Pumpellyite-(Fe^3+^)

Chemical formula: Ca₂Fe₃+Al₂(SiO₄)(Si₂O₇)(OH,O)₂·H₂O

Pumpellyite-(Fe³⁺) is a silicate mineral, a variety of pumpellyite rich in iron, forming characteristic fibrous aggregates of green color.

## Characteristics Pumpellyite-(Fe³⁺) is a mineral belonging to the pumpellyite group, in which trivalent iron (Fe³⁺) dominates one of the positions in the crystal structure, replacing aluminum. It most often forms fibrous, radial, or spherulitic aggregates; poorly developed, tabular, or columnar crystals are less common. Its appearance is very characteristic, often occurring as tiny needles filling voids in the rock. ## Physical Properties This mineral is characterized by a hardness of 6 on the Mohs scale. Its luster is vitreous, and on fracture surfaces, it can be silky. It is a relatively dense mineral, with a density of about 3.23 g/cm³. Crystals are usually translucent to opaque. ## Colors and Varieties Pumpellyite-(Fe³⁺) occurs in various shades of green – from dark green, through blue-green, to greenish-brown. The color is directly related to the iron content. There are no named commercial or gemological varieties, and classification is based on the dominant cation in the structure (e.g., pumpellyite-(Mg), pumpellyite-(Mn²⁺), pumpellyite-(Al)). ## History and Name The name "pumpellyite" was given in 1925 by Charles Palache and Helen E. Vassar in honor of the American geologist Raphael Pumpelly (1837-1923), who first described this mineral (though not as a new species) from the Keweenaw Peninsula in Michigan, USA. The suffix "-(Fe³⁺)" was added later according to the nomenclature rules of the International Mineralogical Association (IMA) to precisely define the chemical composition of this particular variety, in which trivalent iron dominates.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: Greenish white
Density: 3.23
Cleavage: Good on {001}, poor on {100}
Fracture: Uneven, Splintery
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Pumpellyite-(Fe³⁺) is most easily identified by its characteristic mode of occurrence – as radial or fibrous aggregates filling voids and fractures in low-grade metamorphic rocks and altered basalts. The green color and vitreous to silky luster are also important indicative features. ## Distinguishing from Similar Minerals It can be confused with other green silicates, such as epidote, actinolite, or chlorite. It differs from epidote often by its aggregate form (epidote forms more granular or columnar crystals) and slightly different shades of green. Actinolite usually has more elongated, bladed crystals. Chlorites are much softer (hardness 2-2.5) and have perfect cleavage in one direction, forming flakes. ## Crystal Forms Crystals are rare and usually poorly formed, with tabular or columnar habits. Most commonly found are fibrous, radial, spherulitic aggregates, and massive fillings in amygdules and veins in rocks.

Geological environment

## Genesis Pumpellyite-(Fe³⁺) is a characteristic mineral of the zeolite and prehnite-pumpellyite facies – zones of low-grade regional metamorphism. It forms as a result of hydrothermal alteration of basalts and gabbros, often filling gas vesicles (amygdules) and fractures in these rocks. It can also form in sedimentary rocks rich in volcanic fragments (greywackes) subjected to diagenesis and anchimetamorphism. ## Mineral Associations It often co-occurs with minerals such as prehnite, epidote, chlorites, quartz, calcite, laumontite, and other zeolites, as well as native copper (especially in the Great Lakes region of the USA). ## Localities Significant localities for pumpellyite-(Fe³⁺) are found in the USA, particularly on the Keweenaw Peninsula in Michigan, where it occurs in altered lava flows and is associated with copper deposits. Other known localities include Långban in Sweden; Harz in Germany; Val d'Aosta in Italy; and volcanic areas in New Zealand.

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, radial aggregates of intense, emerald-green color, contrasting with a light matrix rock (e.g., with white prehnite or calcite). The size of the aggregates and the absence of damage to delicate fibers are also important. Specimens from classic localities, such as Michigan, are particularly sought after. ## Popular Localities The historical copper mines on the Keweenaw Peninsula in Michigan (USA) are considered the best for this mineral, yielding classic specimens with native copper. Attractive specimens are also found in quarries around Långban in Sweden.

Care and storage

## Cleaning Pumpellyite-(Fe³⁺) specimens can be safely cleaned using a soft brush and distilled water. Ultrasonic cleaners should be avoided, as they can damage delicate, fibrous aggregates. ## What to Avoid This mineral is sensitive to strong acids, which can etch and damage it. Exposure to high temperatures and sudden thermal changes should also be avoided. Prolonged exposure to direct sunlight is not recommended, although it does not cause a rapid color change. ## Storage Collector specimens, especially those with well-formed, acicular crystals, should be stored in closed boxes or display cases to protect them from dust and mechanical damage. Due to its fragility, avoid placing heavier minerals on pumpellyite specimens.