Trigonite

Chemical formula: Pb₂+₃Mn₂+(As₃+O₃)₂(As₃+O₂OH)

Trigonite is a very rare lead and manganese arsenite, forming characteristic, triangular crystals with a strong, adamantine luster.

## Characteristics Trigonite is an extremely rare mineral from the arsenite group, chemically a lead and manganese arsenite. Its most characteristic feature, from which its name is derived, is its tendency to form crystals with a triangular or wedge-like outline. It typically occurs as small, tabular or thick-tabular crystals, which can be single or form small aggregates. Specimens most often have an intense yellow color, transitioning to honey-yellow or yellowish-brown hues. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2-3. It is also very dense due to its high lead content. It possesses an exceptionally strong, adamantine to greasy luster, which gives it a characteristic, vibrant appearance, despite the small size of its crystals. It is transparent to translucent. ## Colors and Varieties Trigonite is known primarily for its intense yellow color, which can range from canary yellow, through honey yellow, to yellowish-brown. No named varieties of this mineral have been distinguished. ## History and Name The name trigonite, given in 1899 by Gustaf Flink, comes from the Greek word "trigonon" (τρίγωνον) meaning "triangle". It directly refers to the typical, triangular outline of its crystals. The mineral was discovered in the famous Långban skarn deposit in Sweden, which is its type locality. ## Uses Due to its extreme rarity, trigonite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals and for scientific institutions.

Properties

Mohs hardness: 2-3
Luster: Adamantine
Streak: Yellow
Density: 8.28
Cleavage: Imperfect on {001}
Fracture: Conchoidal
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Trigonite is identified based on a combination of unique features: triangular crystal outline, intense yellow color, very high, adamantine luster, and extremely high density. Its softness (2-3 on the Mohs scale) is also an important clue. Its occurrence in association with other rare minerals from Långban is key for identification. ## Distinguishing from Similar Minerals Minerals of similar color, such as mimetite or wulfenite, are usually harder and have a different crystal habit. No other mineral combines such a characteristic triangular shape, adamantine luster, and such high density with such low hardness. ## Crystal Forms Trigonite crystallizes in the monoclinic system, but its crystals exhibit pseudo-trigonal symmetry. It forms thick, tabular crystals with a distinctly triangular or wedge-shaped outline. More complex forms or small aggregates of such crystals are less common.

Geological environment

## Genesis Trigonite is a mineral of metamorphic origin. It forms under unique conditions within metamorphosed iron and manganese ore deposits, embedded in carbonate rocks (marbles and skarns). Its formation is associated with hydrothermal processes occurring in the late stage of metamorphism. ## Mineral Associations This mineral occurs in association with other rare minerals, typical of the Långban deposit. It most commonly co-occurs with native lead, hausmannite, tephroite, baryte, phlogopite, and other lead arsenites such as finnemanite or magnussonite. ## Localities The only confirmed and verified occurrence of trigonite in the world is the Långban mine in the Värmland region of Sweden. This is the type locality for this mineral.

Rarity

Extremely rare

For collectors

## Quality Criteria The most desirable trigonite specimens are those with sharply defined, well-formed crystals of a distinct triangular shape. The intensity and purity of the yellow color, as well as a strong luster, significantly increase their value. Specimens where crystals are set on a contrasting matrix, which highlights their form, are highly prized. Size matters, but even millimeter-sized, perfectly formed crystals are considered exceptional. ## Popular Localities The only source of trigonite specimens is the historic Långban mine in Sweden. All specimens available on the collector's market originate from this single, classic locality.

Care and storage

## Cleaning Due to its great rarity and delicacy, cleaning trigonite specimens should be kept to an absolute minimum. If necessary, compressed air can be used to remove dust. Avoid contact with water and any chemical agents. ## What to Avoid Trigonite is a very sensitive mineral. Avoid: - All chemicals, acids, and solvents. - Ultrasonic cleaners and steam cleaning. - Sudden temperature changes. - Prolonged exposure to direct sunlight, which can cause fading. - Contact with water, which can damage the crystal surfaces. - Due to its lead and arsenic content, avoid inhaling dust and wash hands after any contact with the mineral. ## Storage Trigonite specimens should be stored in stable conditions, preferably in a sealed collector's box (e.g., a "membrane box" or a lidded box), to protect them from dust, mechanical damage, and humidity changes. Store away from light and heat sources.

Sources

IMA Mineral List (CNMNC)