Erionite-K

Chemical formula: K<sub>10</sub>(Si<sub>26</sub>Al<sub>10</sub>O<sub>72</sub>)·30H<sub>2</sub>O

Erionite-K is a rare zeolite group mineral, forming aggregates of fine, fibrous crystals with a silky luster.

## Characteristics Erionite-K is a hydrated potassium aluminosilicate, belonging to the zeolite group. It occurs as aggregates of fine, acicular or fibrous crystals, which often form woolly or felted masses. Individual crystals are usually too small to be seen with the naked eye. Aggregates are typically white and have a characteristic silky luster, resulting from their fibrous structure. ## Physical Properties This mineral is characterized by a hardness of 3.5-4 on the Mohs scale. Its crystals are brittle. The density is approximately 2.08 g/cm³. The luster is vitreous on crystal faces and silky on the surfaces of fibrous aggregates. ## Colors and Varieties Erionite-K is usually colorless or white. No colored or commercial varieties are distinguished. ## History and Name The name "erionite" comes from the Greek word *ἔριον* (érion), meaning "wool," in reference to the appearance of its fibrous aggregates. The suffix "-K" indicates the dominance of potassium (K) in its chemical composition, distinguishing it from other minerals in the erionite series (Erionite-Na, Erionite-Ca). It was described as a distinct mineral species in 1997 by the International Mineralogical Association (IMA) as part of the new zeolite classification. ## Uses Erionite-K, like other erionites, is of scientific and collector interest only. It should be emphasized that erionite fibers are classified as a human carcinogen (Group 1 according to IARC), similar to asbestos. Inhaling dust containing erionite fibers is extremely dangerous and can lead to the development of pleural mesothelioma.

Properties

Mohs hardness
3.5-4
Luster
Vitreous, Silky
Streak
White
Density
2.08
Cleavage
None
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Hexagonal

Diagnostic features

## Identification Erionite-K is difficult to unambiguously identify without advanced analysis. It can be preliminarily recognized by its fibrous or woolly appearance, white color, and silky luster. It occurs in vesicles and fissures of volcanic rocks. ## Distinguishing from Similar Minerals It can be confused with other fibrous zeolites, such as mesolite, natrolite, scolecite, or mordenite. Erionite often forms more "woolly" aggregates than the other mentioned minerals. Certain distinction requires chemical analysis (EDS) or X-ray diffraction (XRD) to determine its composition and crystal structure. ## Crystal Forms It forms elongated, acicular or hair-like crystals with a hexagonal cross-section. These crystals aggregate into radial, fibrous or woolly masses, filling voids in the host rock.

Geological environment

## Genesis Erionite-K forms as a result of low-temperature hydrothermal processes, most often in gas vesicles and fissures in volcanic rocks, such as basalts, andesites, and rhyolites. It also forms in volcanic tuffs that have undergone alteration in contact with groundwater. ## Mineral Associations It co-occurs with other zeolites, such as chabazite, phillipsite, heulandite, clinoptilolite, as well as with quartz (chalcedony), opal, calcite, and montmorillonite. ## Localities Significant occurrences of erionite-K have been reported in Orvieto (Terni province, Italy) – this is its type locality. It is also known from many locations in the United States, including Rome in Malheur County (Oregon), as well as from the vicinity of Vesuvius in Italy and some localities in Japan.

Rarity

Rare

For collectors

## Quality Criteria The quality of erionite-K specimens depends on the richness and aesthetics of the fibrous aggregates. The most valued specimens are those in which delicate, white "woolly" aggregates abundantly cover the rock matrix, creating a distinct contrast with it. Due to health hazards, its collector value is limited, and this mineral is sought mainly by specialized zeolite collectors. ## Popular Localities The most well-known specimens come from the type locality in Orvieto, Italy, and from the state of Oregon, USA.

Care and storage

## Cleaning Specimens should not be wet cleaned. Dust can only be removed using compressed air, while wearing appropriate respiratory protection (P3/FFP3 mask). Cleaning should be done in a well-ventilated room or outdoors. ## What to Avoid It is absolutely essential to avoid any activities that could lead to the crushing of the mineral and the creation of dust (inhalation of fibers is extremely dangerous to health). It should not be heated, as, like other zeolites, it contains water that it can easily lose. Avoid contact with acids. ## Storage Erionite-K specimens must be stored in sealed, transparent containers (e.g., "perky box" type) that prevent the release of fibers into the environment. The container should be clearly labeled as containing a mineral hazardous to health. It should not be displayed in open showcases.

External references

Sources

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