Ferro-pargasite

Chemical formula: NaCa₂(Fe₂+₄Al)(Si₆Al₂)O₂₂(OH)₂

Ferro-pargasite is a rare amphibole from the pargasite group, characterized by its dark color resulting from a high iron content.

## Characteristics Ferro-pargasite is a mineral from the amphibole group, belonging to the pargasite series. It is a silicate of sodium, calcium, iron, magnesium, and aluminum. It typically forms prismatic or fibrous crystals, often occurring as granular aggregates or radiating clusters. Its color is usually dark green, greenish-black to black, which is a direct result of the dominance of iron (Fe²⁺) over magnesium in its chemical structure. It rarely forms well-developed, collectible crystals; it is more commonly found as inclusions in rock. ## Physical Properties This mineral is characterized by a hardness ranging from 5-6 on the Mohs scale. It has a vitreous luster, and its transparency varies from translucent on thin edges to almost opaque in more massive fragments. The density is approximately 3.2-3.4 g/cm³, which is typical for iron-rich silicates. ## Colors and Varieties The dominant color of ferro-pargasite is dark green, often transitioning to black. No named color varieties or commercial varieties are distinguished. Variation in shades primarily depends on the iron-to-magnesium ratio and the presence of other trace elements, such as titanium. ## History and Name The name "ferro-pargasite" refers to its chemical composition – it is the iron-rich (Latin: *ferrum*) analogue of pargasite. Pargasite, in turn, was named after its discovery locality in Pargas, Finland. Ferro-pargasite was formally recognized as a distinct mineral by the International Mineralogical Association (IMA) as a result of the redefinition of the amphibole group. ## Uses Ferro-pargasite has no industrial significance. Its occurrence is primarily of scientific importance, serving as an indicator of metamorphic conditions, and is of interest to specialized collectors of rock-forming minerals.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Pale green
Density: 3.2-3.4
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Ferro-pargasite is difficult to identify visually without advanced studies. It is distinguished by its dark green to black color, vitreous luster, and prismatic crystal habit. Characteristic features also include its perfect cleavage in two directions, intersecting at angles of approximately 56° and 124°, typical for amphiboles. ## Distinguishing from Similar Minerals It can be easily confused with other dark amphiboles, such as hornblende, pargasite, hastingsite, or kaersutite. Definitive differentiation requires chemical analysis (e.g., electron microprobe) to determine the exact composition, especially the Fe/Mg ratio. It is distinguished from pyroxenes (e.g., augite) by its cleavage angle (for pyroxenes, it is approximately 90°). ## Crystal Forms It most commonly forms elongated, prismatic crystals with a hexagonal cross-section. It also occurs as granular, fibrous, or radiating aggregates within the host rock.

Geological environment

## Genesis Ferro-pargasite is a rock-forming mineral, forming under conditions of medium to high-grade regional metamorphism, particularly in amphibolites, gneisses, and amphibole schists. It can also crystallize from alkaline magmas in rocks such as syenites and trachytes. ## Mineral Associations It often co-occurs with other metamorphic and igneous minerals, such as plagioclase (especially andesine), potassium feldspars, biotite, epidote, garnets (almandine), quartz, titanite, and other amphiboles and pyroxenes. ## Localities Significant occurrences of ferro-pargasite have been reported in many places around the world, although rarely in the form of well-developed crystals. Known localities include Pargas in Finland (the type locality for pargasite), the Bancroft area in Ontario (Canada), the Ilimaussaq igneous complex in Greenland, as well as numerous sites in Norway, Sweden, Italy, and the USA (e.g., in New York and California states).

Rarity

Not very common

For collectors

## Quality Criteria The collectibility of ferro-pargasite is limited, as it rarely forms large and well-developed crystals. The most prized specimens are those with sharply terminated, lustrous crystals of intense, dark green color, clearly standing out from the host rock. Association with other contrasting minerals is also important. ## Popular Localities Specimens of some collectible value come from alkaline igneous complexes, e.g., from the Bancroft area in Canada, where relatively large crystals have been found. However, most specimens on the market are microminerals or rock fragments with embedded grains, primarily of interest to collectors specializing in rock-forming minerals.

Care and storage

## Cleaning Ferro-pargasite specimens should be cleaned carefully, using a soft brush and distilled water. For specimens embedded in rock, aggressive mechanical methods should be avoided, as they could damage delicate crystals. ## What to Avoid Avoid contact with strong acids, which can react with the mineral. It is not particularly sensitive to light, but prolonged exposure to extreme temperatures is not advisable. ## Storage Store in stable conditions, away from dust and chemicals. Specimens are best displayed in closed display cases or specimen boxes to protect them from mechanical damage.

Sources

IMA Mineral List (CNMNC)