Proto-ferro-anthophyllite
Chemical formula: Fe<sup>2+</sup><sub>7</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>
Proto-ferro-anthophyllite is a rare silicate from the amphibole group, a low-temperature, orthorhombic polymorph of grunerite.
Properties
- Luster
- Vitreous
- Streak
- Pale grayish green
- Cleavage
- Perfect on {210}
- Transparency
- Translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Identification of proto-ferro-anthophyllite is impossible based on visual characteristics alone. It looks identical to other fibrous iron amphiboles, especially grunerite. A key clue is its occurrence environment – metamorphosed iron formations. Definitive identification requires advanced analytical techniques, primarily X-ray diffraction (XRD), which allows for the determination of its unique orthorhombic structure (space group Pnmn). ## Distinguishing from Similar Minerals The most difficult task is distinguishing it from its polymorphs: grunerite and ferro-anthophyllite. Grunerite crystallizes in the monoclinic system, and ferro-anthophyllite in the orthorhombic, but in a different space group (Pnma). Visually, these minerals are indistinguishable. Other fibrous amphiboles, such as kupferite or some varieties of riebeckite, may be similar but usually differ in color and occurrence environment. ## Crystal Forms Proto-ferro-anthophyllite forms fibrous, asbestiform, acicular, and lamellar aggregates. It often occurs as felted aggregates or radial rosettes. Well-formed, single crystals are not known.
Geological environment
## Genesis Proto-ferro-anthophyllite is a mineral of metamorphic rocks. It forms under relatively low-temperature conditions during regional or contact metamorphism of iron-rich sedimentary rocks, such as banded iron formations (BIFs). It is considered a low-temperature phase that transforms into more stable grunerite with increasing temperature. ## Mineral Associations It most commonly co-occurs with minerals typical of metamorphosed iron formations. Its most common associated minerals are grunerite (often as intergrowths), magnetite, and quartz. ## Localities This is a very rare mineral, known from only a few localities worldwide. The type locality, from which the best-studied specimens originate, is the Cheyenne belt in the Granite Mountains, Wyoming, USA. It has also been reported within the Gunflint Iron Formation on the border of the USA (Minnesota) and Canada (Ontario).
Rarity
Very rare
For collectors
## Quality Criteria The collector's value of proto-ferro-anthophyllite is not based on aesthetics, but on its rarity and scientific significance. The most desirable specimens are those whose identity has been confirmed by analytical methods (e.g., XRD). The value of a specimen is enhanced by the richness of the aggregate, a distinct fibrous form, and origin from the type locality (Wyoming). It is a mineral primarily for specialized systematic collectors. ## Popular Localities The vast majority of material available in collections comes from the type locality in the Granite Mountains, Wyoming, USA. Specimens from other locations are extremely rare and represent a significant unique find.
Care and storage
## Cleaning Proto-ferro-anthophyllite specimens are usually brittle and fibrous, so they should only be cleaned mechanically, using a soft brush to remove dust. Wet cleaning, especially with ultrasound, is not recommended, as it could damage delicate aggregates. ## What to Avoid The mineral is thermally unstable – when heated above approximately 230°C, it transforms into grunerite. It should be protected from high temperatures. Avoid contact with strong acids. Due to its fibrous form, caution should be exercised when handling specimens to avoid inhaling potentially released fibers. ## Storage It is recommended to store specimens in closed, padded boxes (e.g., "micromount" type) to protect them from mechanical damage, dust, and to prevent the release of fibers into the environment. They should be kept away from heat sources.