Armellinoite-(Ce)
Chemical formula: Ca<sub>4</sub>Ce<sup>4+</sup>(As<sup>5+</sup>O<sub>4</sub>)<sub>4</sub>·H<sub>2</sub>O
An extremely rare calcium and cerium arsenate, forming characteristic reddish-brown radial aggregates.
Description
## Characteristics Armellinoite-(Ce) is a very rare arsenate mineral with the chemical formula Ca₄Ce⁴⁺(AsO₄)₄·H₂O. It forms small, prismatic or tabular crystals, most commonly occurring as radial or spherulitic aggregates. Its most distinctive visual feature is these fan-shaped or spherical clusters, resembling small sea urchins or rosettes, deposited on the host rock. ## Physical Properties The mineral exhibits a vitreous to resinous luster. It is translucent, and in thicker sections, opaque. Its density, calculated based on chemical composition and crystal structure, is approximately 3.99 g/cm³. Hardness has not yet been precisely determined. ## Colors and Varieties Armellinoite-(Ce) occurs in shades from reddish-brown to brown. The intensity of the color can vary within a single aggregate. No color varieties or commercial forms are known. ## History and Name The mineral was first described in 2011. Its name honors the Italian Armellino family, known mineral collectors who found the first specimens. The suffix "-(Ce)" indicates the dominance of cerium in its chemical composition, in accordance with the classification of rare earth minerals. The type locality (locus typicus) is the Vioolsdrif pegmatite field in the Northern Cape Province, Republic of South Africa. ## Applications Due to its extreme rarity, armellinoite-(Ce) has no industrial applications. It is solely an object of scientific interest and a highly prized specimen in advanced mineral collections.
Diagnostic features
## Identification The key diagnostic feature of armellinoite-(Ce) is its mode of occurrence – radial, fan-shaped, or spherical aggregates of a reddish-brown color. Its geological environment, i.e., miarolitic cavities in granitic pegmatites, is also characteristic. ## Distinguishing from Similar Minerals It can be confused with other rare arsenates or rare earth phosphates that form similar aggregates, e.g., some varieties of agardite or cheralite. Due to its rarity and similarity to other minerals, certain identification is almost exclusively possible using advanced analytical methods, such as X-ray diffraction (XRD) and chemical microanalysis (EDS). ## Crystal Forms Crystals are prismatic to tabular, but rarely occur as single, well-formed individuals. They almost always form radial, spherulitic, or fibrous aggregates.
Geological environment
## Genesis Armellinoite-(Ce) is a hydrothermal mineral, crystallizing in the late stages of granitic pegmatite formation. It forms in rock cavities, known as miaroles, as a result of the action of solutions rich in arsenic, calcium, and rare earth elements, primarily cerium. ## Mineral Associations It most commonly co-occurs with pegmatite-forming minerals such as quartz, albite, and microcline. In miarolitic cavities, it is accompanied by other, often rare, minerals. ## Localities The most important and historically first occurrence is the pegmatites in the Vioolsdrif area in the Northern Cape Province (RSA). Its presence has also been confirmed in pegmatites near the town of Vlastějovice in the Czech Republic.
Rarity
Extremely rare
Collector aspects
## Quality Criteria The most highly prized specimens are those with well-defined, large, and numerous radial aggregates of intense, reddish-brown color. The aesthetics of the host rock (matrix) and the absence of damage to the delicate needles forming the aggregates are also important. Color contrast with light quartz or feldspar enhances visual appeal. ## Popular Localities The vast majority of the highest quality and largest specimens of armellinoite-(Ce) come from the type locality – the Vioolsdrif pegmatite field in the Republic of South Africa.
Care and storage
## Cleaning Specimens should only be dry-cleaned, using a soft brush to remove dust. Contact with water, especially tap water, is not recommended. If absolutely necessary, a small amount of distilled water can be used, and the specimen should be immediately dried with compressed air. ## What to Avoid Ultrasonic cleaners, all chemical agents, and especially acids, which can quickly destroy the mineral, must be absolutely avoided. As an arsenate, it is potentially toxic – avoid inhaling dust and wash hands after any contact with the specimen. ## Storage It is recommended to store in a closed, stable display case or box, away from direct sunlight, moisture, and sudden temperature changes. Specimens should be properly labeled, taking into account potential toxicity.