Potassic-ferro-pargasite

Chemical formula: KCa₂(Fe₂+₄Al)Si₆Al₂O₂₂(OH)₂

Potassic-ferro-pargasite is a mineral from the amphibole group, forming dark green to black, prismatic crystals in metamorphic and igneous rocks.

## Characteristics Potassic-ferro-pargasite is a complex silicate from the amphibole supergroup, belonging to the hornblende series. It forms elongated, prismatic crystals, often with a rhombic cross-section, as well as granular or fibrous aggregates. It typically occurs as embedded grains within the rock. Its color is characteristic—ranging from dark green, through greenish-black, to entirely black. The mineral's luster is vitreous, and on cleavage planes, it can be more intense. ## Physical Properties Its hardness on the Mohs scale is approximately 6, making it a relatively scratch-resistant mineral. It has a density of about 3.28 g/cm³. It is translucent in thin fragments, and in larger pieces or in mass, it becomes opaque. It possesses excellent cleavage in two directions, typical of amphiboles, causing it to break into fragments with smooth, shiny surfaces. ## Colors and Varieties This mineral does not form color varieties or commercial variants. Its coloration is closely related to its chemical composition, mainly due to its high iron content, and is limited to dark shades of green and black. ## History and Name The mineral's name reflects its chemical composition: "potassic" indicates the presence of potassium, "ferro" indicates the dominance of iron (Latin: *ferrum*), and "pargasite" is the root name of the group, derived from the original locality of pargasite in Pargas, Finland. It was defined as a distinct mineral species as part of the amphibole nomenclature revision by the International Mineralogical Association (IMA) in 2012. ## Uses Potassic-ferro-pargasite has no industrial applications. It is solely an object of scientific interest and is valued by advanced collectors of rare and systematic minerals.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: Pale greenish-grey
Density: 3.28
Cleavage: Good on {110}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification is aided by its dark green to black color, vitreous luster, and the form of long, prismatic crystals. A key diagnostic feature, typical of amphiboles, is the angle between cleavage planes, approximately 124° and 56°, visible in the cross-section of crystals. Its occurrence in high-grade metamorphic rocks is also an important clue. ## Distinguishing from Similar Minerals Potassic-ferro-pargasite is difficult to distinguish from other dark amphiboles (such as hornblende or arfvedsonite) and from pyroxenes (e.g., augite) without specialized analysis. It is primarily distinguished from pyroxenes by a different cleavage angle (in pyroxenes, it is close to 90°). Definitive identification and differentiation from other amphiboles require chemical analysis (e.g., by EDS). ## Crystal Forms It most commonly forms columnar, prismatic crystals, single or aggregated into radial clusters. It also occurs as granular, fibrous aggregates or as irregular grains embedded in the rock mass.

Geological environment

## Genesis This is a characteristic mineral of metamorphic rocks that have undergone transformation under conditions of high temperatures and pressures (amphibolite and granulite facies). It forms mainly as a result of the metamorphism of mafic igneous rocks. It is less commonly found in some alkaline igneous rocks, such as syenites, and in skarns. ## Mineral Associations It often co-occurs with minerals such as plagioclase, biotite, garnets (especially almandine), diopside, scapolite, titanite, and other amphiboles from the hornblende group. ## Localities Key localities where well-formed crystals have been found include Långban in Sweden, the Mont Saint-Hilaire alkaline complex in Canada, and the Greenwood Mine in New York State, USA. It is also known from many other localities worldwide, usually in the form of microscopic rock grains.

Rarity

Not very common

For collectors

## Quality Criteria Most valued by collectors are specimens with sharply terminated, well-formed, and lustrous crystals, especially when embedded in a contrasting rock matrix. Due to the rarity of large crystals, even small but perfectly formed specimens achieve high value. Rich aggregates of prismatic crystals are also desirable. ## Popular Localities Långban in Sweden and Mont Saint-Hilaire in Quebec (Canada) are considered classic localities for the best micromount specimens of potassic-ferro-pargasite. Specimens from these locations are standard in systematic collections.

Care and storage

## Cleaning Specimens should be cleaned gently, using a soft brush and distilled water. A brief soak can be used to remove loose contaminants. Ultrasonic cleaners are not recommended, as vibrations can cause crystals to fracture along cleavage planes. ## What to Avoid The mineral is sensitive to strong acids. Avoid sudden temperature changes and mechanical impacts, which can easily damage the brittle crystals. It is stable in sunlight. ## Storage Potassic-ferro-pargasite specimens are best stored in individual display boxes or cabinets to prevent abrasion and scratching by harder minerals. They should be protected from dust, which can accumulate in the depressions of aggregates.

Sources

IMA Mineral List (CNMNC)