Hydroxycalcioroméite

Chemical formula: (Ca,Sb³⁺)₂(Sb⁵⁺,Ti⁴⁺)₂O₆(OH)

Hydroxycalcioromeite is a rare mineral from the romeite group, an oxide of calcium and antimony, valued for its amber-yellow, octahedral crystals.

## Characteristics Hydroxycalcioromeite is a mineral belonging to the romeite group, which is part of the supergroup of pyrochlore. Chemically, it is a hydrated calcium and antimony oxide with possible titanium impurities. It most often forms well-developed, octahedral crystals, which can reach several millimeters in size. Less frequently, it occurs as granular aggregates or as coatings. Its characteristic feature is an intense, warm color. ## Physical properties This mineral is characterized by a hardness of 5.5 on the Mohs scale, placing it in the same category as apatite. Its density is relatively high, at 4.95 g/cm³. Crystals exhibit a luster from vitreous to resinous. They are transparent to translucent. The fracture is subconchoidal, and cleavage is observed along {111} octahedron planes. ## Colors and varieties Hydroxycalcioromeite occurs in shades from amber-yellow and golden-yellow to yellowish-brown. No named color varieties or commercial varieties are distinguished. ## History and name This mineral, originally known as "lewisite", was described as early as 1895. The name hydroxycalcioromeite was introduced in 2010 as part of the redefinition of the pyrochlore supergroup by the International Mineralogical Association (IMA). The new name reflects its chemical composition: "hydroxy" indicates the presence of a hydroxyl group (OH), "calcio" for the dominance of calcium (Ca), and "romeite" for its belonging to the romeite group. The group name comes from the French mineralogist Jean-Baptiste L. Romé de l'Isle. ## Uses Due to its rarity and small crystal size, hydroxycalcioromeite has no industrial application. However, it is a sought-after and valued collector's mineral.

Properties

Mohs hardness: 5.5
Luster: Vitreous to Resinous
Streak: Pale yellow brown
Density: 4.95
Cleavage: {111}
Fracture: Sub-Conchoidal
Transparency: Transparent,Translucent
Crystal system: Isometric

Diagnostic features

## Identification The key diagnostic feature of hydroxycalcioromeite is its characteristic crystal form – they are almost always regular, sharp octahedra. The combination of this form with an amber-yellow color, vitreous or resinous luster, and high density is very suggestive. The streak is pale yellowish-brown. ## Distinguishing from similar minerals Hydroxycalcioromeite can be confused with other minerals from the pyrochlore supergroup, such as microlite or pyrochlore, which also form octahedral crystals. Differentiation often requires advanced chemical analyses (EDS/WDS). It is distinguished from sphalerite or garnets (e.g., spessartine) by its higher density and specific occurrence environment. ## Crystal forms This mineral crystallizes in the isometric system, forming almost exclusively crystals with an octahedral habit. Crystals are usually small, rarely exceeding a few millimeters, but are often well-formed with smooth, lustrous faces.

Geological environment

## Genesis Hydroxycalcioromeite is a mineral of hydrothermal or metamorphic origin. It forms in altered manganese rocks and in calcite veins cutting manganese ore deposits. It can also occur in the oxidation zones of ore deposits. ## Mineral associations This mineral often co-occurs with other oxide minerals and silicates. In its type locality (Tripuí, Brazil), it was found in association with hematite, magnetite, quartz, phlogopite, and other minerals from the pyrochlore group. In other localities, it may occur with braunite, hausmannite, rhodochrosite, and calcite. ## Localities The most important and historical locality, from which the best-known specimens originate, is the Três Cruzes farm in the Tripuí region (Ouro Preto, Minas Gerais) in Brazil. This mineral has also been reported in several other places around the world, including Långban in Sweden, the Prabornaz mine in the Aosta Valley in Italy, and the Fianel mine in Switzerland, although specimens from these localities are usually microscopic in size.

Rarity

Rare

For collectors

## Quality criteria The most highly prized specimens of hydroxycalcioromeite are characterized by sharp, well-formed crystals with an ideal octahedral shape. Intense, amber or golden-yellow color and high transparency are important. Specimens where lustrous crystals are attractively set on a contrasting rock matrix, e.g., on dark hematite, are desirable. Crystal size is a key factor – specimens with crystals larger than 5 mm are already considered exceptional. ## Popular localities By far the most sought-after and classic locality for this mineral is its type locality: Tripuí in Minas Gerais, Brazil. Specimens from there, often described under the historical name "lewisite", are the standard against which other finds are compared and command the highest prices in the collector's market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Ultrasonic cleaners are not recommended. ## What to avoid The mineral is sensitive to strong acids and bases, which can damage its surface. It should be protected from sudden temperature changes and direct, prolonged exposure to sunlight, which could theoretically affect its color. Impacts and scratches should be avoided due to its moderate hardness. ## Storage Collector specimens of hydroxycalcioromeite are best stored in separate, padded boxes or display cases to avoid contact with harder minerals. For display, enclosed cabinets are recommended to protect against dust and mechanical damage.