Proto-anthophyllite

Chemical formula: Mg<sub>7</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>

Protoanthophyllite is a very rare amphibole group mineral, a polymorph of anthophyllite, occurring in fibrous aggregates.

## Characteristics Protoanthophyllite is an amphibole group mineral with a chemical composition almost identical to anthophyllite, but with a different crystal structure (it is its polymorph). It occurs as fibrous or bladed aggregates, often felted and forming asbestos-like masses. Individual fibers are usually flexible. Due to its similarity to anthophyllite, its identification requires advanced research methods. ## Physical Properties This mineral is characterized by a Mohs hardness of approximately 6. It has a vitreous to pearly luster on cleavage surfaces. It is a relatively light mineral, with a density similar to other magnesium amphiboles. ## Colors and Varieties Protoanthophyllite is usually colorless, white, or light gray. No colored varieties are known. ## History and Name The mineral's name refers to its status as a polymorph of anthophyllite with a "proto"-type structure. It was first described by G.V. Gibbs, F.D. Blossom, and G.E. Phillips in 1960. Its status as a distinct mineral species was discredited by the IMA in 2019, as it is structurally identical to protoferroanthophyllite, forming a solid solution series with it. Currently, the name "protoanthophyllite" refers to the magnesium end-member of this series.

Properties

Mohs hardness
6
Luster
Vitreous
Streak
White
Density
2.98
Cleavage
Perfect on {210}
Fracture
Splintery
Transparency
Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Macroscopic identification of protoanthophyllite is practically impossible without specialized examinations. Its fibrous form and color are almost identical to many other amphiboles, especially anthophyllite and tremolite. Definitive identification requires the use of X-ray diffraction (XRD) or transmission electron microscopy (TEM) methods to determine unit cell parameters and symmetry. ## Distinguishing from Similar Minerals Protoanthophyllite is most often confused with anthophyllite, from which it differs only in crystallographic system (orthorhombic for protoanthophyllite, monoclinic for anthophyllite). Visually, it is indistinguishable from other fibrous amphiboles, such as tremolite or some forms of serpentine (chrysotile). ## Crystal Forms This mineral forms almost exclusively fibrous, bladed, or acicular aggregates. Individual, well-formed crystals are not observed.

Geological environment

## Genesis Protoanthophyllite forms under conditions of regional and contact metamorphism of ultramafic rocks (e.g., peridotites) and metamorphosed dolomites. It is a product of hydrothermal alteration of magnesium-rich minerals such as olivine and pyroxene. ## Mineral Associations It most commonly co-occurs with anthophyllite, talc, magnesite, serpentine (antigorite and chrysotile), tremolite, and olivine. ## Localities The most important and best-documented localities from which research specimens originate are the talc mine in Chester, Windsor County, Vermont (USA), and the nickel deposit on St. John's Island in the Red Sea, Egypt. Occurrences have also been reported in several other locations worldwide where serpentinite has undergone metamorphism.

Rarity

Extremely rare

For collectors

## Quality Criteria Due to its extreme rarity and lack of aesthetic appeal, protoanthophyllite is not a mineral of significance in the collector's market. Its value is purely scientific and systematic. For research institutions and specialized systematic collections, specimens with analytically confirmed identity, originating from the type locality (Chester, Vermont), may be valuable.

Care and storage

## Cleaning Protoanthophyllite specimens, due to their fibrous and delicate nature, should be cleaned with the utmost care. It is safest to use compressed air to remove dust. Avoid contact with water, which can weaken the cohesion of fibrous aggregates. ## What to Avoid Ultrasonic cleaners, chemical agents, and vigorous rubbing, which can irreversibly damage or destroy delicate fibers, should be strictly avoided. Like other asbestos-like minerals, inhaling dust or fine fibers that may detach from it should be avoided. ## Storage Specimens should be stored in closed, padded containers or display cases to protect them from mechanical damage and fiber dispersion. Minimizing direct contact and handling of the specimen is recommended.

Sources

Read more