Kellyite

Chemical formula: (Mn₂+,Mg,Al)₃(Si,Al)₂O₅(OH)₄

Kellyite is a very rare, pale yellow mineral of the serpentine group, occurring as tiny, hexagonal platelets in metamorphosed manganese deposits.

## Characteristics Kellyite is a mineral of the kaolinite-serpentine group, being the manganese analogue of amesite. It most commonly occurs as very fine, hexagonal platy crystals, which rarely reach sizes visible to the naked eye. These crystals form lamellar or scaly aggregates, and can also be dispersed within the rock mass. Its characteristic feature is a pale yellow to yellowish-brown color. ## Physical Properties This mineral is relatively soft, with a hardness of about 2.5 on the Mohs scale. It has perfect, unidirectional cleavage, causing it to easily split into thin lamellae. The luster on cleavage surfaces is pearly, and on other surfaces, vitreous. It is a translucent mineral. ## Colors and Varieties Kellyite occurs in shades of yellow – from pale yellow and lemon yellow to yellowish-brown. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1974 by Donald R. Peacor, Eric J. Essene, Willard L. Roberts, and Roland C. Rouse. The name honors William Crowley Kelly (born 1929), a professor of geology at the University of Michigan, in recognition of his contributions to the field of ore deposit geology and ore microscopy. The type locality is the Bald Knob manganese mine in South Carolina, USA. ## Uses Due to its extreme rarity and occurrence as small crystals, kellyite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals and scientists.

Properties

Mohs hardness: 2.5
Luster: Vitreous to Pearly
Streak: Pale yellow
Density: 3.05
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Features facilitating the identification of kellyite include its characteristic pale yellow color, low hardness, perfect cleavage, and occurrence as tiny, hexagonal platelets. A key indicator is its environment of occurrence – metamorphosed manganese-rich deposits. ## Distinguishing from Similar Minerals Kellyite can be confused with brightly colored micas (e.g., muscovite) or other platy minerals. However, micas are usually more elastic. It differs from talc by its slightly higher hardness and hexagonal crystal form. Definitive identification most often requires advanced analytical methods, such as chemical analysis (to confirm the presence of manganese as the dominant component) or X-ray diffraction (XRD). The geological context is extremely helpful in distinguishing it from similar minerals. ## Crystal Forms Kellyite most commonly forms microscopic, hexagonal, platy crystals. They usually occur as scaly or lamellar aggregates. They can also be dispersed in the host rock as individual grains.

Geological environment

## Genesis Kellyite forms under low-grade metamorphic conditions in sedimentary or hydrothermal deposits rich in manganese. It is a secondary mineral, forming in veins and fractures cutting manganese ores. ## Mineral Associations This mineral co-occurs with other manganese minerals. At the type locality (Bald Knob, USA), it was found in association with alleghanyite, galaxite, jacobsite, manganhumite, and sonolite. In other localities, its associated minerals may include tephroite, bementite, caryopilite, and rhodochrosite. ## Localities The most important documented occurrences of kellyite worldwide are few. They include the Bald Knob mine in South Carolina (USA), the Tanohata and Mochi-no-ki mines in Iwate Prefecture (Japan), the Valgraveglia mine in Liguria (Italy), and the Molberget mine in Värmland (Sweden).

Rarity

Very rare

For collectors

## Quality Criteria The quality of kellyite specimens is primarily assessed based on the visibility and development of the crystals. Due to their small size, any specimen with clear, even small, hexagonal crystal outlines is considered valuable. Rich aggregates of the mineral on a rock matrix are highly prized, especially when they create an attractive contrast with other co-occurring minerals, such as pink rhodochrosite. ## Popular Localities Japanese mines, such as Tanohata, are considered the source of the best, most well-formed kellyite crystals. Specimens from there are particularly sought after by collectors specializing in systematic minerals. Examples from the type locality in the USA are mainly of historical significance.

Care and storage

## Cleaning Kellyite specimens should be cleaned with the utmost care due to their low hardness and perfect cleavage. A soft brush is best for removing dust. If necessary, the specimen can be rinsed in distilled water using a very gentle brush, and then immediately dried. Ultrasonic cleaners should be strictly avoided. ## What to Avoid The mineral is very brittle and susceptible to mechanical damage, such as impacts, scratches, or pressure, which can cause the lamellae to delaminate. Contact with acids and other strong chemicals should be avoided. It is stable under normal lighting and temperature conditions. ## Storage It is recommended to store specimens in separate, padded boxes or display cases, away from harder minerals, to prevent scratching. It is best to display it in closed showcases, protecting it from dust and accidental damage.

Sources

IMA Mineral List (CNMNC)