Arisite-(La)

Chemical formula: NaLa<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>[(CO<sub>3</sub>)<sub>1-x</sub>F<sub>2x</sub>]F

Arisite-(La) is a rare carbonate group mineral, forming small, hexagonal crystals with a vitreous luster, found in alkaline complexes.

## Characteristics Arisite-(La) is a complex sodium and lanthanide carbonate, belonging to the arisite group. It forms very small, usually microscopic, hexagonal, tabular or prismatic crystals. Macroscopic specimens are extremely rare. Most often, it occurs as single, dispersed crystals or small aggregates embedded in the host rock. It is a transparent to translucent mineral. ## Physical properties Arisite-(La) crystals exhibit a vitreous luster. The Mohs hardness is estimated at 3 to 3.5, and the density is approximately 4.16 g/cm³. This mineral is brittle and exhibits perfect cleavage in one direction. ## Colors and varieties Arisite-(La) is usually colorless, white, gray, or pale yellow. No color or commercial varieties are distinguished. ## History and name The mineral's name comes from its type locality – the Aris quarry in Namibia. The suffix -(La) indicates the dominant rare-earth element in its composition – lanthanum. The mineral was described by Oleg von Knorring and Th. G. Sahama in 1982, and approved by the International Mineralogical Association (IMA) in 1981. ## Uses Arisite-(La) has no industrial applications. It is of purely scientific and collector's interest as a rare lanthanide mineral.

Properties

Mohs hardness
3-3.5
Luster
Vitreous
Streak
White
Density
4.16
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Hexagonal

Diagnostic features

## Identification Amateur identification of arisite-(La) is practically impossible due to the microscopic size of the crystals and their similarity to many other carbonates. Definitive identification requires advanced analytical methods, such as Raman spectroscopy or X-ray microanalysis (EDS/WDS), which can confirm its chemical composition. ## Distinguishing from similar minerals It can be confused with other rare-earth carbonates, such as bastnäsite, parisite, or ancylite, as well as calcite or dolomite. Differentiation is based on chemical analysis and crystallographic studies. Reaction with hydrochloric acid is a common feature for many carbonates. ## Crystal forms Arisite-(La) crystallizes in the hexagonal system, forming small, tabular crystals with a hexagonal outline. It also occurs as short prisms. Crystals rarely exceed fractions of a millimeter and are usually embedded in other minerals.

Geological environment

## Genesis Arisite-(La) is a hydrothermal mineral, associated with the late stages of crystallization in alkaline igneous complexes, such as carbonatites and nepheline syenites. It forms in rock cavities and druses from solutions rich in sodium, fluorine, and rare-earth elements. ## Mineral associations This mineral co-occurs with other rare alkaline minerals. In the type locality (Aris, Namibia), it was found in association with microcline, nepheline, aegirine, natrolite, willemite, tuperssuatsiaite, and makatite. In Mont Saint-Hilaire (Canada), it is associated with aegirine, rhodochrosite, sericite, microcline, and analcime, among others. ## Localities The most important and well-documented occurrences of arisite-(La) are: - Aris Quarry, Windhoek, Namibia (type locality). - Mont Saint-Hilaire alkaline complex, Quebec, Canada. - Khibiny Massif and Lovozero Massif on the Kola Peninsula, Russia.

Rarity

Very rare

For collectors

## Quality criteria The quality of arisite-(La) specimens is primarily assessed based on the size and development of the crystals, which is a rarity. The most prized specimens are those with visible, sharply terminated, transparent crystals, clearly standing out from the host rock. Color contrast with surrounding minerals and the richness of the mineral association are also important. ## Popular localities The most well-known collector's specimens, though still microscopic, come from the quarries in Mont Saint-Hilaire, Canada. This locality is famous for providing specimens with relatively well-formed crystals in association with other rare minerals.

Care and storage

## Cleaning Specimens should only be cleaned very carefully, using a soft brush to remove dust. Due to the brittleness and small size of the crystals, wet cleaning or the use of ultrasonic cleaners is not recommended. ## What to avoid The mineral is sensitive to acids, in which it reacts violently with the release of carbon dioxide. Contact with chemicals, sudden temperature changes, and mechanical shocks that could damage the delicate crystals should be avoided. ## Storage It is recommended to store specimens in stable conditions, in sealed "micromount" boxes that protect against dust and mechanical damage. Exposure to moisture and chemical contamination should be avoided.

Sources

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