Alluaudite
Chemical formula: ☐NaMn<sup>2+</sup>Fe<sup>3+</sup><sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>
Alluaudite is a sodium, manganese, and iron phosphate, forming granular aggregates ranging in color from yellowish-green to black, characteristic of granitic pegmatites.
Description
## Characteristics Alluaudite is a mineral from the phosphate group, representing a product of alteration of primary phosphate minerals in granitic pegmatites. It most commonly occurs as compact, granular, or massive aggregates, rarely forming visible, small prismatic crystals. Its appearance is often inconspicuous, with specimens embedded in the host rock. A characteristic feature is its vitreous, sometimes greasy or resinous luster. ## Physical Properties The hardness of alluaudite on the Mohs scale is 5 to 5.5, making it a mineral with moderate scratch resistance. Its density ranges from 3.5-3.7 g/cm³. It is a brittle, translucent mineral, becoming almost completely opaque in darker varieties. ## Colors and Varieties The color palette of alluaudite is quite broad and depends on the proportions of iron and manganese and their oxidation state. It ranges from yellowish-green, through brownish-yellow, olive-green, to dark green and almost black. This mineral is the namesake of the alluaudite group, which includes several structurally similar phosphates. ## History and Name The mineral's name, given in 1848 by the French mineralogist Alexis Damour, honors François Alluaud (father) – a French porcelain factory owner, politician, and naturalist, who first found this mineral in pegmatites in Chanteloube, France. ## Uses Alluaudite has no industrial applications. It is solely an object of scientific and collecting interest, especially for those specializing in pegmatite minerals and mineral systematics.
Diagnostic features
## Identification Alluaudite is recognized in a collection by its characteristic granular habit, color (shades of green, yellow, and brown to black), and greasy or resinous luster. A key diagnostic clue is its occurrence environment – alteration zones in granitic pegmatites, often in direct proximity to primary phosphates. ## Distinguishing from Similar Minerals It can be confused with other massive pegmatitic phosphates, such as triphylite, sarcopside, or heterosite. It usually differs from triphylite by a more greenish hue (triphylite can be more reddish-brown). It differs from sarcopside by the absence of a fibrous habit. Certain identification, especially for unusual specimens, often requires advanced studies, e.g., chemical analysis. ## Crystal Forms Well-formed crystals are very rare and usually small, in the form of short prisms. The vast majority of the material occurs as amorphous-looking masses and granular aggregates, often forming veins or nests.
Geological environment
## Genesis Alluaudite is a secondary mineral. It forms in complex granitic pegmatites as a result of hydrothermal alteration and sodium metasomatism of primary phosphates from the triphylite-lithiophilite series. This process involves the oxidation of iron and manganese and enrichment in sodium. ## Mineral Associations This mineral often co-occurs with the minerals from which it formed, i.e., triphylite and lithiophilite. It is also accompanied by other secondary phosphates, such as heterosite, purpurite, arrojadite, sarcopside, and vivianite. Its surroundings also include typical pegmatite minerals: quartz, feldspars (albite), and micas (lepidolite, muscovite). ## Localities Important alluaudite localities worldwide include the historical type locality in Chanteloube (France), numerous pegmatites in the Black Hills region (South Dakota, USA), the Hagendorf-Süd pegmatite (Germany), Varuträsk (Sweden), Buranga mine (Rwanda), and the famous Sapucaia mine (Minas Gerais, Brazil).
Rarity
Not very common
Collector aspects
## Quality Criteria For collectors, the most valuable specimens are those exhibiting an intense, attractive color (e.g., deep olive-green) and a distinct, granular structure. Samples showing a transitional zone and relics of the primary mineral (e.g., triphylite) are highly prized. Exceptionally rare, well-formed crystals achieve significantly higher value. ## Popular Localities The localities in the Black Hills, South Dakota (USA), where alluaudite forms rich, massive aggregates, are considered classic and provide the best specimens. Materials from the Sapucaia mine in Brazil and the Hagendorf pegmatite in Germany, known for interesting mineral associations, are equally valued.
Care and storage
## Cleaning Alluaudite specimens should only be cleaned mechanically, using a soft brush and distilled water. Ultrasonic cleaners should be avoided, as they could damage the brittle structure of the mineral, especially in specimens with visible fractures. ## What to Avoid The mineral is sensitive to strong acids. It should be protected from contact with chemicals and sudden temperature changes. Due to its moderate hardness, it is easily scratched by harder minerals (e.g., quartz), therefore requiring careful handling. ## Storage Collector's specimens of alluaudite are best stored in separate, padded boxes or on stands to avoid abrasions and mechanical damage. It does not require special conditions regarding humidity or lighting, but it is recommended to protect it from dust.