Ordoñezite

Chemical formula: Zn²⁺Sb⁵⁺₂O₆

Ordoñezite is a rare zinc antimony oxide, forming small, transparent crystals with a vitreous luster and characteristic high density.

## Characteristics Ordoñezite is a rare mineral from the oxide group, chemically classified as a zinc antimonate. It occurs as small, well-formed, octahedral or tabular crystals, rarely exceeding a few millimeters in size. It is most often found as single crystals or small aggregates grown on the host rock. Its luster is intense, vitreous to almost adamantine. ## Physical Properties This mineral is characterized by significant hardness, measuring 6.5 on the Mohs scale, which allows it to scratch glass. It is brittle, and its fracture is conchoidal. One of its most distinguishing features is its very high density (approx. 6.64 g/cm³), making specimens surprisingly heavy for their size. It is transparent, which, combined with its luster, gives it an attractive appearance. ## Colors and Varieties Ordoñezite exhibits a wide range of colors. Colorless, pearly gray specimens are found, as well as various shades of brown – from very light, through yellowish and olive, to almost black. Color variability is one of its characteristic features. ## History and Name The mineral was first described in 1953 by George Switzer and William F. Foshag. Its name honors the Mexican geologist Ezequiel Ordóñez (1867-1950), who was the director of the Geological Institute of Mexico and made significant contributions to the geological research of that country. The type locality, from which the first described specimens originated, is the Santín mine in Guanajuato State, Mexico.

Properties

Mohs hardness: 6.5
Luster: Vitreous
Streak: White
Density: 6.635
Cleavage: None
Fracture: Conchoidal
Transparency: Transparent
Crystal system: Tetragonal

Diagnostic features

## Identification Key diagnostic features of ordoñezite are its tetragonal, often octahedral crystals, high hardness (6.5 on the Mohs scale), and very high density. Vitreous to adamantine luster and a wide range of colors (from colorless to dark brown) are also helpful in identification. Its occurrence in association with other antimony minerals in volcanic rocks is a strong indicator. ## Distinguishing from Similar Minerals Ordoñezite can be confused with other minerals of similar appearance, such as cassiterite, rutile, or some garnets. It differs from cassiterite and rutile by its crystal habit and usually lighter color. It differs from garnets by its crystallographic system (tetragonal, not isometric) and significantly higher density. In a hydrothermal environment, it may resemble sphalerite, but it is much harder. ## Crystal Forms Ordoñezite crystals are usually small and take the form of pyramidal, octahedral bipyramids. Tabular crystals are also found. They occur as single, well-formed crystals or in small groups grown on the rock surface.

Geological environment

## Genesis Ordoñezite is a mineral of hydrothermal origin. It forms as a result of hot, antimony-rich solutions acting on zinc-bearing rocks. It is most commonly found in veins and cavities in volcanic rocks, such as rhyolites. ## Mineral Associations This mineral co-occurs with other oxides and sulfides. In its type locality (Mexico), it was found in association with cassiterite, hematite, native bismuth, quartz, and topaz. In other localities, it may occur with other antimony minerals. ## Localities The most important and historical locality for ordoñezite is its type locality – the Santín mine in Santa Catarina, Guanajuato State, Mexico. This is the only place from which well-formed, collectible specimens of this mineral originate.

Rarity

Very rare

For collectors

## Quality Criteria The most prized ordoñezite specimens are those with sharply defined, well-formed, and fully transparent crystals, set on a contrasting rock matrix. Crystals with a pure, light color (e.g., honey or olive) or completely colorless are highly valued. Crystal size is a key factor – specimens with crystals exceeding a few millimeters are exceptionally rare and sought after. ## Popular Localities The only source of valuable collectible specimens remains the type locality in Guanajuato, Mexico. Specimens from this mine set the standard against which all other finds are compared.

Care and storage

## Cleaning Ordoñezite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, followed by thorough drying of the specimen. Due to its brittleness, ultrasonic cleaners should be avoided. ## What to Avoid Avoid contact with strong acids and other aggressive chemicals. Despite its relatively high hardness, the mineral is brittle and sensitive to impacts and sudden temperature changes. It should not be exposed to prolonged direct sunlight, although there are no reports of it fading. ## Storage Ordoñezite is best stored in separate, padded display boxes to prevent scratching by harder minerals or damage to more delicate specimens. Due to the small size of the crystals, it is ideally suited for display in micromounts.