Wyllieite

Chemical formula: NaNaMn₂+(Fe₂+Al)(PO₄)₃

Wyllieite is a rare phosphate mineral with a complex chemical composition, forming granular aggregates of dark green, blue-green, or black color.

## Characteristics Wyllieite is a complex phosphate of sodium, manganese, iron, and aluminum, belonging to the alluaudite group. It typically occurs as granular or massive aggregates, rarely forming visible, platy crystals of small size, not exceeding a few millimeters. Its appearance is most often inconspicuous, and identification requires advanced research methods. ## Physical Properties This mineral is characterized by a Mohs hardness of 4 and a density ranging from 3.55-3.65 g/cm³. It has a vitreous to resinous luster. It is a brittle mineral with an uneven fracture. It exhibits good cleavage in one direction. ## Colors and Varieties Typical colors of wyllieite range from dark green, through blue-green, greenish-black, to black. In thin fragments, it can be transparent to translucent and exhibit pleochroism in shades of blue, yellowish-green, and olive-green. No commercial or colored varieties are distinguished. ## History and Name The mineral is named in honor of Peter John Wyllie (born 1930), a British-American petrologist and geochemist, professor at the University of Chicago and Caltech, for his contributions to experimental petrology. The mineral was first described in 1973 based on material from the Victory pegmatite in Custer, South Dakota (USA). ## Uses Wyllieite has no industrial application. Its significance is purely scientific and as a collector's item, as a rare component of some granitic pegmatites.

Properties

Mohs hardness: 4
Luster: Vitreous to resinous
Streak: Greenish-gray
Density: 3.55-3.65
Cleavage: Good on {100}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Macroscopic identification of wyllieite is very difficult and often impossible without chemical analysis (EDS/WDS) or X-ray diffraction (XRD). Its occurrence in granitic pegmatites, dark green or black color, granular form, and co-occurrence with other phosphates can be indicative. ## Distinguishing from Similar Minerals It can be confused with other dark phosphates from the alluaudite group (e.g., arrojadite, ferroalluaudite) or other black pegmatite minerals, such as triplite, arfvedsonite, or some tourmalines (schorl). Definitive distinction is almost exclusively possible based on advanced laboratory analyses. ## Crystal Forms Crystals are extremely rare, platy, pseudo-octahedral in habit, reaching up to 3 mm. It usually forms untwinned, granular aggregates within the rock mass.

Geological environment

## Genesis Wyllieite is a primary or secondary mineral, crystallizing in the late stages of the evolution of complex granitic pegmatites, rich in phosphorus, lithium, and sodium. It forms in the intermediate zones of these pegmatites. ## Mineral Associations It co-occurs with other phosphates, such as arrojadite-(KFe), sarcopside, graftonite, triplite, wolfeite, as well as with quartz, albite, microcline, and muscovite. ## Localities The most important localities worldwide are those in the United States, in the Black Hills pegmatites of South Dakota (Victory, Tip Top, White Elephant, Big Chief mines). It is also known from the Angarf-Sud pegmatite in Morocco, from the Viitaniemi area in Finland, and from rare pegmatites in Brazil (Minas Gerais) and Australia (New South Wales).

Rarity

Rare

For collectors

## Quality Criteria The quality of a specimen primarily depends on its scientific confirmation and precise locality. Samples with well-formed, even if small, crystals are most valued. Association with other rare phosphates is also important. Color or aggregate size are of secondary importance due to the mineral's inconspicuous appearance. ## Popular Localities The most classic and valued specimens by collectors come from the type locality at the Victory mine and other pegmatites in Custer County, South Dakota, USA.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. Due to possible reactivity and the presence of iron, contact with water and any chemical agents should be avoided. ## What to Avoid Avoid ultrasonic cleaners, chemical agents (acids, bases), and prolonged contact with water, which can accelerate oxidation processes. The mineral is brittle and sensitive to impact. ## Storage It is recommended to store in dry conditions, in a closed display box or cabinet, away from direct sunlight and sources of moisture, to prevent potential chemical changes.

Sources

IMA Mineral List (CNMNC)