Corderoite
Chemical formula: Hg<sub>3</sub>S<sub>2</sub>Cl<sub>2</sub>
Corderoite is a very rare mercury sulfosalt-chloride, forming small, isometric crystals with an adamantine luster in mercury deposits.
Description
## Characteristics Corderoite is a mineral from the sulfide group, chemically a complex mercury sulfosalt-chloride. It occurs as very small, but well-formed, isometric crystals, most often in the form of tetrahedra, less commonly rhombic dodecahedra. They are usually transparent to translucent. It is extremely soft and heavy for its size. ## Physical Properties The hardness of corderoite on the Mohs scale is only 1.5-2, meaning it can be scratched with a fingernail. It is characterized by a strong, adamantine luster on crystal faces. Its density is very high, approximately 7.05 g/cm³, which is noticeable even in small specimens. It does not exhibit cleavage, and its fracture is conchoidal. ## Colors and Varieties This mineral is most often colorless, white, or light gray. Crystals with delicate shades of pink or violet are also found. Under light, corderoite is unstable and transforms into black metacinnabar, which is why many specimens darken over time or become covered with a black tarnish. No varieties are distinguished. ## History and Name Corderoite was first described in 1974 by Eugene E. Foord, Frank W. Dickson, and Gordon E. Brown Jr. Its name comes from the discovery locality – the Cordero Mine in Humboldt County, Nevada (USA), which was a significant mercury producer. ## Uses Due to its rarity, small crystal size, and toxicity, corderoite has no industrial applications. It is solely an object of scientific and collector interest.
Diagnostic features
## Identification Characteristic features of corderoite include its isometric, most often tetrahedral crystals, adamantine luster, very low hardness (1.5-2), and high density. A key diagnostic feature is its red streak, which distinguishes it from many similarly looking minerals. It usually occurs as microcrystals. ## Distinguishing from Similar Minerals Corderoite is sometimes confused with other mercury minerals. It is distinguished from cinnabar by its color (cinnabar is red) and crystal habit. It differs from calomel by its crystal habit and lack of fluorescence under UV light. From other gray or white minerals with adamantine luster (e.g., some diamonds, sphalerite), it is distinguished by its extreme softness and red streak. ## Crystal Forms It crystallizes in the isometric system, forming mainly sharp, well-formed tetrahedra, often with visible modifications of other forms. Rhombic dodecahedra are less common. Crystals rarely exceed 1-2 mm in size.
Geological environment
## Genesis Corderoite is a secondary mineral, forming in the oxidation zones of hydrothermal mercury deposits. It forms under low-temperature conditions as a result of the alteration of primary mercury sulfides, mainly cinnabar, in the presence of chloride-rich solutions. ## Mineral Associations It most commonly co-occurs with cinnabar, metacinnabar, native mercury, and calomel. Other secondary minerals such as gypsum, quartz, chalcedony, and perhamite also accompany it. ## Localities This is a very rare mineral, known from only a few localities worldwide. The most important and type locality is the Cordero Mine in Humboldt County, Nevada (USA). Significant specimens also come from the nearby McDermitt Mine, located on the border of Nevada and Oregon. Practically all collector material originates from these two localities.
Rarity
Very rare
Collector aspects
## Quality Criteria The most highly valued specimens are those with sharp, well-formed, and transparent crystals larger than 1 mm. The absence of black metacinnabar tarnish, indicating mineral degradation, is important. Aesthetic compositions where colorless or light gray corderoite crystals contrast with an intensely red cinnabar matrix are particularly sought after. ## Popular Localities Almost all collector specimens come from two adjacent mines in the USA: Cordero Mine (Humboldt County, Nevada) and McDermitt Mine (Malheur County, Oregon). Specimens from these localities are considered classic and the best for this species.
Care and storage
## Cleaning Corderoite specimens should only be dry-cleaned, using a soft brush to remove dust. Any contact with water, and especially with chemicals and ultrasound, which can damage it, should be avoided. ## What to Avoid Corderoite is a mercury mineral, which makes it toxic. Avoid inhaling dust and always wash hands after contact with a specimen. It is sensitive to light, under which it darkens, transforming into metacinnabar. It should be protected from direct sunlight and high temperatures. ## Storage It is recommended to store corderoite in a closed, airtight container (e.g., a "perky box"), in a dark and dry place. This form of storage protects it from light, moisture, and reduces the risk of accidental contact with the toxic mineral.