Magnesio-hastingsite

Chemical formula: NaCa₂(Mg₄Fe₃+)Si₆Al₂O₂₂(OH)₂

Magnesiohastingsite is a complex silicate from the amphibole group, forming dark green to black, prismatic crystals in metamorphic and igneous rocks.

## Characteristics Magnesiohastingsite is a mineral from the silicate group, belonging to the calcic amphiboles. It forms elongated, prismatic, or columnar crystals, often terminated by steep faces. It also occurs as granular, fibrous aggregates or as embedded grains within the rock. Its crystals are rarely well-formed and usually do not reach large sizes. ## Physical Properties This mineral is characterized by a hardness of 5-6 on the Mohs scale. It has a vitreous luster and is typically opaque, rarely translucent on thin edges. Its density ranges from 3.0 to 3.3 g/cm³, depending on the precise chemical composition, especially the iron content. ## Colors and Varieties Its color is usually dark green, greenish-black, or almost black. Sometimes it can also be brownish-green. There are no named color varieties or commercial varieties. Color variability is mainly a function of the magnesium to iron ratio. ## History and Name The name "magnesiohastingsite" refers to its chemical composition – the dominance of magnesium (magnesio-) and its relation to the mineral hastingsite. Hastingsite was named after Hastings County in Ontario (Canada), where it was first described. Magnesiohastingsite was formally recognized as a distinct mineral species by the International Mineralogical Association (IMA) as part of the amphibole nomenclature revision. ## Uses Magnesiohastingsite has no industrial significance. However, it is an important rock-forming mineral whose presence and composition provide geologists with information about the pressure and temperature conditions under which a given rock formed. It has only scientific and collectible value.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Greenish-gray
Density: 3.0-3.3
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Magnesiohastingsite in its matrix rock is identified by its dark green to black color, vitreous luster, and the characteristic elongated crystal form typical of amphiboles. A key feature is its perfect cleavage in two directions, intersecting at angles of approximately 56° and 124°, which is visible in cross-sections of the crystals. ## Distinguishing from Similar Minerals Magnesiohastingsite is difficult to distinguish from other dark amphiboles (such as hornblende, ferrohastingsite, pargasite) and from pyroxenes (e.g., augite) without advanced analysis. It is distinguished from pyroxenes by its cleavage angle (for pyroxenes, it is approximately 90°). Final identification and differentiation from other amphiboles require chemical analysis (e.g., EDS) to determine the magnesium to iron ratio and other elements. ## Crystal Forms Crystals are most often prismatic, elongated along the crystallographic c-axis. They occur as single crystals embedded in rock, as well as in granular, fibrous, or radial aggregates.

Geological environment

## Genesis Magnesiohastingsite is a characteristic mineral of medium to high-grade metamorphic rocks, such as amphibolites, gneisses, and amphibole schists. It also forms in igneous rocks, especially in alkaline and calcium-rich intrusions, such as syenites, diorites, and gabbros. Its presence indicates crystallization under conditions of elevated pressure and temperature. ## Mineral Associations In metamorphic rocks, it most commonly coexists with plagioclase, scapolite, titanite, epidote, diopside, and garnets. In igneous rocks, it is accompanied by nepheline, alkali feldspars, biotite, and other amphiboles and pyroxenes. ## Localities Significant occurrences of magnesiohastingsite have been reported in many places worldwide. In Canada, it occurs in the type locality in Hastings County and in the Bancroft area of Ontario. Other important localities include the Kola Peninsula in Russia, the Ilímaussaq igneous complex in Greenland, the Arendal area in Norway, as well as numerous localities in the USA (e.g., in New York and California states).

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most desirable specimens are those with well-formed, sharply terminated crystals with a distinct luster and an intense, dark green color. Crystals that contrast with a light matrix rock (e.g., with white marble or feldspars) are particularly prized. Large, single crystals or aesthetic crystal groups are rare and sought after. ## Popular Localities The most famous collectible specimens come from historical localities in Ontario, Canada (Bancroft, Hastings Co.), where large, well-formed crystals were found. Specimens from Greenland and the Kola Peninsula are also valued, although they often occur as embedded grains rather than freestanding crystals.

Care and storage

## Cleaning Magnesiohastingsite specimens can be cleaned using a soft, dry brush to remove dust. For heavier soiling, distilled water is permissible. Ultrasonic cleaners should be avoided, as they can damage fragile crystals or cause them to detach from the matrix rock. ## What to Avoid This mineral is sensitive to strong acids, which can etch it and alter its surface. Avoid sudden temperature changes, which can lead to internal fractures. Prolonged exposure to direct sunlight is not recommended, although its distinct effect on color is not known. ## Storage Specimens should be stored in stable conditions, away from sources of vibration and chemicals. It is best to keep them in sealed boxes or display cases to protect them from dust and mechanical damage. As it is relatively brittle, contact with harder minerals should be avoided.

Sources

IMA Mineral List (CNMNC)