Coutinhoite

Chemical formula: Th<sup>4+</sup><sub>x</sub>Ba<sub>1-2x</sub>(U<sup>6+</sup>O<sub>2</sub>)<sub>2</sub>Si<sub>5</sub>O<sub>13</sub>·3H<sub>2</sub>O

Coutinhoite is an extremely rare, radioactive mineral from the uranyl silicate group, discovered in Brazil, forming microscopic, yellow crystals.

## Characteristics Coutinhoite is a hydrated uranyl, thorium, and barium silicate. It occurs as very small, acicular or platy crystals, rarely exceeding one millimeter in length. These crystals form radial aggregates or rosettes of an intensely yellow color. Due to its chemical composition, this mineral is highly radioactive. ## Physical Properties Coutinhoite crystals exhibit a vitreous luster. Hardness and density have not been precisely measured due to the small size and rarity of samples; however, the density calculated based on the chemical formula and unit cell parameters is approximately 4.35 g/cm³. The mineral is brittle. ## Colors and Varieties This mineral occurs in a characteristic, bright yellow color. No color varieties or commercial forms are known. ## History and Name Coutinhoite was first described in 2011 by a team of mineralogists (Atencio et al.) based on samples found in a granite quarry in Ubatuba, São Paulo state, Brazil. The mineral is named in honor of José Moacyr Vianna Coutinho (1924–2007), a professor of mineralogy at the University of São Paulo, in recognition of his contributions to Brazilian mineralogy. ## Uses Coutinhoite has no industrial or commercial applications. Its significance is purely scientific and as a collector's item, as an extremely rare representative of uranyl minerals.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Light yellow
Density
4.35
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Coutinhoite can be identified by its intensely yellow color, characteristic radial aggregates composed of acicular crystals, and strong radioactivity, easily detectable with a Geiger counter. It occurs in vugs in granitic rocks. Under shortwave UV light, it exhibits weak, greenish fluorescence. ## Distinguishing from Similar Minerals It can be confused with other yellow secondary uranium minerals, such as uranophane, beta-uranophane, or boltwoodite. Uranophane often forms similar radial aggregates but has a different chemical composition (it is a calcium uranyl silicate). Precise differentiation of these minerals requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Crystal Forms Coutinhoite crystals are elongated, acicular or platy, with a rhombic cross-section. They typically occur as radial or stellate aggregates, as well as disordered clusters on the surface of the host rock.

Geological environment

## Genesis Coutinhoite is a secondary mineral, forming in the oxidation zone (weathering) of uranium deposits. It forms as a result of hydrothermal processes or the weathering of primary uranium minerals, such as uraninite, in the presence of solutions rich in silica, thorium, and barium. It occurs in fissures and small vugs in granitic rocks and pegmatites. ## Mineral Associations This mineral co-occurs with other secondary uranium minerals. In its type locality (Ubatuba, Brazil), it was found in association with uraninite, beta-uranophane, boltwoodite, gypsum, hematite, kaolinite, and quartz. ## Localities The only confirmed and described locality of coutinhoite in the world is the "Pedreira do Piquiri" granite quarry in Ubatuba, São Paulo state, Brazil. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality Criteria The collector's appeal of coutinhoite primarily depends on the quality and definition of the crystal aggregates. Specimens with well-formed, radial rosettes of intense yellow color, contrasting with the darker host rock, are most valued. Due to the microscopic size of the crystals, the size of the aggregate itself is also important. The purity of the specimen and the absence of mechanical damage are crucial. ## Popular Localities The only source of coutinhoite specimens is its type locality in Ubatuba, Brazil. Material from this location is extremely difficult to acquire and appears on the collector's market sporadically, mainly through specialized channels.

Care and storage

## Cleaning Coutinhoite specimens should not be wet cleaned. Due to their fragility and small size, dust is best removed using compressed air from a safe distance or a very soft brush. Always wash hands after contact with the mineral. ## What to Avoid Avoid contact with water and chemicals. As a secondary mineral, it may be unstable in a humid environment. Due to strong radioactivity, exposure should be limited, and inhalation of dust should be avoided. It should not be heated or exposed to ultrasound. ## Storage Coutinhoite specimens must be stored in specialized, sealed, and appropriately labeled containers designed for radioactive minerals. These containers should be kept away from other minerals that could be damaged by radiation, and in a place inaccessible to children and pets.

Sources

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