Clinojimthompsonite

Chemical formula: Mg<sub>5</sub>Si<sub>6</sub>O<sub>16</sub>(OH)<sub>2</sub>

Clinojimthompsonite is a very rare magnesium silicate, occurring as fibrous or lamellar aggregates, and is the monoclinic polymorph of jimthompsonite.

## Characteristics Clinojimthompsonite is a mineral from the silicate group, belonging to the so-called biopyriboles – minerals with a structure intermediate between pyroxenes and amphiboles. It forms fibrous aggregates, often felted (asbestos-like), as well as lamellae and radial aggregates. It usually occurs as microscopic inclusions in other minerals, mainly in amphiboles. ## Physical Properties It is a relatively soft mineral, with a Mohs hardness of 2.5-3. It has a vitreous luster, and a pearly luster on cleavage surfaces. It is transparent to translucent. Its density is approximately 2.81 g/cm³. ## Colors and Varieties Clinojimthompsonite is most often white, grayish, or pale green. No color varieties or commercial varieties are distinguished. ## History and Name The mineral's name, given in 1977 by Hannah E. Veblen and Charles W. Burnham, refers to its monoclinic crystallographic symmetry and to the related orthorhombic mineral, jimthompsonite. The "jimthompsonite" part honors Professor James B. Thompson, Jr. (1921-2011), an American petrologist from Harvard University, who was a pioneer in biopyribole research. ## Uses Clinojimthompsonite has no industrial applications. It is solely an object of scientific and collecting interest.

Properties

Mohs hardness
2.5-3
Luster
Vitreous, Pearly
Streak
White
Density
2.81
Cleavage
Perfect on {010} and {100}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of clinojimthompsonite is extremely difficult and impossible to perform based on visual characteristics alone. Its fibrous or lamellar appearance within metamorphic rocks is only an indication. Certain identification requires advanced analytical techniques, such as X-ray diffraction (XRD) or electron microprobe analysis (EDS/WDS). ## Distinguishing from Similar Minerals Clinojimthompsonite is visually indistinguishable from its orthorhombic polymorph, jimthompsonite, as well as from another biopyribole – chesterite. It can also be easily confused with fibrous varieties of talc, serpentine, tremolite, or anthophyllite. Differences between them can only be determined based on analysis of crystal structure and chemical composition. ## Crystal Forms This mineral does not form well-developed, distinct crystals. It occurs as microscopic fibers or lamellae, which often form felted, radial, or parallel aggregates. It is most commonly found as intergrowths with other chain silicates.

Geological environment

## Genesis Clinojimthompsonite forms under conditions of low-grade retrograde metamorphism, mainly within metamorphosed ultramafic rocks (e.g., serpentinites). It forms as a result of hydrothermal alteration of other magnesium silicates, such as enstatite or anthophyllite. It is less commonly found in metamorphosed carbonate rocks. ## Mineral Associations It most often co-occurs with other biopyriboles – jimthompsonite and chesterite. It is also accompanied by anthophyllite, talc, minerals from the serpentine group, magnesite, and tremolite. ## Localities The most important and classic occurrence (type locality) is the Carlton talc quarry in Chester, Vermont (USA). Other confirmed localities include the Balmat mine in New York State (USA), the Allalin area near Zermatt in Switzerland, and several occurrences in Finland (e.g., Polvijärvi and Outokumpu).

Rarity

Very rare

For collectors

## Quality Criteria As a microscopic mineral, clinojimthompsonite is primarily valued by specialized collectors. The value of a specimen is determined by its abundance (amount of mineral on the sample), distinct formation of fibers or lamellae, and confirmed co-occurrence with other rare biopyriboles. The most important factor increasing value is its origin from the type locality. ## Popular Localities The most sought-after specimens by collectors are those from the Carlton quarry in Chester (Vermont, USA), as they represent reference material for this mineral.

Care and storage

## Cleaning Clinojimthompsonite specimens are very delicate, especially in fibrous form. They should be cleaned only with a soft brush to remove dust. If necessary, compressed air can be used from a safe distance. Avoid washing with water and using any chemical agents. ## What to Avoid The mineral is soft and brittle, susceptible to mechanical damage. It should be protected from scratching, impacts, and vibrations. Ultrasonic cleaners should not be used. Avoid contact with acids. ## Storage It is recommended to store specimens in sealed "micromount" boxes, which protect them from dust and mechanical damage. Each specimen should be properly labeled.

Sources

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