Schmitterite
Chemical formula: (U<sup>6+</sup>O<sub>2</sub>)Te<sup>4+</sup>O<sub>3</sub>
Rare uranyl tellurite forming lemon-yellow, microcrystalline coatings and crusts in oxidation zones.
Description
## Characteristics Schmitterite is a secondary mineral belonging to the tellurite group. It occurs as microscopic crystals aggregated into spherical masses (spherulites), and most often forms thin coatings and crusts on the host rock. Its appearance is very characteristic due to its intense, lemon-yellow or straw-yellow color. Due to the presence of uranium, this mineral is radioactive. ## Physical Properties Schmitterite crystals are too small to assess their macroscopic properties. This mineral is relatively soft, with a hardness of about 2 on the Mohs scale. It exhibits a strong, almost adamantine luster. It is translucent. Its density, calculated based on chemical composition and unit cell parameters, is 6.88 g/cm³, which is a very high value, typical for uranium minerals. ## Colors and Varieties Schmitterite is monochromatic. Its color ranges from lemon-yellow to straw-yellow. No color varieties or commercial varieties are known. ## History and Name The mineral was described in 1971 by Richard V. Gaines. Its name honors Ernst Schmitter (1881–1964), a German mining engineer who worked for many years in Mexico and studied the deposits where this mineral was discovered. The type locality (place of first discovery) is the Moctezuma mine (also known as Bambolla) in Sonora, Mexico. ## Applications Schmitterite has no industrial applications. Due to its rarity, attractive appearance, and origin, it is sought after by advanced mineral collectors, especially those specializing in rare minerals, micromounts, and uranium minerals.
Diagnostic features
## Identification Key diagnostic features of schmitterite are its intense, lemon-yellow color, occurrence in the form of microcrystalline coatings and spherulites, and strong radioactivity, detectable with a Geiger counter. Its paragenesis, i.e., co-occurrence with other tellurides and tellurites in the oxidation zone, is also an important indicator. ## Distinguishing from Similar Minerals Schmitterite can be confused with many other yellow secondary uranium minerals, such as carnotite, tyuyamunite, or autunite. Distinguishing them based on visual characteristics is practically impossible. Certain identification is only provided by chemical analysis (e.g., EDS), which will show the simultaneous presence of uranium and tellurium, and X-ray diffraction (XRD). From other yellow tellurites (e.g., emmonsite), it is distinguished by the presence of uranium and strong radioactivity. ## Crystal Forms Schmitterite crystallizes in the orthorhombic system. It forms microscopic, tabular or bladed crystals, which usually occur as radial aggregates, spherulites, and thin crusts and coatings.
Geological environment
## Genesis Schmitterite is a secondary mineral, formed as a result of weathering and oxidation of primary minerals in hydrothermal deposits rich in tellurium and uranium. It forms under desert conditions, in the oxidation zone (iron cap), above the groundwater level. ## Mineral Associations This mineral occurs in association with other, often rare, tellurium and uranium minerals. It most often co-occurs with paratellurite, native tellurium, emmonsite, uraninite, as well as quartz and hematite. ## Localities Schmitterite is an extremely rare mineral, known practically only from one locality in the world. The best and only significant specimens come from its type locality – the Moctezuma mine (Bambolla) in the municipality of Moctezuma, Sonora, Mexico.
Rarity
Very rare
Collector aspects
## Quality Criteria The quality of schmitterite specimens, being a typical micromineral, is assessed according to different criteria than in the case of minerals forming large crystals. Most valued are specimens where the crusts are thick, have an intense, lemon-yellow color, and cover a significant surface of the matrix. Samples with well-formed spherulites visible under a microscope are exceptionally sought after. The attractiveness of a specimen is also enhanced by the presence of rare associated minerals. ## Popular Localities The only source of valuable collector specimens of schmitterite is the Moctezuma mine in Sonora, Mexico. Specimens from this locality are considered the global standard for this mineral.
Care and storage
## Cleaning Schmitterite specimens should generally not be wet cleaned. Dust and loose contaminants can be removed very carefully with a soft brush or a photographic air blower. Contact with water can lead to damage to the delicate crystals. ## What to Avoid Contact with chemicals, especially acids and bases, should be strictly avoided. As a uranium mineral, schmitterite is radioactive – avoid inhaling dust and direct, prolonged skin contact. After each contact with the specimen, hands should be thoroughly washed. It should not be heated or exposed to prolonged moisture. ## Storage Schmitterite specimens must be stored in a manner that ensures radiological safety. It is recommended to keep them in sealed, lead or glass containers, away from areas of permanent human presence, especially bedrooms and kitchens. The container should be clearly marked with the international radioactivity symbol. It should be stored away from other minerals that may be sensitive to radiation (e.g., causing darkening of smoky quartz).