Axinite-(Fe)

Chemical formula: Ca₄Fe₂+₂Al₄[B₂Si₈O₃₀](OH)₂

This is an iron-rich borosilicate from the axinite group, valued for its characteristic, sharply terminated crystals with a vitreous luster and colors ranging from clove brown to violet.

## Characteristics Axinite-(Fe), formerly known as ferroaxinite, is the most common mineral of the axinite group. It forms flattened, sharp-edged crystals with a characteristic, axe-like shape, often gathered into fan-shaped or rosette aggregates. The crystal surfaces are typically strongly striated. It is a hard and relatively heavy mineral, and its name refers to the sharp, axe-like (Greek *axine*) shape of its crystals. ## Physical Properties It is characterized by a hardness of 6.5-7 on the Mohs scale and a density of 3.26-3.36 g/cm³. The luster is strongly vitreous, and the mineral can be transparent to translucent. Some specimens exhibit strong pleochroism, changing color depending on the direction of observation (e.g., from violet to colorless and light brown). It also possesses piezoelectric and pyroelectric properties, meaning it generates an electrical charge under pressure or temperature change. ## Colors and Varieties The typical color of axinite-(Fe) is clove brown, but brown-violet, yellowish-brown, gray, greenish, and more rarely blue or almost colorless specimens are also found. Color variation depends mainly on the content of iron, manganese, and magnesium. No named commercial or gemmological varieties are distinguished, and classification is based on the dominant cation (Fe, Mn, Mg). ## History and Name The name "axinite" was given by the French mineralogist René Just Haüy in 1797 and comes from the Greek word *ἀξίνη* (*axine*), meaning axe, in reference to the shape of the crystals. The suffix "-(Fe)" was added in 2008 as part of a group nomenclature revision to precisely indicate iron as the dominant element in this particular variety. ## Uses Due to its hardness, luster, and interesting colors, transparent axinite-(Fe) crystals are sometimes faceted and used as gemstones, although their use is limited due to their perfect cleavage. It is primarily a valued and sought-after collector's stone.

Properties

Mohs hardness: 6.5-7
Luster: Vitreous
Streak: White
Density: 3.26-3.36
Cleavage: Good on {100}, poor on {001}, {110} and {-111}
Fracture: Conchoidal to uneven
Transparency: Transparent to translucent
Crystal system: Triclinic

Diagnostic features

## Identification The key diagnostic feature of axinite-(Fe) is its unique, flattened, and sharp-edged crystal shape, resembling an axe blade. Characteristic features also include its color (most often clove brown), strong vitreous luster, and distinct striations on the crystal faces. A helpful property is the strong pleochroism visible in transparent fragments. ## Distinguishing from Similar Minerals It can be confused with brown epidote, dravite (a variety of tourmaline), or smoky quartz. It differs from epidote primarily by its different, less columnar crystal habit and often violet hue. From tourmaline, it differs in crystal shape (tourmalines have a triangular cross-section) and hardness. Smoky quartz does not exhibit cleavage or pleochroism and has a different crystal habit. ## Crystal Forms Crystals are tabular, flattened, with sharp edges, often with a triangular outline. They usually occur as rosette, fan-shaped, or radial aggregates. Granular and massive aggregates are also found.

Geological environment

## Genesis Axinite-(Fe) is a mineral typical of contact and low-grade regional metamorphism processes. It forms in skarns, hornfelses, and alpine fissures as a result of metasomatism of carbonate rocks (limestones, dolomites) under the influence of boron-rich hydrothermal solutions originating from granitoid intrusions. It can also crystallize in alpine-type veins. ## Mineral Associations It often co-occurs with minerals such as quartz, calcite, epidote, clinozoisite, diopside, hedenbergite, garnets (mainly andradite and grossular), vesuvianite, prehnite, scapolite, titanite, and sulfides, e.g., pyrite and chalcopyrite. ## Localities The most famous and classic locality, from which large, well-formed crystals originate, is Le Bourg d'Oisans in the Dauphiné Alps, France. Other important localities include Obira in Japan (large, gem-quality crystals), Dalnegorsk in Russia, the vicinity of the Baja California Peninsula in Mexico, as well as numerous localities in Switzerland, USA (California, Nevada), Brazil, and Tanzania.

Rarity

Not very common

For collectors

## Quality Criteria The most highly valued collector's specimens are those with large, sharp, undamaged crystals of high transparency and intense, desirable color (e.g., deep violet or saturated brown). Their aesthetic arrangement on a rock matrix, especially in association with contrasting minerals like white quartz or calcite, enhances their attractiveness. Complete, fan-shaped aggregates are also highly prized. ## Market Prices The prices of axinite-(Fe) vary widely. Small, single crystals or fragments can be purchased for several tens of Polish zlotys. Good quality specimens from classic localities, several centimeters in size, cost from several hundred to several thousand Polish zlotys. Exceptional, large, and aesthetic crystal groups on matrix, especially from historic localities like Bourg d'Oisans, can reach prices of several tens of thousands of Polish zlotys or more on the specialized market. ## Popular Localities For collectors, specimens from the classic locality of Le Bourg d'Oisans in France are the most sought-after, serving as a benchmark for this mineral. Large, gem-quality crystals from the Obira mine in Japan and well-formed crystals from Dalnegorsk in Russia are also highly valued.

Care and storage

## Cleaning Axinite specimens should be cleaned carefully, using a soft brush and lukewarm water, possibly with a mild soap. After washing, they should be thoroughly rinsed with distilled water and left to dry completely. Ultrasonic cleaners are not recommended, as they can damage crystals along cleavage planes. ## What to Avoid Avoid contact with strong acids and bases. The mineral is sensitive to rapid temperature changes (thermal shock), which can cause cracks. Although relatively hard, it is also brittle and susceptible to mechanical damage due to its cleavage. ## Storage Collector's specimens of axinite are best stored in separate, padded boxes or on stands to prevent scratches and chips from other minerals. They should be protected from dust and sudden temperature changes. Gemstone specimens require careful wearing and storage.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FAxinite