Potassic-chloro-hastingsite

Chemical formula: KCa₂(Fe₂+₄Fe₃+)(Si₆Al₂)O₂₂Cl₂

A silicate from the amphibole group, characterized by the dominance of potassium and chlorine; forms dark green to black, prismatic crystals.

## Characteristics Potassic-chloro-hastingsite is a complex silicate belonging to the amphibole supergroup. It is a dark mineral, most often black or very dark green. It forms elongated, prismatic crystals with a vitreous luster, which may show visible striations on their faces. It occurs as single, well-formed crystals embedded in rock, as well as in granular aggregates or radial clusters. It is part of an isomorphic series with other minerals from the hastingsite group, meaning its chemical composition may vary slightly depending on the conditions of formation. ## Physical Properties This mineral has a hardness ranging from 5.5-6 on the Mohs scale, making it relatively scratch-resistant. It has a vitreous luster on crystal faces and fracture surfaces. It is an opaque mineral; only on very thin edges or in thin fragments can it appear translucent green. Its density is approximately 3.36 g/cm³. ## Colors and Varieties The dominant and essentially only encountered color is black or very dark, greenish-black. Due to its specific chemical definition, no color or commercial varieties are distinguished for it. ## History and Name The mineral's name directly reflects its chemical composition: the "potassic" component indicates the dominance of potassium (K) in a specific crystallographic site, "chloro" indicates the dominance of chlorine (Cl), and "hastingsite" refers to its relation to the mineral hastingsite. It was officially recognized as a distinct mineral species by the International Mineralogical Association (IMA) in 2005. ## Uses Potassic-chloro-hastingsite has no industrial applications. Its significance is purely scientific and collectible, being an object of interest for specialists studying the amphibole group and collectors of rare and systematic minerals.

Properties

Mohs hardness: 5.5-6
Luster: Vitreous
Streak: Greenish gray
Density: 3.36
Cleavage: Perfect on {110}
Fracture: Uneven to conchoidal
Transparency: Opaque, translucent in thin splinters
Crystal system: Monoclinic

Diagnostic features

## Identification Field identification is difficult. The main visual features are black color, prismatic crystal habit, and vitreous luster. A key diagnostic feature, typical of amphiboles, is excellent cleavage in two directions, intersecting at angles of approximately 56° and 124°. These angles are often visible in the cross-section of the crystal. ## Distinguishing from Similar Minerals It is practically impossible to distinguish potassic-chloro-hastingsite from other black amphiboles (such as hornblende, arfvedsonite, or other minerals from the hastingsite group) without advanced laboratory analyses (EDS/WDS chemical analysis, X-ray diffraction). Identification of associated minerals and knowledge of the geology of the given locality can be helpful. ## Crystal Forms It most often forms well-developed, columnar or prismatic crystals, often terminated by simple faces. It also occurs as granular aggregates, fibrous or radial clusters in the host rock.

Geological environment

## Genesis This is a mineral typical of metamorphic rocks that have undergone transformation under conditions of high temperature and pressure. It forms mainly in skarns – carbonate rocks contact-metamorphosed at the interface with magmatic intrusions, as well as in iron-rich rock formations. ## Mineral Associations It often co-occurs with minerals such as magnetite, diopside, hedenbergite, garnets (mainly andradite), scapolite, titanite, and other amphiboles and pyroxenes. ## Localities The most important and classic locality, which is also the type locality, is the Greenwood Mine (also known as Patterson Mine) in Tuxedo, Orange County, New York, USA. Other known occurrences include the Östanmossa mine in Sweden. These are few, well-documented localities worldwide.

Rarity

Very rare

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, and lustrous crystals that stand out clearly from the host rock (matrix). Large, single crystals and aesthetic compositions with associated minerals, such as pink calcite or magnetite, are also highly prized. ## Popular Localities By far the most sought-after and highly valued specimens come from the type locality – the Greenwood Mine in New York State, USA. Crystals from this location are considered the global standard for this mineral.

Care and storage

## Cleaning Specimens should be cleaned gently, using a soft brush or toothbrush. Distilled water can be used, possibly with a small amount of neutral soap. After cleaning, the specimen should be thoroughly rinsed and allowed to dry completely. ## What to Avoid The mineral is sensitive to impact due to its cleavage – it can easily fracture along specific planes. Contact with strong acids and other aggressive chemicals should be avoided. Ultrasonic cleaners are not recommended, as they can cause crystals to crack along cleavage planes. ## Storage Potassic-chloro-hastingsite specimens are best stored in separate, padded boxes or display cases to prevent abrasion and mechanical damage from contact with harder minerals. It is stable under normal conditions and does not require protection from light or humidity.

Sources

IMA Mineral List (CNMNC)