Ferrowyllieite

Chemical formula: NaNaFe₂+(Fe₂+Al)(PO₄)₃

A rare phosphate mineral, forming granular aggregates of dark green to black color, found in granitic pegmatites.

## Characteristics Ferrowyllieite is a complex sodium, iron, and aluminum phosphate, belonging to the wyllieite group. It most commonly occurs as anhedral (irregular) grains and massive aggregates, strongly intergrown with the host rock. Well-formed crystals are extremely rare. It typically has a dark green, greenish-black, or black color. It is an opaque mineral; only on thin edges can it be slightly translucent. ## Physical Properties The Mohs hardness of ferrowyllieite is approximately 4, classifying it as a relatively soft mineral. It exhibits a vitreous to resinous luster. Its density is significant, around 3.59 g/cm³. ## Colors and Varieties This mineral does not show much color variation, remaining in the range from dark green to black. No named color varieties or commercial forms are known. ## History and Name Ferrowyllieite was described as a new mineral in 1979 by Paul B. Moore and Jun Ito. Its name refers to its chemical composition – with dominant iron (Latin: *ferrum*) – and its structural similarity to the mineral wyllieite. The Victory Mine in Custer County, South Dakota, USA, was designated as the type locality (the place of its first discovery and description). ## Applications Due to its rarity, ferrowyllieite has no industrial applications. It is solely an object of scientific interest and is valued by specialized collectors of pegmatite minerals.

Properties

Mohs hardness: 4
Luster: Vitreous to Resinous
Streak: Grayish green
Density: 3.59
Cleavage: Good on {011}
Fracture: Uneven to Subconchoidal
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Helpful features for identifying ferrowyllieite include its dark color, vitreous or resinous luster, relatively high density, and specific occurrence environment – granitic pegmatites. A Mohs hardness of 4 is also a significant indicator. ## Distinguishing from Similar Minerals Ferrowyllieite can be confused with other dark phosphate minerals found in pegmatites, such as arrojadite, triphylite (often covered with weathering products), or some varieties of apatite. It can also be mistaken for black tourmaline (schorl), which is, however, much harder (hardness approx. 7-7.5). Final and certain differentiation often requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis. ## Crystal Forms Ferrowyllieite crystals are extremely rare. It almost always forms granular, massive aggregates or irregular grains intergrown with quartz, feldspar, or other pegmatite minerals.

Geological environment

## Genesis Ferrowyllieite is a primary hydrothermal mineral, crystallizing in the late stages of the formation of complex granitic pegmatites, rich in phosphorus. It forms under conditions of relatively low temperatures and high pressures, in the presence of volatile components. ## Mineral Associations This mineral occurs in association with other phosphates typical of pegmatites, such as triphylite, sarcopside, graftonite, arrojadite-(KFe), and wolfeite. It also co-occurs with the main rock-forming minerals of pegmatites: quartz, albite (a variety of feldspar), and muscovite. ## Localities The most important and well-documented occurrences of ferrowyllieite in the world are found in the pegmatites of the Black Hills region in South Dakota, USA, with particular emphasis on the Victory Mine near Custer, which is its type locality. The occurrence of this mineral in other parts of the world is very limited and poorly confirmed.

Rarity

Rare

For collectors

## Quality Criteria The collector's appeal of ferrowyllieite is primarily related to its rarity. Specimens rich in this mineral, forming distinct aggregates with an intense, dark green color, are most valued. Samples with visible, even partially formed, crystal faces are of exceptional value. Association with other rare phosphates, forming aesthetic and scientifically interesting compositions, is also of great importance. ## Popular Localities The vast majority of specimens available on the collector's market come from classic localities in the Black Hills pegmatites of South Dakota, USA. Specimens from the Victory Mine are particularly sought after by specialists due to their historical significance as type material.

Care and storage

## Cleaning Ferrowyllieite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, it can be rinsed with distilled water and immediately thoroughly dried. Ultrasonic cleaners are not recommended. ## What to Avoid As a phosphate, this mineral can be sensitive to acids. Contact with all chemicals and detergents should be avoided. Prolonged exposure to moisture may promote its slow weathering. It is stable under typical collecting conditions. ## Storage It is recommended to store specimens in a dry place, in closed boxes or display cases, to protect them from dust and sudden changes in humidity. Avoid placing it in direct contact with minerals that could scratch it.

Sources

IMA Mineral List (CNMNC)