Coalingite

Chemical formula: Mg<sub>10</sub>Fe<sup>3+</sup><sub>2</sub>CO<sub>3</sub>(OH)<sub>24</sub>·2H<sub>2</sub>O

Coalingite is a rare magnesium and iron hydroxycarbonate, forming tiny, flexible lamellae and fibrous aggregates of a golden-brown color.

## Characteristics Coalingite is a mineral from the hydroxycarbonate group, chemically a hydrated basic magnesium and iron carbonate. It occurs as very small, hexagonal plates and lamellae, which often form rosette-like or fibrous aggregates. These lamellae are flexible. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties This mineral is characterized by very low hardness, approximately 1-2 on the Mohs scale, meaning it is very soft. Its lamellae are flexible but not elastic. It has a luster described as waxy to silky. It is translucent. ## Colors and Varieties Coalingite occurs in shades ranging from reddish-brown and golden-brown to brown. No distinct color varieties or commercial varieties are recognized. ## History and Name The mineral's name comes from its discovery location near the city of Coalinga in Fresno County, California (USA). It was described and approved as a new mineral in 1965 by F. A. Mumpton, H. W. Jaffe, and C. S. Thompson. The discovery was made in the New Idria serpentinite massif. ## Uses Coalingite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals or minerals from serpentinite rocks.

Properties

Mohs hardness
1-2
Luster
Waxy
Streak
Light brown
Density
2.25
Cleavage
Perfect on {0001}
Fracture
Micaceous
Transparency
Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Coalingite can be identified by its characteristic mode of occurrence – tiny, flexible lamellae forming fibrous or rosette-like aggregates. Its golden-brown color, silky luster, and very low hardness are also key features. It reacts with hydrochloric acid, which is typical for carbonates. ## Distinguishing from Similar Minerals It can be confused with other secondary minerals found in serpentinites, such as pyroaurite or sjögrenite, which have a similar appearance and chemical composition. Differentiation often requires advanced analytical methods, such as X-ray diffraction (XRD). From visually similar micas, it is distinguished by its significantly lower hardness and reaction with acids. ## Crystal Forms It forms very fine, hexagonal crystals in the form of plates and lamellae. These crystals aggregate into rosette-like forms, as well as fibrous or radial clusters.

Geological environment

## Genesis Coalingite is a secondary mineral, formed as a result of low-temperature hydrothermal processes or weathering of ultramafic rocks, mainly serpentinites. It forms as an alteration product of magnesium-rich minerals, such as olivine or pyroxene, in the presence of carbon dioxide-rich solutions. ## Mineral Associations It most commonly co-occurs with minerals typical of the serpentinite environment. These include chrysotile, lizardite, antigorite (minerals from the serpentine group), as well as brucite, hydromagnesite, artinite, pyroaurite, and magnesite. ## Localities The most important and type locality is the New Idria serpentinite massif in Fresno and San Benito counties, California, USA, where it was discovered. It is also known from several other occurrences worldwide, including Okayama Prefecture in Japan, the vicinity of Kraubath an der Mur in Styria (Austria), and Val Malenco in Italy. All these localities are associated with ultramafic rock massifs.

Rarity

Very rare

For collectors

## Quality Criteria Coalingite's collector appeal primarily depends on the richness and aesthetics of the aggregates. Specimens with well-formed, distinct rosettes or dense, silky fibrous clusters are most valued. An intense, golden-brown color and contrast with the rock matrix (most often dark serpentinite) are also important. Due to the microscopic nature of the crystals, specimens are usually small (so-called "micromounts"). ## Popular Localities By far the most classic and valued specimens come from the type locality – the New Idria mercury deposit in California, USA. Specimens from this location are considered exemplary for this mineral.

Care and storage

## Cleaning Coalingite specimens are extremely delicate and should be handled with the utmost care. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove loose dust particles. Avoid contact with water, which can damage the fine, fibrous aggregates. ## What to Avoid Absolutely avoid all chemicals, especially acids, which react violently with carbonates. The mineral is very soft, susceptible to scratches and mechanical damage. It should be protected from moisture, high temperatures, and direct sunlight. ## Storage It is recommended to store coalingite specimens in sealed, airtight "micromount" boxes, which protect them from dust, moisture, and physical damage. Due to its fragility and small size, it is not a mineral suitable for display outside of a specialized container.

Sources

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