Potassic-pargasite

Chemical formula: KCa₂(Mg₄Al)(Si₆Al₂)O₂₂(OH)₂

Potassic-pargasite is a rare amphibole group mineral, forming dark green to black, prismatic crystals in metamorphic rocks.

## Characteristics Potassic-pargasite is a complex silicate belonging to the amphibole supergroup. It represents the potassium-rich end-member of the pargasite series. It forms elongated, prismatic crystals, often with a hexagonal cross-section. It also occurs as granular aggregates or as a rock-forming component within the rock mass. Specimens are typically dark green, brownish-green, or almost black. ## Physical Properties It is characterized by a hardness of 5-6 on the Mohs scale and a vitreous luster on crystal faces. It is a relatively dense mineral, with a density of approximately 3.18 g/cm³. It is usually opaque, and in thin fragments, it can be translucent. ## Colors and Varieties The dominant colors are dark green, brownish-green, and black. Color variability depends on minor elemental impurities, mainly iron. No named commercial or gemological varieties are distinguished. ## History and Name The mineral's name reflects its chemical composition and its connection to another mineral. The term "potassic" indicates the dominance of potassium in its structure, while "pargasite" refers to the mineral pargasite, named after its type locality in Pargas, Finland. Potassic-pargasite has been formally recognized as a distinct mineral species by the International Mineralogical Association (IMA). ## Applications Due to its rarity and lack of significant accumulations, potassic-pargasite has no industrial applications. However, it is an object of scientific interest and is valued by collectors specializing in mineral systematics and rare species.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: White to light grey
Density: 3.18
Cleavage: Good on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Field identification is based on the prismatic crystal habit, dark color, vitreous luster, and occurrence in metamorphic rocks. The characteristic amphibole cleavage at approximately 124°/56° is a key feature, although difficult to observe without proper equipment. ## Distinguishing from Similar Minerals Potassic-pargasite is visually indistinguishable from other dark amphiboles, such as hornblende, pargasite, or magnesio-hastingsite. Distinguishing it from pyroxenes (e.g., augite) is possible based on cleavage angles (pyroxenes have cleavage at angles close to 90°). Definitive identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS). ## Crystal Forms It most commonly forms columnar or prismatic crystals, which may be terminated by simple or complex faces. It also occurs as radial aggregates, granular masses, or as chaotic masses within the rock.

Geological environment

## Genesis It forms under conditions of medium to high-grade regional metamorphism, mainly in amphibolites, gneisses, and marbles. It is also found in contact skarns and in some alkaline igneous rocks. ## Mineral Associations It often co-occurs with minerals such as calcite, diopside, phlogopite, scapolite, titanite, as well as other amphiboles and pyroxenes. ## Localities The type locality is in Pargas, Finland. Other known occurrences include the Bancroft area in Ontario (Canada), New York and New Jersey in the USA, and some Alpine localities in Italy.

Rarity

Rare

For collectors

## Quality Criteria The most sought-after specimens are those with well-formed, sharp, and lustrous crystals of an intense, dark green color. The value of a specimen is enhanced by its size, lack of damage, and a contrasting mineral matrix, such as white calcite or marble. ## Popular Localities Classic specimens come from Pargas, Finland. High-quality crystals, often of considerable size, are found in quarries in the Bancroft area, Canada, and in Franklin and Sterling Hill, New Jersey, USA.

Care and storage

## Cleaning Specimens should be cleaned gently with a soft brush and distilled water. Ultrasonic cleaners are not recommended, as they can damage crystals along cleavage planes. ## What to Avoid The mineral is sensitive to strong acids. Avoid sudden temperature changes and mechanical impacts, which can cause fracturing. It is stable in sunlight. ## Storage Store in separate boxes or on stands to prevent scratches and damage from harder minerals. Well-formed crystals are best displayed in showcases that protect against dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)