Almeidaite

Chemical formula: Pb2+Zn2+2(Mn2+,Y)(Ti4+,Fe3+)18O36(OH,O)2

Almeidaite is an extremely rare mineral from the crichtonite group, an oxide with a complex chemical composition, forming black, tabular crystals.

Description

## Characteristics Almeidaite is a complex oxide belonging to the crichtonite group. It forms well-developed, tabular or short prismatic crystals with a hexagonal outline, reaching up to several millimeters in size. Its color is uniformly black, and the crystal surfaces are characterized by a submetallic, sometimes slightly resinous luster. It is an opaque mineral. ## Physical Properties Almeidaite is a relatively hard mineral, with a hardness of approximately 6 on the Mohs scale. Its density is significant, around 4.63 g/cm³, which is noticeable even in small specimens. It does not exhibit cleavage, and its fracture is conchoidal. ## Colors and Varieties This mineral occurs exclusively in black. No color varieties or commercial varieties are known. ## History and Name Almeidaite was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 2013. Its name honors Fernando Flávio Marques de Almeida (1916–2013), a distinguished Brazilian geologist, for his contributions to the geology of South America. It was described by a research team led by Luiz A. D. Menezes Filho. ## Uses Due to its extreme rarity, almeidaite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for specialized collectors of rare minerals.

Diagnostic features

## Identification Characteristic features of almeidaite include its black color, submetallic luster, hexagonal crystal habit, and high density. Its occurrence in granite pegmatites in association with quartz and feldspars is also an important clue. However, definitive identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS), due to its similarity to other minerals in the crichtonite group. ## Distinguishing from Similar Minerals Almeidaite can be confused with other black, heavy minerals from the crichtonite group, such as senaite or crichtonite. Distinguishing them is practically possible only on the basis of chemical composition analysis, which in almeidaite is characterized by the presence of lead and zinc. From more common minerals, such as ilmenite or hematite, it differs in crystal habit and specific occurrence environment. ## Crystal Forms Almeidaite crystallizes in tabular, hexagonal crystals, often flattened parallel to the basal plane. It occurs as single, overgrown crystals in rock cavities (miaroles).

Geological environment

## Genesis Almeidaite is a mineral of magmatic origin, crystallizing in the late stages of granite pegmatite formation. It forms in rare-element-rich, differentiated intrusions, often within miaroles, which are small cavities in the rock. ## Mineral Associations This mineral occurs in association with quartz (often smoky), albite, riebeckite, and zircon. ## Localities The only confirmed and described locality for almeidaite in the world is its type locality – the Novo Horizonte pegmatite in the municipality of Novo Horizonte, in the state of Bahia, Brazil.

Rarity

Extremely rare

Collector aspects

## Quality Criteria The most prized almeidaite specimens are those with sharp, well-formed, lustrous crystals with a distinct hexagonal outline. The size of the crystals is of great importance, as most of them are very small. Specimens in which the black almeidaite crystal is set on a contrasting, light matrix (e.g., on white albite or transparent quartz) are particularly sought after by collectors. ## Popular Localities The only source of collector specimens is the Novo Horizonte pegmatite in Brazil. Material from this locality is extremely limited, making it one of the rarest minerals available on the market.

Care and storage

## Cleaning Almeidaite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, crystals can be rinsed with distilled water. Ultrasonic cleaners should be avoided, as they could cause cracks in brittle crystals. ## What to Avoid The mineral is sensitive to strong acids. It should be protected from impacts and falls, as it is brittle despite its considerable hardness. It is stable under normal temperature conditions and is not sensitive to sunlight. ## Storage It is recommended to store almeidaite specimens in separate, padded boxes to prevent scratching by other minerals. Protect from dust and moisture. It is best displayed in enclosed display cases.