Potassic-fluoro-hastingsite

Chemical formula: KCa₂(Fe₂+₄Fe₃+)(Si₆Al₂)O₂₂F₂

Potassic-fluoro-hastingsite is a rare, dark green to black amphibole, distinguished by the presence of potassium and fluorine in its complex chemical structure.

## Characteristics Potassic-fluoro-hastingsite is a complex silicate from the amphibole group, belonging to the hastingsite series. It forms elongated, columnar, or acicular crystals, often gathered in radial or granular aggregates. Its color is typically dark green, greenish-black to almost black. It is an opaque mineral with a vitreous luster on crystal surfaces. ## Physical Properties It is characterized by a hardness in the range of 5-6 on the Mohs scale, which is typical for amphiboles. Its density is relatively high, approximately 3.29 g/cm³. It exhibits perfect cleavage in two directions, intersecting at angles of approximately 56° and 124°, which is a characteristic feature of the entire amphibole group. The fracture is uneven. ## Colors and Varieties This mineral does not form color varieties or commercial varieties. Its coloration is uniform and ranges from dark green to black, depending on the exact chemical composition, especially the ratio of iron Fe²⁺ to Fe³⁺. ## History and Name The mineral's name refers to its chemical composition – indicating the dominance of potassium ("potassic") and fluorine ("fluoro") and its association with hastingsite, which in turn was named after the locality of Hastings in Ontario (Canada). It is a relatively recently defined member of the amphibole supergroup, and its status has been formally approved by the International Mineralogical Association (IMA). ## Uses Potassic-fluoro-hastingsite has no industrial applications. Its significance is purely scientific and as a collector's item, as a rare and well-defined member of the large and complex amphibole group.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Greenish-gray
Density: 3.29
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying potassic-fluoro-hastingsite in field conditions is practically impossible without advanced analyses. In a collection, it can be preliminarily identified based on its dark green to black color, columnar crystal habit, and co-occurrence with other minerals typical of skarns and syenites. The characteristic amphibole cleavage angle (approx. 124°/56°) is a key feature if visible. ## Distinguishing from Similar Minerals It can be easily confused with other dark amphiboles (hornblende, ferro-hornblende, other hastingsites) and pyroxenes (e.g., augite). Distinguishing it from other amphiboles requires chemical analysis (EDS, WDS) to confirm the dominance of potassium and fluorine. It is distinguished from pyroxenes by the cleavage angle – in pyroxenes, it is approximately 90°. ## Crystal Forms Most often, it forms elongated, prismatic (columnar) crystals with a hexagonal cross-section. It also occurs as granular, fibrous, or radial aggregates within the host rock.

Geological environment

## Genesis It is an igneous and metamorphic mineral. It forms in silica-undersaturated, alkaline igneous rocks, such as syenites and nepheline syenites. It also forms during contact metamorphism processes, in skarns created at the contact of alkaline intrusions with carbonate sedimentary rocks. ## Mineral Associations It most commonly co-occurs with minerals such as nepheline, microcline, albite, aegirine, titanite, apatite, fluorite, calcite, and other amphiboles and pyroxenes from the aegirine-augite group. ## Localities The most important and well-documented occurrences of this mineral worldwide are in alkaline complexes. Key localities include the Kola Peninsula in Russia (especially the Khibiny and Lovozero massifs), the Ilímaussaq complex in Greenland, and some alkaline intrusions in Canada (e.g., Mont Saint-Hilaire in Quebec).

Rarity

Very rare

For collectors

## Quality Criteria Most valued by collectors are specimens with well-formed, sharp, and terminated crystals with a distinct luster. Contrast with a light host rock (e.g., white nepheline or albite) is of great importance. Specimens from classic, well-documented localities, such as the Kola Peninsula or Mont Saint-Hilaire, are more highly rated. Crystal size is significant – those exceeding a centimeter in length are already rare and sought after. ## Popular Localities Undoubtedly, the most desirable specimens come from the alkaline massifs of the Kola Peninsula in Russia, from where large, well-formed crystals are known. Specimens from Mont Saint-Hilaire in Canada are also highly prized, known for their excellent development and often accompanying rare minerals.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. For heavier dirt, compressed air can be used. Avoid contact with water and any chemical agents. ## What to Avoid Absolutely avoid contact with acids and other chemicals that may react with the mineral's complex composition. Do not heat it or expose it to sudden temperature changes. Prolonged exposure to strong sunlight is not advisable, although there is no evidence of fading. ## Storage Specimens are best stored in closed display boxes or cabinets to protect them from dust and mechanical damage. Due to its cleavage, avoid placing heavier minerals on its surface.

Sources

IMA Mineral List (CNMNC)