Magnesio-fluoro-hastingsite

Chemical formula: NaCa₂(Mg₄Fe₃+)(Si₆Al₂)O₂₂F₂

A dark green or black amphibole group mineral, the magnesium-fluorine analogue of hastingsite, forming prismatic crystals.

## Characteristics Magnesio-fluoro-hastingsite is a complex silicate from the amphibole supergroup, belonging to the hastingsite series. It most commonly forms columnar or prismatic crystals, dark green to black in color, often embedded in the host rock. Specimens are rarely fully developed and terminated; they more often occur as granular aggregates or fibrous masses. It is an opaque mineral or at most translucent on thin edges. ## Physical Properties This mineral is characterized by a hardness of 5.5-6 on the Mohs scale and a vitreous luster. Its density is approximately 3.2-3.3 g/cm³. It exhibits excellent cleavage in two directions, typical for amphiboles, intersecting at angles of approximately 56° and 124°. ## Colors and Varieties Its color is uniform, ranging from dark green, through greenish-black, to completely black. No color varieties or commercial names are distinguished. ## History and Name The mineral's name directly refers to its chemical composition – indicating the dominance of magnesium (magnesio) and fluorine (fluoro) and its relation to hastingsite, another mineral from the amphibole group. The name of the parent mineral, hastingsite, comes from Hastings County in Ontario (Canada). Magnesio-fluoro-hastingsite was formally recognized as a distinct mineral species by the International Mineralogical Association (IMA) in 2005. ## Applications Magnesio-fluoro-hastingsite has no industrial applications. It is solely an object of scientific interest, as an indicator of specific metamorphic conditions, and for collectors.

Properties

Mohs hardness: 5.5-6
Luster: Vitreous
Streak: White to pale gray
Density: 3.2-3.3
Cleavage: Perfect on {110}
Fracture: Uneven to Sub-Conchoidal
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Helpful identification features include: dark green to black color, vitreous luster, prismatic crystal habit, and the characteristic amphibole cleavage. Occurrence in high-grade metamorphic rocks is also an important clue. ## Distinguishing from Similar Minerals Magnesio-fluoro-hastingsite is difficult to distinguish from other dark amphiboles (e.g., hornblende, ferro-hastingsite) and from pyroxenes (e.g., augite) without specialized studies. It is distinguished from pyroxenes by the cleavage angle (approx. 56/124° for amphiboles, approx. 90° for pyroxenes). Certain distinction from other amphiboles requires chemical analysis to confirm the dominance of magnesium and fluorine. ## Crystal Forms It forms elongated, columnar crystals with a hexagonal or octagonal cross-section. It often occurs as chaotic granular aggregates, fibrous masses, or as individual grains within the rock.

Geological environment

## Genesis This is a mineral typical of high-grade metamorphic rocks, such as amphibolites and granulites, which formed under conditions of high temperature and pressure, with the involvement of fluorine-rich fluids. It can also occur in some alkaline igneous rocks, e.g., in syenites. ## Mineral Associations It often co-occurs with minerals such as plagioclase, biotite, titanite, apatite, diopside, and other amphiboles and pyroxenes. ## Localities The most important occurrences of this mineral are in the USA (Greenwood Mine, Orange County, New York – this is the type locality), as well as in Pargas, Finland, and in some regions of Canada (Ontario, Quebec).

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp, and lustrous crystals, especially if they are set against a contrasting rock matrix. Large, single crystals with distinct terminations are rare and sought after. The presence of associated minerals also increases value. ## Popular Localities The classic and best collecting localities are considered to be the historical type locality – Greenwood Mine in New York State, USA.

Care and storage

## Cleaning Specimens can be cleaned using a soft brush and distilled water. Due to the presence of cleavage planes, ultrasonic cleaners should be avoided, as they could cause crystals to fracture along these planes. ## What to Avoid The mineral is sensitive to strong acids. It should be protected from sudden temperature changes and mechanical impacts, which can cause damage. ## Storage Store under standard conditions, in separate display boxes or cabinets, to avoid scratches from harder minerals. It is not sensitive to light.

Sources

IMA Mineral List (CNMNC)