Retzian-(La)

Chemical formula: Mn₂+₂LaAs₅+O₄(OH)₄

A rare manganese and lanthanum arsenate, forming small, radial aggregates of a reddish-brown color.

## Characteristics Retzian-(La) is a rare mineral from the retzian group, belonging to the arsenate class. It forms small, most often radial or fan-shaped aggregates, as well as crusts and druses. Well-formed, single crystals with a platy or short-prismatic habit are rare. It usually occurs as aggregates composed of numerous, tiny crystals. ## Physical properties This mineral is characterized by a hardness of 4 on the Mohs scale, which makes it relatively soft. It has a vitreous luster, transitioning to greasy on some surfaces. It is translucent to opaque. Its density is approximately 4.16 g/cm³. ## Colors and varieties Retzian-(La) ranges in color from reddish-brown and brown to yellowish-brown. The name Retzian-(La) distinguishes it from other members of the retzian group, in which the dominant rare earth element is cerium (Retzian-(Ce)) or neodymium (Retzian-(Nd)). Color varieties do not have separate trade names. ## History and name The mineral (as a species) was named in 1894 by Hjalmar Sjögren in honor of the Swedish naturalist Anders Johan Retzius (1742-1821). The suffix -(La) was added later, in accordance with the rules for naming minerals containing rare earth elements, to indicate lanthanum as the dominant element in this particular mineral. ## Applications Retzian-(La) has no industrial application. It is solely an object of scientific interest and a valued acquisition in advanced systematic and regional mineral collections.

Properties

Mohs hardness: 4
Luster: Vitreous to greasy
Streak: Yellowish brown
Density: 4.16
Cleavage: Good on {100}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Characteristic features helpful in identification are: reddish-brown color, radial or fan-shaped aggregates, vitreous to greasy luster, and occurrence in a specific geological environment (metamorphosed Mn-Fe deposits). Relatively low hardness is also key. ## Distinguishing from similar minerals Retzian-(La) can be confused with other brown secondary minerals found in the same localities. It is distinguished from goethite by a lighter, yellowish-brown streak. Distinguishing it from other minerals of the retzian group, such as Retzian-(Ce), is impossible without advanced chemical analyses (e.g., EDS). ## Crystal forms Crystals are orthorhombic, usually with a platy or short-prismatic habit. Most often they form radial, fan-shaped or rosette-like aggregates, as well as druses and crusts on the surface of other minerals.

Geological environment

## Genesis It is a secondary mineral, formed in metamorphosed, stratiform iron and manganese ore deposits. It forms in fractures and small caverns, crystallizing from hydrothermal solutions rich in arsenic and rare earth elements. ## Mineral associations In the type locality (Sterling Hill, USA), it coexists with calcite, franklinite, willemite, and other rare arsenates. In Swedish mines (Nordmark), it is accompanied by hausmannite, pyrochroite, and calcite. ## Localities The most important and well-known occurrences of this mineral in the world are the Sterling Mine in Sterling Hill, Ogdensburg, New Jersey, USA (type locality) and mines in the Nordmark Odal Field area near Filipstad, in the Värmland region of Sweden.

Rarity

Very rare

For collectors

## Quality criteria The most highly valued specimens are those with well-formed, radial aggregates of intense, reddish-brown color, set on a contrasting matrix. Association with other rare minerals from the given locality is also of great importance. Due to its rarity, even small but distinct aggregates are valuable to collectors. ## Popular localities The most sought-after specimens by collectors come from two classic localities: Sterling Hill in the USA and Nordmark in Sweden. They set the standard against which other finds are compared.

Care and storage

## Cleaning Specimens should be cleaned very carefully, preferably dry with a soft brush. If necessary, brief cleaning in distilled water with a soft brush is permissible. After wet cleaning, the specimen should be thoroughly dried immediately. Ultrasonic cleaners should not be used. ## What to avoid As an arsenate, this mineral is potentially toxic – inhalation of dust and ingestion should be avoided. Due to its low hardness (4 on the Mohs scale), it is susceptible to scratching. It should be protected from contact with harder minerals. Contact with acids and other strong chemicals should be avoided. ## Storage It is recommended to store specimens in separate, closed display boxes, away from harder minerals, to avoid mechanical damage. The label should include a warning about arsenic content.

Sources

IMA Mineral List (CNMNC)