Cotunnite

Cotunnite is a naturally occurring mineral composed of lead chloride, with the chemical formula PbCl2. It is a rare secondary mineral that forms in arid environments or in the oxidized zones of lead-rich ore deposits. The mineral is named after the Italian mineralogist Giovanni Antonio Colonna.

Cotunnite typically crystallizes in the orthorhombic system, often appearing as acicular or prismatic crystals. It can also form as massive or granular aggregates. The color of cotunnite ranges from colorless to white, pale yellow, or grayish. It has a vitreous luster and is transparent to translucent. The mineral is relatively soft, with a Mohs hardness of 1.5 to 2, and has a high specific gravity, typically around 7.4 to 7.5.

One of the notable properties of cotunnite is its solubility in water, especially when the water contains dilute acids. This solubility contributes to its formation in weathering environments. Pure cotunnite is not found in large quantities, making it a mineralogical curiosity rather than an economically important ore. Its presence can sometimes indicate the past or present existence of lead-bearing minerals that have undergone alteration.

Cotunnite has been found in various locations worldwide, including Italy, Greece, Spain, and the United States. It is often associated with other secondary lead minerals such as cerussite and anglesite, as well as goethite and other iron oxides. Due to its rarity and softness, cotunnite is not commonly used in any industrial applications. It is primarily of interest to mineral collectors and geologists studying mineral formation processes.