Potassic-magnesio-hastingsite

Chemical formula: KCa₂(Mg₄Fe₃+)(Si₆Al₂)O₂₂(OH)₂

Potassic-magnesio-hastingsite is a rare amphibole group mineral, forming dark, almost black crystals in metamorphic rocks.

## Characteristics Potassic-magnesio-hastingsite is a complex silicate from the amphibole supergroup, belonging to the hastingsite series. It typically forms small, anhedral to subhedral prismatic crystals embedded in rock, less commonly occurring as granular aggregates. Its color is most often dark green, brownish-green to almost black, which makes it macroscopically difficult to distinguish from other dark amphiboles, such as hornblende. ## Physical Properties This mineral is characterized by a hardness of 5.5-6 on the Mohs scale and a density of approximately 3.2-3.3 g/cm³. It has a vitreous luster, which can be slightly more intense on cleavage planes. It is a brittle mineral, translucent only on thin edges, and opaque in larger fragments. ## Colors and Varieties Dark green to black is the dominant color. No distinct color varieties or commercial names are recognized. Color variability results from minor fluctuations in chemical composition, mainly in the proportions of iron and magnesium. ## History and Name The mineral's name directly refers to its chemical composition – indicating the dominance of potassium (potassic) and magnesium (magnesio) and its relationship to the previously known hastingsite. It was formally defined as part of the amphibole nomenclature revision in 2012 by the International Mineralogical Association (IMA). The original hastingsite was named after its discovery locality in Hastings County, Ontario, Canada. ## Applications Due to its rarity, potassic-magnesio-hastingsite has no industrial applications. It is solely an object of scientific and collector interest, particularly for those specializing in mineral systematics or rare minerals.

Properties

Mohs hardness: 5.5-6
Luster: Vitreous
Streak: Greenish grey
Density: 3.2-3.3
Cleavage: Perfect on {110}
Fracture: Uneven to subconchoidal
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Helpful identification features include: dark color (green to black), prismatic crystal habit, vitreous luster, and typical amphibole cleavage in two directions. Occurrence in metamorphic rocks, such as skarns, is also an important clue. However, definitive identification is impossible without advanced chemical analyses (e.g., EDS, WDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Potassic-magnesio-hastingsite is visually indistinguishable from other dark amphiboles of the hornblende and hastingsite groups (e.g., magnesio-hastingsite, ferro-hastingsite) and from some pyroxenes (e.g., augite). Pyroxenes are distinguished by a cleavage angle close to 90°, while amphiboles have angles of approximately 56°/124°. Tourmaline (schorl) has a similar color and habit but is characterized by a lack of cleavage and its typical triangular cross-section of crystals. ## Crystal Forms It most often forms columnar, prismatic crystals, which may be terminated by simple or oblique faces. It usually occurs as anhedral, poorly formed grains within the rock mass. Well-formed, freestanding crystals are rarely found.

Geological environment

## Genesis This is a mineral typical of medium- to high-grade metamorphic rocks. It forms under conditions of contact metamorphism, in calcium-rich skarns, and also in regionally metamorphosed amphibolites and gneisses that are characterized by high potassium and magnesium content. ## Mineral Associations It most commonly co-occurs with minerals typical of calc-silicate rocks and skarns, such as calcite, diopside, scapolite, titanite, phlogopite, as well as with other amphiboles and pyroxenes. ## Localities As a rare mineral, it is known from few localities worldwide. Confirmed occurrences include, among others, the vicinity of Kragerø in the Telemark region, Norway (type locality), as well as some occurrences in Canada (Ontario, Quebec) and the USA (e.g., in the Adirondack Mountains in New York State), where it occurs in metamorphic rocks of the Grenville Complex.

Rarity

Very rare

For collectors

## Quality Criteria The most important value criterion for this mineral is the certainty of its identification using analytical methods. Specimens with well-formed, lustrous crystals, even if small, are highly prized. A contrasting placement of dark crystals on a lighter rock matrix (e.g., on white calcite) significantly enhances the visual appeal and value of the specimen. ## Popular Localities The most sought-after by specialized collectors are specimens from the type locality in Norway and from classic localities in Canada and the USA, which provided material for research on the entire hastingsite group.

Care and storage

## Cleaning Specimens should be cleaned gently, using a soft brush and distilled water. A brief soak in water can be used to remove loose contaminants. Ultrasonic cleaners are not recommended, as they can damage the specimen along cleavage planes. ## What to Avoid The mineral is sensitive to strong acids. Avoid sudden temperature changes and prolonged exposure to direct sunlight, although it is not known to be prone to fading. Protect from impacts due to its brittleness. ## Storage Store in stable conditions, away from dust and moisture. It is best to place the specimen in a separate display box or cabinet to prevent scratches from harder minerals and mechanical damage.

Sources

IMA Mineral List (CNMNC)