Arrojadite-(SrFe)

Chemical formula: SrFe₂+(CaNa₂)Fe₂+₁₃Al(PO₄)₁₁(PO₃OH)(OH)₂

Arrojadite-(SrFe) is a very rare, dark green to black mineral from the arrojadite group, distinguished by the presence of strontium and iron in its complex chemical structure.

## Characteristics Arrojadite-(SrFe) is a complex phosphate belonging to the arrojadite group. It typically forms granular or massive aggregates, less frequently occurring as imperfectly formed crystals. Its color ranges from dark green, through greenish-black, to black. It is an opaque mineral with a vitreous or resinous luster. ## Physical Properties This mineral is characterized by a hardness of approximately 5 on the Mohs scale. Its density is relatively high, around 3.55 g/cm³. It exhibits good cleavage in one direction. The fracture is uneven to conchoidal. ## History and Name The name "arrojadite" honors the Brazilian geologist Miguel Arrojado Ribeiro de Lisboa (1872–1932). The "(SrFe)" suffix refers to the dominant elements in specific positions within its crystal structure – strontium (Sr) and iron (Fe). It was formally described as a new mineral species in 2005, after re-examination of material from its type locality in Brazil.

Properties

Mohs hardness: 5
Luster: Vitreous to resinous
Streak: Grayish green
Density: 3.55
Cleavage: Good on {010}
Fracture: Uneven to subconchoidal
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Arrojadite-(SrFe) is difficult to identify visually. Its diagnosis requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical microanalysis (EDS/WDS), which allow for the determination of its unique chemical composition and crystal structure. Dark green color, relatively high density, and occurrence in granitic pegmatites are indicative features. ## Differentiation from Similar Minerals It can be confused with other dark phosphate minerals found in pegmatites, such as triphylite, graftonite, or other minerals from the arrojadite group. Definitive differentiation is only possible through detailed laboratory analysis. ## Crystal Forms Crystals are rare and usually poorly formed, with a tabular or prismatic habit. Most often, it forms massive, granular aggregates within the host rock.

Geological environment

## Genesis It is a primary mineral, crystallizing in the late stages of the evolution of granitic pegmatites, rich in phosphorus and lithium. ## Mineral Associations It co-occurs with other phosphate minerals typical of pegmatites, such as triphylite, as well as with quartz, microcline, and albite. ## Localities Its type locality, from which the best-studied specimens originate, is the Sapucaia mine in Galileia, Minas Gerais state, Brazil. This is the only confirmed and well-documented occurrence of this mineral in the world.

Rarity

Very rare

For collectors

## Quality Criteria As a mineral of primarily scientific and systematic importance, its collector's value is small. The most desirable specimens for specialized collections would be those rich in the mineral, exhibiting visible, even if imperfect, crystalline forms, originating from the type locality, and with analytically confirmed identity.

Care and storage

## Cleaning It is recommended to clean only with a soft, dry brush to remove dust. The use of water, especially with detergents, is inadvisable due to the risk of chemical reactions. ## What to Avoid Avoid contact with all chemicals, acids, and bases. The mineral is sensitive to rapid temperature changes. It should not be exposed to prolonged strong sunlight. ## Storage Specimens should be stored in stable, dry conditions, preferably in sealed boxes or display cases, to protect them from dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)