Cinnabar is a mineral composed of mercury sulfide, represented by the chemical formula HgS. It stands as the single most important ore from which elemental mercury is extracted. The defining characteristic of cinnabar is its strikingly vibrant coloration, which has captured human attention for millennia.
The Signature Visual Traits
The most distinctive feature of cinnabar is its remarkable color, which ranges from a bright scarlet to a brownish-red, historically known as vermilion. This intense, almost glowing red is exceptionally rare among minerals, making a pure sample of cinnabar visually unique. The hue is a result of the specific crystalline arrangement of mercury and sulfur atoms within the mineral’s structure.
The surface quality, or luster, of cinnabar varies significantly depending on its form. Highly pure, crystalline specimens exhibit an adamantine luster, meaning they reflect light with a brilliance similar to a cut diamond. However, the mineral is more commonly found in massive, granular, or earthy forms, where the luster appears duller or even earthy due to the presence of impurities.
Cinnabar rarely forms large, distinct crystals; when it does, they belong to the trigonal crystal system, often appearing as small rhombohedrons or tabular shapes. More frequently, it occurs as crusts, coatings, or veins filling fractures within a host rock.
Key Physical Properties for Identification
A simple yet highly diagnostic test involves checking its streak, which is the color of the mineral’s powder when rubbed across an unglazed porcelain plate. Cinnabar produces a brilliant scarlet-red streak, a color often brighter than the mineral itself, making it a reliable indicator.
The mineral is also notably soft, possessing a low Mohs hardness value that typically falls between 2.0 and 2.5. This means a sharp tool or even a copper coin can easily scratch the surface of cinnabar. This low hardness allows the mineral to be easily ground into the fine powder that was historically used as the pigment vermilion.
A major distinguishing feature is the mineral’s exceptionally high specific gravity, which is approximately 8.1 to 8.2. This density is remarkably high for a nonmetallic mineral, causing a sample of cinnabar to feel unusually heavy when held, a sensation referred to as “heft.” Additionally, cinnabar exhibits perfect cleavage in three prismatic directions, meaning it tends to break into smooth, distinct fragments along these planes.
Geological Context and Formation
Cinnabar is classified as a hydrothermal mineral, meaning it forms through precipitation from hot, ascending water solutions and vapors. This process occurs at relatively shallow depths and low temperatures, typically less than 200 degrees Celsius, often associated with geologically recent volcanic activity or hot springs. The mercury-rich fluids move through fractures in the rock, depositing the mercury sulfide.
The mineral is commonly found as thin coatings or as impregnations within the host rock, which is frequently a shale, limestone, or schist. Cinnabar is often found alongside other minerals that form in these low-temperature hydrothermal systems.
These associated materials may include common minerals like quartz, calcite, and dolomite. It is also found with other sulfide minerals such as pyrite, marcasite, or stibnite, and occasionally, small droplets of native liquid mercury may be present nearby.