Pumpellyite-(Al)

Chemical formula: Ca₂Al₃(SiO₄)(Si₂O₇)(OH,O)₂·H₂O

Pumpellyite-(Al) is a calcium aluminosilicate, forming characteristic acicular or fibrous aggregates of a greenish color in metamorphic rocks.

## Characteristics Pumpellyite-(Al) is a mineral from the silicate group, belonging to the pumpellyite subgroup. It forms typical, small, acicular or fibrous crystals, which often occur as radial or tangled aggregates. Less common are short-prismatic, tabular crystals. Due to the small size of the crystals, its features are often difficult to observe with the naked eye, and aggregates appear as dull or silky masses. ## Physical Properties This mineral is characterized by a hardness of 5.5 on the Mohs scale. Its luster is most often vitreous, and on the surfaces of fibrous aggregates, it can be silky. It is a transparent to translucent mineral. The theoretical density is approximately 3.2 g/cm³, however, empirical data are often imprecise. ## Colors and Varieties Pumpellyite-(Al) occurs in shades from emerald green to white, sometimes with a bluish tint. The green color is most common and results from iron impurities. No named commercial or gemological varieties are distinguished. ## History and Name The name "pumpellyite" was given in honor of the American geologist Raphael Pumpelly (1837-1923). The suffix "-(Al)" refers to the dominance of aluminum at a specific position in the crystal structure, which distinguishes it from other minerals in this subgroup, such as pumpellyite-(Fe²⁺) or pumpellyite-(Mg). The type locality for pumpellyite-(Al) is located in the La Flèche quarry in Belgium. ## Uses Pumpellyite-(Al) has no significant industrial application. However, it is an important indicator mineral in petrology, helping to determine metamorphic conditions (pumpellyite-actinolite facies). For collectors, it is an interesting example of a rock-forming mineral with characteristic forms of occurrence.

Properties

Mohs hardness: 5.5
Luster: Vitreous
Streak: White
Density: 0
Cleavage: {100}
Fracture: Uneven
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Pumpellyite-(Al) is most easily recognized by its characteristic forms of occurrence: radial, fibrous, or acicular aggregates filling fissures and amygdales in rocks. Its greenish color is also typical. Identification of single, small crystals usually requires magnification. ## Distinguishing from Similar Minerals It can be confused with other green silicates occurring in similar conditions, such as epidote, actinolite, or chlorite. It usually differs from epidote by a lighter, more "grassy" shade of green and more often forms fibrous rather than granular aggregates. Actinolite also forms acicular crystals, but they are often longer and thicker. Final distinction often requires chemical analysis or optical methods. ## Crystal Forms Crystals are usually small, acicular or hair-like. They form radial, fan-shaped, spherulitic aggregates or tangled, felted masses. Single, short-prismatic or tabular crystals with a rhombic cross-section are less common.

Geological environment

## Genesis Pumpellyite-(Al) is a characteristic mineral for rocks that have undergone regional or hydrothermal metamorphism under conditions of low temperatures and relatively high pressures. It forms within the zeolite and pumpellyite-actinolite facies. It often forms in altered basalts, gabbros, diabases, and volcanic tuffs, where it fills gas vesicles (amygdales) and fissures. ## Mineral Associations It co-occurs with minerals such as prehnite, epidote, chlorites, actinolite, quartz, calcite, laumontite, and other zeolites. ## Localities Significant occurrences of this mineral are found in many places around the world. In Europe, it is known from Belgium (type locality La Flèche), Italy (Ligurian Apennines), Austria, and Scotland. In the United States, beautiful specimens come from the Keweenaw Peninsula region in Michigan, where they fill amygdales in prehistoric lava flows. Other known localities include New Zealand and Japan.

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, radial aggregates of intense, emerald-green color. Contrast with light host rock (e.g., with prehnite or quartz) enhances attractiveness. The size of the aggregates and the absence of damage to delicate, acicular crystals are also important. Specimens from Michigan, where pumpellyite forms amygdale fillings (so-called "thundereggs"), are particularly sought after. ## Popular Localities Among the most known and valued localities from which collector specimens originate are the copper mines on the Keweenaw Peninsula in Michigan (USA). Specimens from there often appear as beautiful, radial aggregates in basaltic amygdales.

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 Avoid contact with acids and strong chemical agents. The mineral is sensitive to rapid temperature changes. Prolonged exposure to direct sunlight is not recommended, although it does not cause a rapid color change. ## Storage Specimens, especially those with well-formed, acicular crystals, are brittle and susceptible to mechanical damage. It is best to store them in separate, padded boxes or display cases, away from harder minerals that could scratch them.