Arrojadite-(KFe)

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

Arrojadite-(KFe) is a rare, dark green to black phosphate mineral found in granitic pegmatites.

## Characteristics Arrojadite-(KFe) is a complex phosphate belonging to the arrojadite group. It typically forms anhedral masses or granular aggregates, less frequently imperfectly formed crystals. Its color is most often dark green, greenish-black, or 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, ranging from 3.51-3.55 g/cm³. It exhibits good cleavage in one direction. ## Colors and Varieties The dominant colors are shades of dark green, transitioning to greenish-black and black. In thin splinters, it may be translucent green or brownish-green. There are no named commercial or color varieties. ## History and Name The arrojadite group was named in honor of Miguel Arrojado Ribeiro Lisboa (1872-1932), a Brazilian geologist and engineer. Arrojadite-(KFe) is a member of this group with a dominance of potassium (K) and iron (Fe), which is reflected in its name. It was formally described as a distinct mineral species during the revision of the arrojadite group in 2005 by the International Mineralogical Association (IMA). ## Uses Arrojadite-(KFe) has no industrial applications. Its significance is purely scientific and as a collector's item, being a rare and chemically complex phosphate.

Properties

Mohs hardness: 5
Luster: Vitreous to resinous
Streak: Greenish gray
Density: 3.51-3.55
Cleavage: Good on {001}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Arrojadite-(KFe) is difficult to identify visually without advanced research methods. Preliminary identification is based on its characteristic dark green to black color, vitreous or resinous luster, and occurrence in granitic pegmatites. Its relatively high density can be a helpful indicator. ## Distinguishing from Similar Minerals It can be confused with other dark phosphates found in pegmatites, such as triphylite, graftonite, or especially other minerals from the arrojadite group. Definitive differentiation from these minerals requires chemical analysis (e.g., EDS) or X-ray diffraction (XRD) to determine composition and structure. ## Crystal Forms Crystals are rare and usually poorly formed, with a tabular or thick-tabular habit. It most often occurs as granular or massive aggregates embedded in the host rock.

Geological environment

## Genesis It is a primary mineral, crystallizing in the late stages from phosphorus- and lithium-rich post-magmatic fluids in granitic pegmatites, especially of the LCT (lithium-cesium-tantalum) type. ## Mineral Associations It often co-occurs with other pegmatite-typical phosphates, such as triphylite, lithiophilite, graftonite, sarcopside, wolframite, as well as with quartz, microcline, albite, and muscovite. ## Localities The most important and well-documented occurrences of arrojadite-(KFe) worldwide include: - Palermo No. 1 Mine in North Groton, New Hampshire, USA (a classic locality for many phosphates). - White Elephant Mine, Custer, South Dakota, USA. - Localities in the Hagendorf-Pleystein region in Bavaria, Germany. - Serra Branca, Paraíba, Brazil (the type locality of original arrojadite).

Rarity

Rare

For collectors

## Quality Criteria Arrojadite-(KFe) specimens are valued primarily by collectors specializing in rare, systematic, and pegmatite minerals. The most desirable specimens are those with well-formed, even if partially, crystals. Rich aggregates with an intense, dark green color, clearly contrasting with the lighter host rock (matrix), are also highly prized. Association with other rare phosphates is also important. ## Popular Localities The most classic and valued specimens, often subjects of scientific research, come from the Palermo No. 1 Mine in New Hampshire, USA. Material from this locality is considered reference material for this mineral.

Care and storage

## Cleaning Specimens should be cleaned only mechanically, using a soft, dry brush to remove dust. Due to possible reactivity with acids and potential water content, wet cleaning and the use of any chemicals should be avoided. ## What to Avoid Avoid contact with water, acids, and other cleaning agents. The mineral is relatively soft and brittle, so it must be protected from impacts and scratching. It should not be heated or exposed to prolonged strong light. ## Storage It is recommended to store specimens in a dry place, in closed collector boxes or display cases, to protect them from dust, humidity, and mechanical damage. It should be kept separate from harder minerals.

Sources

IMA Mineral List (CNMNC)