Oxo-magnesio-hastingsite

Chemical formula: NaCa₂(Mg₂Fe₃+₃)(Si₆Al₂)O₂₂O₂

Oxy-magnesio-hastingsite is a rare amphibole from the hornblende group, distinguished by the presence of magnesium and trivalent iron in its complex chemical structure.

## Characteristics Oxy-magnesio-hastingsite is a complex silicate belonging to the amphibole group, specifically to the hornblende subgroup. It is a dark-colored mineral, typically black or dark green to blackish-green. It forms prismatic or fibrous crystals, which rarely occur as well-formed, isolated individuals. It is most commonly found as granular aggregates or intergrown within the host rock. Its visual identification without specialized equipment is practically impossible. ## Physical Properties This mineral is characterized by a hardness of 5-6 on the Mohs scale. It has a vitreous luster and is usually opaque, although thin fragments may be translucent. Its density ranges from 3.24 to 3.45 g/cm³, which is typical for iron- and magnesium-rich amphiboles. ## Colors and Varieties The dominant color is black, sometimes with a green or brown tint. Due to its rarity and complex chemical composition, no distinct color or commercial varieties are recognized. ## History and Name The mineral's name reflects its chemical composition and its relationship to other minerals in the hastingsite group. The prefix "oxy-" indicates the presence of an additional oxygen atom (O²⁻) in the structure, which is not part of the silicate group. The "magnesio-" component emphasizes the dominance of magnesium over divalent iron in a specific structural site. The name "hastingsite" comes from the Hastings region in Ontario (Canada), where minerals of this series were first described. Oxy-magnesio-hastingsite has been formally recognized by the International Mineralogical Association (IMA) as a distinct mineral species. ## Applications Oxy-magnesio-hastingsite has no industrial applications. Its significance is purely scientific and as a collector's item, as a rare member of the large and complex amphibole group.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Greenish-gray
Density: 3.24-3.45
Cleavage: Good on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Macroscopic identification of oxy-magnesio-hastingsite is extremely difficult. Features such as black color, prismatic habit, and occurrence in metamorphic or igneous rocks can only suggest that it is an amphibole. Certain identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS) combined with X-ray diffraction (XRD) to accurately determine the chemical composition and unit cell parameters. ## Distinguishing from Similar Minerals This mineral is practically impossible to distinguish with the naked eye from other black amphiboles, such as hornblende, arfvedsonite, riebeckite, or other minerals from the hastingsite series (e.g., magnesio-hastingsite, potassic-magnesio-hastingsite). All these minerals have similar hardness, luster, color, and mode of occurrence. The differences between them are subtle and mainly concern chemical composition, which can only be examined in a laboratory. ## Crystal Forms Crystals are usually prismatic, often elongated, and can form fibrous, radial, or granular aggregates. Well-formed, terminated crystals are rare. It most often occurs as a rock-forming component, in the form of intergrown, anhedral (irregular) grains.

Geological environment

## Genesis Oxy-magnesio-hastingsite is a mineral formed under conditions of high-grade metamorphism, particularly in the amphibolite and granulite facies. It forms in skarns, which are rocks formed at the contact of a magmatic intrusion (usually granitoid) with carbonate rocks (limestones, dolomites). It can also occur in some alkaline igneous rocks, such as syenites. ## Mineral Associations This mineral co-occurs with other minerals typical of skarn environments and high-temperature metamorphism. The most common associations include: diopside, grossular (garnet), vesuvianite, scapolite, titanite, apatite, and calcite. ## Localities Confirmed occurrences of this rare mineral are few. The most important include: the Jakobsberg mine in the Värmland region, Sweden (type locality), skarns in the Bancroft area, Ontario (Canada), and some localities in Norway and Russia (Kola Peninsula).

Rarity

Very rare

For collectors

## Quality Criteria The quality of a collector's specimen of oxy-magnesio-hastingsite is primarily determined by the degree of crystal development. The most desirable specimens are those with visible, well-formed prismatic crystals, even if they are small. Samples where the mineral forms aesthetic contrasts with lighter associated minerals, such as calcite or diopside, are also highly valued. However, the key element of value is certainty of identification – specimens without analytical confirmation have negligible collector's value. ## Popular Localities The most prized specimens by collectors, although still very rare on the market, come from the classic locality at the Jakobsberg mine in Sweden. Specimens from this location are considered reference material for this mineral species.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. For heavier soiling, compressed air can be used. The use of water is not recommended due to the risk of reaction with inclusions or associated minerals. ## What to Avoid Contact with acids and other chemicals that may damage the mineral's surface should be absolutely avoided. It should not be heated or subjected to sudden temperature changes. As a dark-colored mineral, it may fade with prolonged exposure to direct sunlight. ## Storage Specimens should be stored under stable conditions, in a dry place, away from direct sunlight and heat sources. It is best to place them in closed display boxes or cabinets to protect them from dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)