Triangulite

Chemical formula: Al<sub>3</sub>(U<sup>6+</sup>O<sub>2</sub>)<sub>4</sub>(PO<sub>4</sub>)<sub>4</sub>(OH)<sub>5</sub>·5H<sub>2</sub>O

Triangulite is a very rare, bright yellow uranyl mineral, forming characteristic, triangular crystals.

## Characteristics Triangulite is a hydrated aluminum uranyl phosphate. Its most characteristic feature, from which its name is derived, is the formation of tabular crystals with a distinct triangular outline. These crystals are very small, usually not exceeding 1 mm, and often occur in radial or rosette-like aggregates, forming visually attractive clusters on the host rock. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. It has a vitreous luster and is transparent to translucent. Due to its uranium content, it is highly radioactive. ## Colors and Varieties Triangulite occurs in a characteristic, bright, canary-yellow color. No color varieties or commercial varieties are known. ## History and Name The mineral's name, given in 1983 by Michel Deliens and Paul Piret, refers to the triangular shape of its crystals (Latin: *triangulus* - triangle). It was discovered in the Kobokobo mine in Kivu province, Democratic Republic of Congo, which remains its type locality. ## Uses Due to its extreme rarity and small crystal size, triangulite has no industrial application. It is solely a mineral of scientific and collector's importance, sought after by specialized collectors of uranium minerals.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Pale yellow
Density
4.05
Cleavage
Perfect on {100}
Fracture
Micaceous
Transparency
Transparent to Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification The key diagnostic feature of triangulite is the unique, triangular shape of its canary-yellow crystals. Strong radioactivity is also an important clue. This mineral fluoresces green under ultraviolet light. ## Distinguishing from Similar Minerals It can be confused with other secondary uranium minerals of similar color, such as autunite or torbernite. However, none of these form characteristic, triangular crystals. Final differentiation from very similar minerals of the phosphuranylite group (e.g., phosphuranylite, althupite) requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal Forms Triangulite forms thin, tabular crystals with a triangular outline. These crystals often combine into fan-shaped, rosette-like, or radial aggregates.

Geological environment

## Genesis Triangulite is a secondary uranium mineral, forming in the oxidation (weathering) zone of granitic pegmatite deposits containing uraninite. It forms as a result of the interaction of phosphate- and aluminum-rich waters with primary uranium minerals. ## Mineral Associations It most often co-occurs with other secondary uranium minerals, such as phosphuranylite, althupite, autunite, torbernite, as well as with host rock minerals like quartz, feldspars, and micas. ## Localities The only confirmed and most important occurrence of triangulite in the world is its type locality – the Kobokobo mine in South Kivu province, Democratic Republic of Congo. This is the sole source of collector specimens.

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued specimens are those with sharp, well-formed, triangular crystals forming aesthetic, rosette-like aggregates. Good contrast with the rock matrix and the absence of damage to these extremely delicate structures are important. Crystal size, although always small, also affects value – larger (above 0.5 mm) and clearer crystals are more desirable. ## Popular Localities The only source of triangulite specimens is the Kobokobo mine in the Democratic Republic of Congo. All specimens available on the collector's market originate from this single locality.

Care and storage

## Cleaning Triangulite specimens are generally not cleaned. Any attempts at mechanical or chemical cleaning will almost certainly destroy the delicate crystals. Only very careful dust removal with a soft brush or a photographic air blower is permissible. ## What to Avoid Avoid contact with water, chemicals, ultrasound, and any vibrations or impacts. The crystals are extremely brittle. As a uranium mineral, it requires special precautions – avoid inhaling dust and wash hands after each contact. ## Storage Store in stable conditions, in a closed, lined "micromount" box, away from sources of vibration. Due to radioactivity, specimens should be kept away from areas of permanent human presence, in a well-ventilated room.

Sources

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