Molysite

Chemical formula: Fe<sup>3+</sup>Cl<sub>3</sub>

Molysite is a rare, hygroscopic iron(III) chloride, forming reddish-brown coatings in volcanic exhalations.

## Characteristics Molysite is an iron(III) chloride, most often occurring as fine-grained, earthy, or fibrous coatings and crusts. It rarely forms small, tabular crystals with a hexagonal outline. Its most characteristic feature is strong hygroscopicity – the mineral absorbs moisture from the air, becoming damp and eventually dissolving into a solution. For this reason, it is extremely unstable under conditions other than those in which it formed. ## Physical Properties It is a very soft and brittle mineral. It has a resinous luster, and its density is approximately 2.9 g/cm³. It is translucent to opaque. Due to its nature, physical properties are difficult to study on stable samples. ## Colors and Varieties Molysite ranges in color from reddish-brown and brown to yellow and yellowish-brown, depending on the degree of hydration and the presence of impurities. No commercial or color varieties are distinguished. ## History and Name The mineral was first described in 1868 by the Italian mineralogist Arcangelo Scacchi. Its name comes from the Greek word *mólysis*, meaning "stain" or "dirt," which refers to the fact that the mineral stains fingers when touched. The type locality is Mount Vesuvius in Italy. ## Uses Molysite has no industrial application. It is solely an object of scientific interest and a rare acquisition for specialized mineralogical collections.

Properties

Luster
Resinous
Streak
Yellow to yellow-brown
Density
2.9
Cleavage
Perfect on {0001}
Transparency
Translucent to Opaque
Crystal system
Trigonal

Diagnostic features

## Identification The key diagnostic feature of molysite is its occurrence environment – active volcanic fumaroles. In collecting conditions, its most characteristic property is strong hygroscopicity (deliquescence), meaning the absorption of moisture from the air until dissolution. The reddish-brown color and form of coatings are also helpful in identification. ## Distinguishing from similar minerals Molysite is sometimes confused with iron oxides and hydroxides, such as hematite or goethite, which may have a similar color. The fundamental difference, however, is molysite's complete solubility in water and its much lower hardness. Hematite and goethite are hard and completely insoluble. ## Crystal forms It most often occurs as earthy, powdery, or fibrous coatings and crusts. Well-formed, small (usually less than 1 mm) hexagonal tabular crystals are very rarely observed.

Geological environment

## Genesis Molysite is a typical mineral of volcanic origin. It forms by sublimation, i.e., direct crystallization from hot volcanic gases (exhalations) at temperatures from 200 to 400°C. It forms on crater walls and within active fumaroles. ## Mineral associations It often co-occurs with other fumarolic minerals, such as halite, sylvine, sassolite, tenorite, as well as with hematite and native sulfur. ## Localities The most important and classic localities for this mineral are active volcanoes in Italy – Vesuvius (type locality) and Etna. It is also found in exhalations from Mount St. Helens (USA), on volcanoes of the Kamchatka Peninsula (Russia), and in other active volcanic regions worldwide.

Rarity

Rare

For collectors

## Quality criteria Due to its extreme instability, the main criterion for a specimen's value is its very existence and correct identification. Samples rich in molysite, forming distinct, colored coatings on the host rock, are highly prized. Specimens with visible, even microscopic, crystals are exceptionally rare and the most desired form. Association with other rare fumarolic minerals also increases its value. ## Popular localities The localities on Vesuvius in Italy are considered classic and provide the best historical material. Specimens from this location are of greatest importance to collectors specializing in rare minerals and those from classic localities.

Care and storage

## Cleaning The mineral is extremely sensitive to moisture and water. **Under no circumstances should it be cleaned wet.** Any contact with water or even a damp cloth will cause it to dissolve and be destroyed. Only very careful, mechanical removal of dust with a soft, completely dry brush is permissible. ## What to avoid Contact with water, water vapor, and humid air must be strictly avoided. The specimen should not be touched with bare hands, as skin moisture can initiate its decomposition process. It must be protected from all chemicals. ## Storage Molysite specimens require specialized storage conditions. They must be kept in tightly sealed, airtight containers (e.g., "perky box" type) along with a moisture-absorbing agent (e.g., silica gel). Exposure to open air, even in a dry room, leads to rapid degradation of the mineral.

External references

Sources

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