Perroudite
Chemical formula: Ag¹⁺₄Hg²⁺₅S²⁻₅(I¹⁻,Br)₂Cl₂
Perroudite is a rare silver and mercury sulfosalt-halide, forming characteristic acicular crystals of intense red color and metallic luster.
Properties
- Mohs hardness
- 2
- Color
- Bright red
- Luster
- Metallic
- Streak
- Reddish-orange
- Density
- 0
- Cleavage
- {100}
- Fracture
- Irregular/Uneven
- Transparency
- Transparent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Key diagnostic features of perroudite include its intense red color with a violet hue, metallic luster, and characteristic form of small, acicular crystals. A red streak is also an important indicator. Field identification is difficult due to the microscopic size of the crystals and requires specialized equipment. ## Distinguishing from Similar Minerals Perroudite is sometimes confused with cinnabar (HgS), which also has a red color and occurs in similar environments. However, cinnabar typically has a more brick-red streak and a adamantine or earthy luster, in contrast to perroudite's metallic luster. It is distinguished from kermesite by its environment of occurrence and mineral paragenesis. ## Crystal Forms Perroudite crystallizes as very small, elongated, acicular, or hair-like crystals, often with a square cross-section. They typically form disordered aggregates, entangled masses, or radial fans on the surface of the host rock.
Geological environment
## Genesis Perroudite is a secondary mineral, formed in the oxidation zones (weathering) of ore deposits rich in silver and mercury. It forms as a result of the reaction of hydrothermal solutions with primary minerals in the presence of chlorine, bromine, and iodine. ## Mineral Associations This mineral often co-occurs with other secondary silver and mercury minerals. Depending on the location, it is associated with minerals such as capgaronnite, iodargyrite, anglesite, quartz, barite, as well as rare copper and lead halides. ## Localities The most important and classic localities for perroudite are the Cap-Garonne mine in Var, France (type locality), and the Broken Hill area in New South Wales, Australia. It is also known from the Tsumeb mine in Namibia and several locations in Chile (e.g., San Rafael mine, Atacama region) and Germany (Palatinate).
Rarity
Very rare
For collectors
## Quality Criteria The most highly prized perroudite specimens are those that feature rich, dense clusters of well-formed, intensely red crystals, contrasting with a light host rock (most often quartz). Due to the microscopic size of the crystals, specimens are primarily evaluated under magnification. The intensity and purity of the color, as well as the absence of damage to the delicate needles, are important. ## Popular Localities Specimens from the Cap-Garonne mine in France and Broken Hill in Australia are considered classic and most sought after by collectors specializing in rare minerals (so-called "micromounts").
Care and storage
## Cleaning Perroudite specimens should be cleaned only very carefully, using compressed air to remove dust. Due to its extreme brittleness and chemical reactivity, the use of water, and especially any chemical agents, is discouraged. Ultrasonic cleaning is absolutely forbidden. ## What to Avoid The mineral is sensitive to sunlight, which can cause it to darken and gradually decompose. Exposure to high temperatures, humidity, and all chemicals, including acids and bases, should be avoided. Due to its mercury content, caution should be exercised, and dust inhalation should be avoided. ## Storage Perroudite requires storage in complete darkness, in a dry place, and at a stable temperature. It is best kept in a closed, opaque display box. Due to its brittleness, vibrations and contact with harder minerals should be avoided.