany, Italy. It also occurs in the Jakobsberg mine in Sweden, in antimony mines in the St-Pons-Mares region of France, and in the Clara mine in the Black Forest, Germany.

Rarity

Rare

For collectors

## Quality Criteria The most important criterion for collectors is the quality and size of the crystals. The most desirable are sharp, undamaged, lustrous octahedra with intense orange or red coloration. Specimens featuring many well-formed crystals contrasting with the rock matrix are highly valued. ## Popular Localities Specimens from Långban (Sweden) and Buca della Vena (Italy) are considered classic and most sought after in the collector's market due to their historical significance and crystal quality.

Care and storage

## Cleaning Caution is advised. It is best to use compressed air to remove dust. If wet cleaning is necessary, use a soft brush and distilled water, then dry the specimen thoroughly. ## What to Avoid Avoid contact with strong acids and other aggressive chemicals. The mineral is brittle, so it must be protected from impacts and falls. Ultrasonic cleaners should not be used. ## Storage Specimens should be stored in separate, padded boxes to prevent scratches and mechanical damage. Due to the small size of the crystals, "micromount" type boxes are ideal.

Sources

IMA Mineral List (CNMNC)