Pumpellyite-(Fe^2+^)

Chemical formula: Ca₂Fe₂+Al₂(SiO₄)(Si₂O₇)(OH)₂·H₂O

Pumpellyite-(Fe²⁺) is a silicate mineral forming characteristic fibrous or radial aggregates, ranging in color from green to bluish-green.

## Characteristics Pumpellyite-(Fe²⁺) is a mineral belonging to the pumpellyite group, in which iron (Fe²⁺) dominates at a specific position in the crystal structure. It most often forms fibrous, acicular, or radial aggregates; small, tabular crystals are less common. Specimens are usually small, and individual crystals are often microscopic, intergrown into dense aggregates filling fissures or amygdales in rocks. ## 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 approximately 3.2 g/cm³. It is brittle and exhibits good cleavage in one direction. ## Colors and Varieties Pumpellyite-(Fe²⁺) occurs in shades from green, through bluish-green, to dark green and almost black. Its color is directly related to its 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 "-(Fe²⁺)" was added later in accordance with the nomenclature rules of the International Mineralogical Association (IMA) to precisely specify the dominant divalent cation at that position. ## Uses Pumpellyite-(Fe²⁺) has no industrial significance. However, it is an interesting mineral for collectors specializing in metamorphic rocks and for scientists studying low-grade metamorphism processes.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: Greenish gray
Density: 3.18 - 3.25
Cleavage: Good on {001}, Poor on {100}
Fracture: Uneven, Splintery
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Pumpellyite-(Fe²⁺) is most easily recognized by its characteristic forms of occurrence – radial or fibrous aggregates filling voids in rocks. The green to bluish-green color is also an important indicator. Its hardness (6 on the Mohs scale) helps distinguish it from many softer secondary minerals. ## Distinguishing from Similar Minerals Pumpellyite-(Fe²⁺) can be confused with epidote, actinolite, or chlorite. Epidote usually forms better-developed, prismatic crystals with a characteristic pistache-green color. Actinolite also forms acicular or fibrous aggregates, but often has a darker, more grayish-green color. Chlorites are much softer (hardness 2-2.5) and have perfect cleavage in one direction, forming flakes. ## Crystal Forms Crystals are rare, usually tabular or prismatic, elongated along the b-axis. It is most commonly found in the form of radial, spherulitic, or fibrous aggregates, as well as massive and granular aggregates.

Geological environment

## Genesis Pumpellyite-(Fe²⁺) is a characteristic mineral for rocks that have undergone low-grade metamorphism, under zeolite and prehnite-pumpellyite facies conditions. It forms as a result of hydrothermal alteration of basalts, gabbros, diabases, and other mafic rocks. It often fills amygdales, veins, and fissures in these rocks. ## Mineral Associations This mineral often coexists with prehnite, epidote, chlorites, quartz, calcite, laumontite, and other zeolites. In metamorphic rocks, it is also accompanied by actinolite, albite, and titanite. ## Localities Significant occurrences of pumpellyite-(Fe²⁺) are found in the Lake Superior region of the USA (Keweenaw Peninsula, Michigan), where it was first described. It also occurs in the Alps (Switzerland, Austria, Italy), New Zealand, Japan, and many other places worldwide where metamorphosed volcanic rocks are exposed.

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, radial aggregates of intense, bluish-green color, contrasting with the light background of the host rock (e.g., with prehnite or quartz). The size of the aggregates and the absence of damage to delicate fibers are also important. Well-formed, macroscopic crystals are exceptionally rare and sought after. ## Popular Localities Classic and most desirable specimens come from the copper mines on the Keweenaw Peninsula in Michigan, USA. These are historically important and often characterized by excellent quality. Other known localities providing collector material include Långban in Sweden and some localities in the Alps.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used. Ultrasonic cleaners should be avoided, as they can damage delicate, fibrous aggregates. ## What to Avoid The mineral is sensitive to strong acids, which can etch it. Avoid sudden temperature changes and prolonged exposure to direct sunlight, although its distinct effect on fading is not known. ## Storage Collector specimens of pumpellyite-(Fe²⁺), especially those with a delicate, acicular structure, should be stored in closed boxes or display cases to protect them from mechanical damage and dust. It does not require special humidity conditions.