Ferro-anthophyllite

Chemical formula: &#₉₇₂₃;Fe²⁺₂Fe²⁺₅Si₈O₂₂(OH)₂

Ferro-anthophyllite is an iron-rich amphibole group mineral, forming fibrous or radiating aggregates ranging in color from greenish-brown to black.

## Characteristics Ferro-anthophyllite is an amphibole group mineral, representing the iron-rich (Fe²⁺) end-member of an isomorphous series with magnesium-rich anthophyllite. It rarely forms well-developed, single crystals. It typically occurs as fibrous, bladed, platy, or radiating aggregates. Its habit is often asbestiform, which is characteristic of many amphiboles. ## Physical Properties This mineral is characterized by a hardness of 5.5-6 on the Mohs scale. It exhibits perfect cleavage in two directions, intersecting at an angle of approximately 125°, which is typical for amphiboles. Its luster is vitreous, and on cleavage surfaces, it can be pearly. It is a relatively dense mineral, with a density of about 3.44 g/cm³. ## Colors and Varieties The color of ferro-anthophyllite is usually dark, ranging from clove-brown, through greenish-brown, to almost black. It directly depends on its high iron content. No named commercial or color varieties are distinguished. ## History and Name The name "ferro-anthophyllite" refers to its chemical composition – high iron content (Latin: *ferrum*) – and its structural and chemical relationship with anthophyllite. It was disqualified as a distinct mineral species by the IMA in 2012 as part of the redefinition of the amphibole supergroup and is now classified as an iron-rich variety of anthophyllite.

Properties

Mohs hardness: 5.5-6
Color: Clove-brown to dark brown, grey to white, pale green.
Luster: Vitreous, silky
Streak: White to greyish-white.
Density: 3.24
Cleavage: Perfect on {210}, imperfect on {010}, {100}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Ferro-anthophyllite is most easily identified by its fibrous or bladed habit, dark, brownish-green color, and vitreous luster. The amphibole-characteristic cleavage at ~125° is a key feature, though difficult to observe in compact aggregates. Co-occurrence with other metamorphic minerals, such as garnets or magnetite, is an important clue. ## Distinguishing from Similar Minerals It can be confused with other dark amphiboles, such as hornblende, grunerite, or cummingtonite. Grunerite and cummingtonite have similar compositions but crystallize in the monoclinic system (ferro-anthophyllite in orthorhombic). Differentiation often requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). It usually differs from hornblende by a lighter, more brownish color and often a more fibrous habit. ## Crystal Forms Crystals are rare, prismatic or bladed. Most commonly found are fibrous, radiating, platy, and massive, granular aggregates.

Geological environment

## Genesis This is a characteristic mineral of metamorphic rocks formed under medium to high-grade metamorphic conditions. It forms in iron-rich formations, such as metamorphosed iron-rich sediments (e.g., BIF formations - banded iron formations) and in amphibolites and gneisses. It can also occur in some igneous rocks, such as norites. ## Mineral Associations It often co-occurs with minerals such as almandine (garnet), magnetite, quartz, hornblende, cummingtonite, grunerite, hedenbergite, and fayalite. ## Localities Significant occurrences of ferro-anthophyllite (or iron-rich anthophyllite) have been reported in iron formations in the Lake Superior region of the USA (Michigan and Minnesota). Other known localities include Krivoy Rog in Ukraine, Labrador in Canada, and some regions of Norway and Sweden.

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most valuable specimens are those exhibiting well-formed, radiating aggregates or clearly visible, lustrous fibrous clusters. Contrast with associated minerals, such as red garnets, significantly enhances the visual appeal of the specimen. Purity and lack of damage to delicate fibers are crucial. ## Popular Localities Specimens from classic localities for iron formations, such as the Mesabi Range in Minnesota (USA), are prized by collectors specializing in metamorphic minerals and mineral systematics.

Care and storage

## Cleaning Specimens should be cleaned very carefully, preferably using compressed air to remove dust from fibrous aggregates. A soft brush can be used. Contact with water, especially with detergents, is not recommended, as it can penetrate micro-cracks and weaken the structure of the aggregates. ## What to Avoid Avoid contact with acids and other chemicals that may react with the mineral. Brittle, fibrous specimens are sensitive to impact and vibrations. Due to its potentially fibrous nature, avoid inhaling dust generated during processing or cleaning. ## Storage It is recommended to store specimens in closed, padded boxes or display cases to protect them from mechanical damage and dust. Avoid areas with high humidity.