Faujasite-Ca

Chemical formula: (Ca,Na,Mg)<sub>2</sub>(Si,Al)<sub>12</sub>O<sub>24</sub>·15H<sub>2</sub>O

Faujasite-Ca is a rare zeolite group mineral, forming characteristic, colorless or white octahedral crystals.

## Characteristics Faujasite-Ca is a hydrated calcium, sodium, and magnesium aluminosilicate, belonging to the zeolite group. It is a rare mineral, representing the calcium-dominant end-member of the faujasite series. It most often forms very well-developed, single crystals in the shape of perfect octahedra. They are usually small, ranging in size from fractions of a millimeter to several millimeters. They occur as colorless or milky-white crystals grown in cavities and fissures of volcanic rocks. ## Physical Properties This mineral is characterized by a hardness in the range of 4.5-5 on the Mohs scale, making it relatively brittle. It has a vitreous luster and is transparent to translucent. Its density is very low, approximately 1.91 g/cm³, which is typical for zeolites with an open, "cage-like" crystalline structure filled with water molecules. ## Colors and Varieties Faujasite-Ca is most often colorless or white. It does not form colored varieties or have commercial names. Together with faujasite-Na (sodium) and faujasite-Mg (magnesium), it forms the faujasite group, in which individual minerals differ in the dominant cation. ## History and Name The name of the "faujasite" group was given in 1842 by Augustin Alexis Damour in honor of Barthélemy Faujas de Saint-Fond (1741-1819), a French geologist and volcanologist, who was one of the first to study the volcanic rocks of Kaiserstuhl in Germany, where this mineral was discovered. The prefix "-Ca" indicates the dominance of calcium in the chemical composition of this particular mineral within the group. ## Applications Natural faujasite-Ca, due to its rarity and small crystal size, is of purely scientific and collectible interest. Its synthetic counterparts with the faujasite structure (known as zeolites type X and Y) are, however, among the most important catalytic materials in the chemical industry, especially in petroleum cracking processes.

Properties

Mohs hardness
4.5-5
Luster
Vitreous
Streak
White
Density
1.91
Cleavage
Poor/Indistinct on {111}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Cubic

Diagnostic features

## Identification The most important diagnostic feature of faujasite-Ca is its crystallographic form - it almost always occurs as sharply terminated, perfect octahedra. Other important features include very low density, vitreous luster, and its typical occurrence in gas vesicles in volcanic rocks. ## Distinguishing from Similar Minerals Faujasite-Ca is sometimes confused with other minerals that form octahedra. It is distinguished from spinel and magnetite by its significantly lower hardness and density. From fluorite, which also forms octahedra, it differs by the absence of perfect cleavage and significantly lower density. From other zeolites, such as analcime or chabazite, it can be distinguished by the perfectly octahedral form of its crystals (analcime forms icositetrahedra, and chabazite pseudo-cubes). ## Crystal Forms Crystals are almost exclusively in the form of simple, regular octahedra {111}. Twinning according to the spinel law sometimes occurs. Crystals rarely form larger aggregates, most often occurring as single, overgrown individuals.

Geological environment

## Genesis Faujasite-Ca is a hydrothermal mineral, crystallizing at low temperatures. It forms in the late stages of volcanic activity, filling gas vesicles and small fissures in volcanic rocks, such as phonolites, basalts, and tephrites. ## Mineral Associations It most often co-occurs with other zeolite group minerals, such as phillipsite, chabazite, thomsonite, and gismondine. It is also accompanied by augite, olivine, nepheline, and calcite. ## Localities The most important and classic locality for faujasite-Ca is the Kaiserstuhl volcanic complex in Baden-Württemberg, Germany, particularly the Sasbach locality, which is its type locality. Specimens have also been found in other places around the world with similar geology, but the German localities remain the main source of collectible specimens.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized faujasite-Ca specimens are those with sharp, well-formed, transparent crystals with a vitreous luster. The size of the crystals is of great importance - specimens exceeding several millimeters are rare. The aesthetic arrangement of single, lustrous octahedra on a contrasting rock matrix enhances its attractiveness. ## Popular Localities Specimens from the classic locality in the Kaiserstuhl massif in Germany are by far the most sought after by collectors. This is where exemplary examples of this mineral originate.

Care and storage

## Cleaning Faujasite specimens should be cleaned very carefully, using a soft brush and distilled water. Due to its porous structure and brittleness, ultrasonic cleaners are not recommended, as they can cause cracking or complete destruction of the crystals. ## What to Avoid The mineral is sensitive to high temperatures, which cause rapid dehydration and destruction of its crystalline structure. Contact with acids, which dissolve it, and ammonia should be avoided. It is relatively soft and susceptible to scratching. ## Storage Collectible faujasite specimens are best stored in closed, padded boxes to protect them from dust, mechanical damage, and sudden temperature changes. Prolonged exposure to direct sunlight should be avoided.

External references

Sources

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