Eglestonite

Chemical formula: Hg<sup>1+</sup><sub>6</sub>OCl<sub>3</sub>(OH)

Eglestonite is a rare mercury(I) oxide hydroxide chloride, forming regular crystals with an adamantine luster that darken upon exposure to light.

## Characteristics Eglestonite is a mineral from the halide group, chemically classified as mercury(I) oxide hydroxide chloride. It occurs as well-formed, isometric crystals, most commonly in the habit of a rhombic dodecahedron or octahedron, reaching up to 2 mm in size. It is also found in granular aggregates or as coatings. A characteristic feature of eglestonite is its reaction to light – freshly exposed surfaces have a color ranging from canary yellow to yellowish-brown, but upon exposure to light, they quickly darken, becoming brown, and eventually black. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of approximately 2.5. It is very dense, which is typical for mercury minerals; its specific gravity is 8.33 g/cm³. It exhibits a strong, adamantine luster. It is transparent to translucent. It does not show cleavage, and its fracture is conchoidal. ## Colors and Varieties Eglestonite does not have named varieties. Its color is its characteristic and diagnostic feature, changing from canary yellow, through yellowish-brown, brown, to black as a result of light exposure. The streak is pale yellow. ## History and Name The mineral was first described in 1904 by Arthur Sutcliffe Eakle. The name "eglestonite" was given in honor of Thomas Egleston (1832-1900), professor of mineralogy and metallurgy at the School of Mines at Columbia University in New York and founder of the American Institute of Mining Engineers. ## Uses Due to its rarity, small crystal size, and toxicity (mercury content), eglestonite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals or mercury minerals.

Properties

Mohs hardness
2.5
Luster
Adamantine
Streak
Light yellow
Density
8.33
Cleavage
None
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Cubic

Diagnostic features

## Identification The key diagnostic feature of eglestonite is its characteristic, rapid color change from yellow to black upon exposure to light. Other helpful identification features include very high density, adamantine luster, and the regular, isometric crystal form (most often rhombic dodecahedra). ## Distinguishing from Similar Minerals Eglestonite can be confused with other rare, secondary mercury minerals of yellow color, such as terlinguaite. However, none of them exhibit such a rapid and dramatic color change upon exposure to light. High density and crystal form also help distinguish it from other yellow minerals, e.g., sulfur (much lighter) or orpiment (different crystal habit, lower density).

Geological environment

## Genesis Eglestonite is a secondary mineral, forming in the oxidation zones of mercury deposits. It forms as a result of weathering and alteration of primary mercury ores, such as cinnabar, in the presence of chloride-rich solutions, often in desert or arid conditions. ## Mineral Associations This mineral often co-occurs with other secondary mercury minerals. The most common associations include: native mercury, calomel (Hg₂Cl₂), cinnabar (HgS), terlinguaite (Hg₂OCl), montroydite (HgO), as well as calcite (CaCO₃) and gypsum (CaSO₄·2H₂O). ## Localities The most important and classic locality for eglestonite, from which the world's best specimens originate, is the Terlingua area in Brewster County, Texas, USA. Significant finds have also been reported in mercury mines in San Bernardino County, California (USA), and in Landsberg, Bavaria (Germany).

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued by collectors are specimens with well-formed, sharp crystals of intense, canary-yellow color, which indicates a lack of light exposure. Rich groups of crystals on a contrasting matrix are highly prized, especially in association with other rare mercury minerals, such as terlinguaite. Crystal size is a key factor – specimens with crystals exceeding 1 mm are already considered significant. ## Popular Localities By far the most desirable and classic locality for eglestonite is the Terlingua district in Texas. Specimens from this locality, especially those from old, historical collections, command the highest prices and are considered the standard for this species.

Care and storage

## Cleaning Cleaning eglestonite is not recommended due to its toxicity and sensitivity. If absolutely necessary, compressed air can be used to remove loose dust particles. Avoid contact with water and any chemicals. ## What to Avoid Eglestonite is extremely sensitive to light, which causes irreversible darkening and decomposition. Exposure to sunlight and intense artificial lighting must be strictly avoided. The mineral is also very toxic due to its high mercury content – avoid inhaling dust and direct skin contact. Do not heat. ## Storage Eglestonite specimens must be stored in complete darkness, in a tightly sealed, labeled container, to prevent light exposure and accidental contact. Keep away from children, pets, and food. It is recommended to store in dedicated systematic boxes, away from other minerals that could be contaminated by mercury.

Sources

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