Potassic-hastingsite

Chemical formula: KCa₂(Fe₂+₄Fe₃+)(Si₆Al₂)O₂₂(OH)₂

Potassic-hastingsite is a dark, almost black mineral from the amphibole group, forming prismatic crystals mainly in alkaline igneous rocks.

## Characteristics Potassic-hastingsite is a mineral from the amphibole supergroup, belonging to the hastingsite series. It forms prismatic crystals, ranging from short and stout to elongated, acicular. It typically occurs as disseminated grains within rocks or as compact, granular, or fibrous aggregates. Its color is most often black, dark green, or greenish-black. It is a brittle mineral. ## Physical Properties The Mohs hardness is 5-6, which is a typical value for amphiboles. The luster is vitreous, and on fracture surfaces, it can be dull. The mineral is most often opaque, and in thin fragments or on crystal edges, it can be translucent. The density ranges from 3.26-3.43 g/cm³. ## Colors and Varieties Black and dark green colors dominate. Shades depend on subtle variations in chemical composition, mainly the ratio of iron in the ferrous and ferric states. No distinct color varieties or commercial varieties are recognized. ## History and Name The mineral's name refers to its chemical composition – the dominant role of potassium (potassic) in its structure – and its connection to hastingsite, which in turn was named after its discovery locality in Hastings County, Ontario (Canada). Potassic-hastingsite was formally recognized as a distinct mineral species during the revision of the amphibole group nomenclature. ## Uses Potassic-hastingsite has no direct industrial application. It has scientific significance as a rock-forming mineral, helping to determine the formation conditions of alkaline rocks. It is also an interesting mineral for advanced collectors specializing in mineral systematics or minerals from specific localities.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Grayish green
Density: 3.26-3.43
Cleavage: Good on {110}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Key diagnostic features include crystal habit (elongated prisms) and the characteristic amphibole cleavage, with two planes intersecting at angles of approximately 56° and 124°. The dark color and vitreous luster are also helpful in identification. ## Distinguishing from Similar Minerals Potassic-hastingsite is difficult to distinguish from other black amphiboles, such as hornblende, without advanced chemical analysis. It is primarily distinguished from minerals of the pyroxene group (e.g., augite) by its cleavage angle – in pyroxenes, it is approximately 90°. Arfvedsonite, another alkaline amphibole, can be similar but often exhibits weak pleochroism in bluish or purplish hues, which is usually absent in potassic-hastingsite. ## Crystal Forms It most often forms columnar, prismatic crystals, often terminated with simple faces. It also occurs as granular, fibrous, or radial aggregates. In rocks, it often forms irregular, anhedral grains.

Geological environment

## Genesis This is a characteristic mineral of alkaline igneous rocks, poor in silica, such as nepheline syenites and associated pegmatites. It also forms in contact metamorphism processes, in skarns, and in some iron-rich metamorphic rocks. ## Mineral Associations It co-occurs with minerals typical of alkaline environments, such as nepheline, microcline, albite, aegirine, biotite, titanite, apatite, and magnetite. ## Localities Significant occurrences of well-formed potassic-hastingsite crystals include the Mont Saint-Hilaire alkaline complex in Quebec (Canada), where it occurs as excellent crystals. Other important localities include Iron Hill in Colorado (USA), which is the type locality, the Ilímaussaq complex in Greenland, and alkaline massifs on the Kola Peninsula in Russia.

Rarity

Not very common

For collectors

## Quality Criteria The most highly valued specimens are those with sharp, well-formed, and terminated crystals with a strong luster. Large crystal size and their occurrence on a rock matrix in association with other rare minerals (e.g., from Mont Saint-Hilaire) significantly increase their collector appeal. ## Popular Localities Specimens from Mont Saint-Hilaire, Canada, are considered the best in the world, distinguished by their perfect crystal form. Materials from the type locality at Iron Hill, USA, also hold historical significance.

Care and storage

## Cleaning Specimens can be cleaned with a soft brush under running water or in distilled water. Ultrasonic cleaners should be avoided, as they can cause fracturing along cleavage planes. ## What to Avoid The mineral is sensitive to strong acids. It should be protected from sudden temperature changes and strong impacts, which can easily lead to damage due to its distinct cleavage. ## Storage Store under standard conditions, protecting from dust and mechanical damage. It is best to keep specimens in separate boxes or on stands to avoid scratches and chipping from other minerals.

Sources

IMA Mineral List (CNMNC)