Erionite-Ca

Chemical formula: Ca₅(Si₂₆Al₁₀O₇₂)·₃₀H₂O

Erionite-Ca is a hydrated calcium aluminosilicate from the zeolite group, forming colorless or white, silky fibrous aggregates.

## Characteristics Erionite-Ca is a mineral from the zeolite group, belonging to the erionite subgroup. Chemically, it is a hydrated calcium aluminosilicate. It forms characteristic, woolly or asbestiform aggregates composed of very fine, acicular or fibrous crystals. Individual crystals are usually too small to be seen with the naked eye, and the aggregates have the appearance of delicate, silky fibers. ## Physical Properties This mineral is characterized by low hardness, ranging from 3.5-4 on the Mohs scale. Its density is also low, approximately 2.08 g/cm³. Crystals exhibit a vitreous luster, while their aggregates often have a silky luster, which is typical for minerals with a fibrous structure. It is transparent. ## Colors and Varieties Erionite-Ca occurs in a limited color range. Most often, it is colorless or white. No named color varieties or commercial varieties are distinguished. ## History and Name The name "erionite" comes from the Greek word *ἔριον* (erion), meaning "wool," in reference to the woolly appearance of its aggregates. The "-Ca" suffix indicates calcium as the dominant cation in its structure. The mineral was described from its type locality in Maze, Niigata Prefecture, Japan. ## Uses Erionite-Ca, like other zeolites, has potential applications as a molecular sieve, catalyst, or in ion exchange processes. Due to its fibrous nature, similar to asbestos, its use in some forms may be limited due to health hazards. It is primarily a mineral of scientific and collector's interest.

Properties

Mohs hardness: 3.5-4
Luster: Vitreous , Silky
Streak: White
Density: 2.08
Transparency: Transparent
Crystal system: Hexagonal

Diagnostic features

## Identification Erionite-Ca is most easily recognized by its characteristic, woolly or felted aggregates with a silky luster. The aggregates consist of very fine, flexible fibers. Its occurrence in gas vesicles in volcanic rocks is also an important clue. ## Distinguishing from Similar Minerals Erionite-Ca can be confused with other fibrous zeolites, such as mesolite, natrolite, or scolecite. Differentiation often requires advanced analytical methods (e.g., XRD, EDS). It can also be confused with chrysotile asbestos, which also forms fibrous aggregates but is usually more flexible and elastic. ## Crystal Forms Crystals are strongly elongated, acicular or hair-like, hexagonal. They usually occur as radial or tangled aggregates, forming woolly, asbestiform masses filling cavities in the rock.

Geological environment

## Genesis Erionite-Ca is a low-temperature hydrothermal mineral. It forms in gas vesicles and fractures in volcanic rocks, such as basalts, andesites, and rhyolites. It crystallizes from aqueous solutions during the final stages of lava cooling. ## Mineral Associations It often co-occurs with other zeolites, such as chabazite, heulandite, stilbite, as well as with calcite and quartz (chalcedony, opal). ## Localities The most important occurrences of this mineral are in Japan (Maze, Niigata Prefecture - type locality). It is also known from various localities in the USA (e.g., Oregon, Arizona, Nevada), as well as from Iceland, Italy, and Germany, where it occurs in volcanic rocks.

Rarity

Not very common

For collectors

## Quality Criteria The most prized Erionite-Ca specimens are those that form rich, dense, and "fluffy" fibrous aggregates with a pure, white color and silky luster. It is important that the delicate fibers are as undamaged as possible. Contrast with a dark host rock (matrix) also enhances the visual appeal of the specimen. ## Popular Localities Specimens from the classic locality in Maze, Japan, are historically important. Good quality collector material also comes from localities in Oregon, USA, where erionite forms attractive, snow-white aggregates in basaltic vesicles.

Care and storage

## Cleaning Erionite-Ca specimens are very delicate and brittle. They should only be cleaned with compressed air to remove dust. Any contact with water or brushes can irreversibly damage the delicate, fibrous aggregates. ## What to Avoid Avoid contact with water, chemicals, and especially acids, which can damage the mineral's structure. Fibrous aggregates are extremely sensitive to touch and vibrations. Also, avoid inhaling dust or fine fibers that may break off from the specimen, due to potential health risks similar to asbestos. ## Storage Specimens should be stored in closed, padded boxes or display cases to protect them from dust, mechanical damage, and vibrations. It is best to place them in a stable location where they will not be exposed to accidental bumping.