Comancheite
Chemical formula: Hg<sup>2+</sup><sub>55</sub>N<sup>3-</sup><sub>24</sub>(N<sup>3-</sup>H<sub>2</sub>,OH)<sub>4</sub>(Cl,Br)<sub>34</sub>
Comancheite is an extremely rare, reddish-brown mercury mineral, found exclusively in Texas as microscopic crystals.
Properties
- Mohs hardness
- 2
- Luster
- Adamantine
- Streak
- Light reddish-brown
- Density
- 8.25
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Translucent to Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Key diagnostic features of comancheite are its reddish-brown color, adamantine luster, very high density, and occurrence as microcrystalline coatings. However, the most important indicator is its origin – this mineral is known from only one locality in the world. Its association with other rare mercury minerals from Terlingua is also a strong clue. ## Distinguishing from Similar Minerals Comancheite can be confused with other reddish or brownish mercury minerals found in the same environment, such as kleinite, terlinguaite, or eglestonite. Often, the only way to definitively distinguish them is through advanced chemical analysis (EDS) or X-ray diffraction (XRD). It usually differs from cinnabar by a more brownish hue and stronger luster. ## Crystal Forms It most commonly forms earthy or granular aggregates and coatings. More rarely, it is found as very small, individual trigonal crystals, which are only visible under high magnification.
Geological environment
## Genesis Comancheite is a secondary mineral, forming in the oxidation zone of mercury deposits. It forms as a result of weathering processes and alterations of primary mercury minerals in an environment rich in chlorides and bromides. It occurs in cavities and fractures of carbonate rocks (limestones) that serve as the host rock for the deposit. ## Mineral Associations This mineral co-occurs with a whole range of other, often rare, mercury minerals. Its typical associations include: native mercury, calomel, terlinguaite, eglestonite, montroydite, and kleinite. ## Localities The only confirmed and described occurrence of comancheite in the world is its type locality: the Mariposa mine (also known as California Mountain Mine) in the Terlingua mining district, Brewster County, Texas, USA.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of comancheite specimens, which are almost always micromounts, is assessed based on the richness and intensity of the coating's color on the rock matrix. Samples with visible, even microscopic, crystals of good development and strong luster are more highly valued. The attractiveness of a specimen is also enhanced by the presence of well-formed crystals of other rare associated minerals. ## Popular Localities The only source of specimens of this mineral is historical finds from the Mariposa mine in Terlingua, Texas. All comancheite available on the collector's market originates from this single locality.
Care and storage
## Cleaning Comancheite specimens should only be dry-cleaned, using a soft brush to remove dust. Avoid water, ultrasonic cleaners, and any chemical agents. Due to its toxicity, all care activities must be performed with the utmost caution. ## What to Avoid **The mineral is extremely toxic due to its high mercury content.** It is absolutely essential to avoid inhaling dust and direct skin contact. Hands must be thoroughly washed after any contact with a specimen. It must not be heated, as this leads to the release of toxic mercury vapors. It should be protected from acids and other chemicals. ## Storage Comancheite must be stored in a sealed, permanently closed, and clearly labeled container, away from other minerals to prevent contamination. It is recommended to keep it in a cool, dry, and dark place, inaccessible to children and pets.