Cayalsite-(Y)

Chemical formula: CaY₆Al₂Si₄O₁₈F₆

Cayalsite-(Y) is an extremely rare mineral from the kainosite group, found as microscopic, colorless crystals in volcanic tuffs.

## Characteristics Cayalsite-(Y) is an extremely rare silicate mineral belonging to the kainosite group. It occurs as very small, elongated, platy crystals, typically not exceeding 20 micrometers in length and 5 micrometers in width. These crystals are colorless and transparent, which, combined with their microscopic size, makes them impossible to observe with the naked eye. They usually form randomly oriented clusters or radial aggregates within the host rock. ## Physical Properties Due to the microscopic size of the crystals, most physical properties, such as hardness, density, or luster, have not been precisely determined. Hardness is estimated at approximately 5 on the Mohs scale, and the density calculated based on the chemical formula and unit cell parameters is 4.15 g/cm³. The luster is likely vitreous, and the mineral is transparent. ## Colors and Varieties The mineral is colorless. No colored or commercial varieties are known. ## History and Name Cayalsite-(Y) was first described in 2014 by Japanese mineralogists Koichi Watenabe, Sakae Sano, et al. The name is derived from its chemical composition: **Ca** (calcium), **Y** (yttrium), **Al** (aluminum), and **Si** (silicon), with the "-(Y)" suffix indicating the dominance of yttrium as the rare-earth element. It was approved by the International Mineralogical Association (IMA) under number 2013-128. ## Uses Cayalsite-(Y) has no commercial or industrial applications. Its significance is purely scientific and as a collector's item, as an extremely rare representative of its mineral group.

Properties

Mohs hardness: 5
Luster: Vitreous
Streak: White
Density: 4.15
Cleavage: None
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of cayalsite-(Y) is only possible using advanced laboratory techniques, such as scanning electron microscopy (SEM) with chemical composition analysis (EDS) and X-ray diffraction (XRD). Recognition is impossible under collector or field conditions. ## Distinguishing from Similar Minerals Due to its unique chemical composition and structure, it is easily distinguishable from other minerals using analytical methods. Visually, on a microscopic scale, it may resemble other colorless, acicular minerals, but without analysis, it cannot be differentiated from them. ## Crystal Forms It forms microscopic, platy to acicular crystals, elongated along the crystallographic b-axis. These crystals occur as single, dispersed individuals or form small, radial aggregates.

Geological environment

## Genesis Cayalsite-(Y) forms as a result of hydrothermal processes in the late stage of crystallization of alkaline volcanic rocks. At its type locality (Saga Prefecture, Japan), it was found in voids and fissures in trachyte tuff, where it crystallized from solutions rich in rare-earth elements and fluorine. ## Mineral Associations This mineral co-occurs with minerals such as quartz, fluorite, hematite, hiortdahlite, and other rare minerals, including keiviite-(Y) and fluocerite-(Ce). ## Localities The only confirmed occurrence of cayalsite-(Y) in the world is its type locality: Mitsukoshi Quarry, Hizen Volcano, Karatsu City, Saga Prefecture, Japan.

Rarity

Extremely rare

For collectors

## Quality Criteria As a micromount mineral, the value of a specimen is primarily determined by the abundance and quality of crystals visible under the microscope, as well as the presence of well-defined aggregates. Association with other rare minerals is also important. Confirmation of identification through analysis (e.g., EDS) significantly increases the value and credibility of the specimen. ## Popular Localities The only source of specimens is the Mitsukoshi Quarry in Japan. Material from this locality is extremely difficult to acquire and appears on the collector's market sporadically.

Care and storage

## Cleaning Specimens containing cayalsite-(Y) should not be cleaned mechanically or chemically due to their microscopic nature and the fragility of the crystals. Any attempts at cleaning will almost certainly destroy the mineral. They should be treated exclusively as micromount specimens. ## What to Avoid Avoid contact with any chemicals, ultrasound, and all forms of physical impact, such as abrasion or scratching. The host rock may be sensitive to acids. ## Storage Specimens should be stored exclusively in specialized micromount boxes to protect them from dust, vibrations, and mechanical damage. Storage under stable room conditions is sufficient.

Sources

IMA Mineral List (CNMNC)