Chestermanite

Chemical formula: Mg₂(Fe₃+,Mg,Al,Sb₅+)O₂BO₃

Chestermanite is an extremely rare, black borate mineral found exclusively in contact metamorphic zones in California.

## Characteristics Chestermanite is a complex borate of magnesium, iron, aluminum, and antimony, belonging to the ludwigite group. It forms small, prismatic crystals up to 1 mm long, usually elongated in one direction. It also occurs as aggregates with a fibrous structure. The mineral has a black color and a characteristic, submetallic luster. It is opaque, although the thinnest fragments may show slight translucency. ## Physical Properties The Mohs hardness of chestermanite is 6. Its density is relatively high, approximately 3.71 g/cm³. The mineral is characterized by a submetallic luster and a reddish-brown streak. It exhibits perfect cleavage in one direction and good cleavage in another. ## Colors and Varieties This mineral occurs exclusively in black. No colored or commercial varieties are known. ## History and Name Chestermanite was first described in 1986 by a team of mineralogists (J.A. Mandarino, D.C. Harris, A.C. Roberts, P.J. Dunn). The mineral is named in honor of Charles Wesley Chesterman (1913–1991), an American geologist who worked for the California Division of Mines and Geology, in recognition of his contributions to the study of California geology. ## Applications Due to its extreme rarity and occurrence as small crystals, chestermanite has no industrial applications. It is solely an object of scientific interest and is valued by specialized collectors of rare minerals.

Properties

Mohs hardness: 6
Luster: Sub-metallic
Streak: Reddish-brown
Density: 3.71
Cleavage: Perfect on {010}, good on {100}
Transparency: Translucent to Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Helpful features for identifying chestermanite include its black color, submetallic luster, reddish-brown streak, and prismatic or fibrous habit. However, the key is the environment of occurrence – skarn at the contact with dolomite – and the presence of characteristic associated minerals, such as other borates. ## Distinguishing from Similar Minerals Chestermanite is very similar to other black minerals of the ludwigite group, especially ludwigite itself. Certain differentiation of these minerals in field conditions is practically impossible and requires advanced chemical analyses (e.g., EDS) to determine the elemental composition. It is distinguished from black tourmaline (schorl) by its lower hardness (6 versus 7-7.5 for tourmaline) and different crystal form. ## Crystal Forms Crystals are prismatic, elongated along one axis, reaching up to 1 mm in length. They often form fibrous aggregates or radial clusters.

Geological environment

## Genesis Chestermanite is a metamorphic mineral. It forms under conditions of contact metamorphism, in skarn zones developed at the contact of granodiorite intrusion with dolomitic marbles. ## Mineral Associations This mineral occurs in association with other borates, such as ludwigite, fluoborite, and szaibelyite. It also co-occurs with calcite, phlogopite, and spinel. ## Localities The only confirmed locality for chestermanite in the world is its type locality: the area around the Potter-Huffman Dam on the North Fork Kings River in Fresno County, California (USA).

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of chestermanite specimens, being a typical micromineral, is primarily assessed based on the development and sharpness of the crystals, even if they are submillimeter. The most valued are single, well-formed prismatic crystals embedded in a rock matrix. The value of a specimen is enhanced by the presence of other rare associated minerals from the same locality. ## Popular Localities The only source of chestermanite specimens is its type and so far only known locality in California, USA. For this reason, all specimens available on the collector's market come from there.

Care and storage

## Cleaning Chestermanite specimens should be cleaned with the utmost care, preferably using a soft, dry brush to remove dust. If necessary, distilled water and a very soft brush can be used, after which the specimen should be immediately and thoroughly dried. ## What to Avoid Avoid contact with all chemical agents, especially acids. Ultrasonic cleaners are not recommended. As a borate, the mineral may be sensitive to prolonged exposure to water. It should be protected from high temperatures and sudden temperature changes. ## Storage Due to the rarity and small size of the crystals, chestermanite specimens are best stored in specialized micromount boxes, which protect against dust and mechanical damage. A stable, dry environment is recommended.

Sources

IMA Mineral List (CNMNC)