Gianellaite
Chemical formula: [(N<sup>3-</sup>Hg<sup>2+</sup><sub>2</sub>)<sub>2</sub>](S<sup>6+</sup>O<sub>4</sub>)(H<sub>2</sub>O)<sub><i>x</i></sub>
Gianellaite is a very rare mercury nitride sulfate, forming characteristic, cubic crystals of an intense canary-yellow color.
Description
## Characteristics Gianellaite is a secondary mineral from the sulfate group, chemically classified as mercury nitride sulfate. It occurs as well-formed, sharp, cubic crystals, which can reach up to 2 mm in size. Less commonly, it forms granular aggregates or coatings. Its most characteristic features are its intense, canary-yellow or pale yellow color and strong, adamantine luster, giving the crystals a vibrant appearance. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3. It is characterized by a high density of approximately 7.18 g/cm³, which is typical for mercury-bearing minerals. It is transparent to translucent. It exhibits no cleavage, and its fracture is conchoidal. ## Colors and Varieties Gianellaite occurs in uniform shades of yellow, from pale yellow to intense canary yellow. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1972 by Eugene E. Foord, Willard L. Roberts, and George R. Rapp Jr. The name honors Vincent Paul Gianella (1886–1974), a professor of geology at the Mackay School of Mines (University of Nevada, Reno), in recognition of his contributions to the study of Nevada geology, particularly the mercury deposits in the Terlingua district, Texas, where the mineral was discovered. ## Uses Due to its extreme rarity, toxicity, and instability, gianellaite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for advanced systematic and rare mineral collections.
Diagnostic features
## Identification Key diagnostic features of gianellaite include its canary-yellow color, cubic crystal form, adamantine luster, and very high density. Its paragenesis – co-occurrence with other rare mercury minerals in a specific geological environment – is also an important indicator. ## Distinguishing from Similar Minerals Gianellaite can be confused with **native sulfur**, which has a similar color but significantly lower density, a different crystal habit (orthorhombic), and is softer. From other secondary mercury minerals, such as **terlinguaite** or **eglestonite**, it is primarily distinguished by its regular (cubic) crystallographic system and characteristic, pure yellow color. ## Crystal Forms Gianellaite crystallizes almost exclusively in the form of simple cubes {100}. Crystals are usually small but well-formed, often growing on other minerals, especially calomel or cinnabar.
Geological environment
## Genesis Gianellaite is a secondary mineral, formed in the oxidation zone of mercury deposits. It forms under low-temperature conditions, near the Earth's surface, as a result of the alteration of cinnabar (HgS) by hydrothermal solutions in an alkaline environment. ## Mineral Associations This mineral occurs in association with other mercury minerals. It most commonly co-occurs with cinnabar, native mercury, calomel, eglestonite, terlinguaite, montroydite, and hanawaltite. ## Localities The most important and classic locality for gianellaite is its type locality – the Mariposa mine in the Terlingua district, Brewster County, Texas (USA). It is also known from several other localities worldwide, including the Clear Creek deposit in San Benito County, California (USA).
Rarity
Very rare
Collector aspects
## Quality Criteria The most highly prized gianellaite specimens are those with sharp, well-formed cubic crystals of intense, canary-yellow color and strong luster. The size of the crystals and their placement on a contrasting matrix, such as white calomel, are of great importance. Undamaged specimens with a clearly visible crystalline form achieve the highest collector's value. ## Popular Localities Specimens from the type locality – the mine in the Terlingua district, Texas – are by far the most sought after by collectors. Material from this location is considered the global standard for this mineral.
Care and storage
## Cleaning Gianellaite specimens should only be dry-cleaned, using a soft brush to remove dust. Any contact with water, which can damage the mineral, should be avoided. Ultrasonic or chemical cleaning is absolutely forbidden. ## What to Avoid Gianellaite is **extremely toxic** due to its high mercury content. Inhaling dust and direct skin contact should be avoided. Hands should be thoroughly washed after any contact with the mineral. It is also light-sensitive and may darken when exposed to light, so it should be protected from prolonged exposure to sunlight and intense lighting. Moisture and high temperatures should be avoided. ## Storage It is recommended to store specimens in a sealed, airtight container (e.g., a "membrane box"), in a dark and dry place. The container should be clearly labeled as toxic material. Keep away from children and pets.