Yugawaralite

Chemical formula: Ca(Si<sub>6</sub>Al<sub>2</sub>)O<sub>16</sub>·4H<sub>2</sub>O

Yugawaralite is a rare, colorless or white zeolite with a vitreous luster, forming tabular crystals in vesicles and fissures of volcanic rocks.

## Characteristics Yugawaralite is a hydrated calcium aluminosilicate from the zeolite group. It forms characteristic, flattened, tabular crystals, often grouped into fan-shaped or radial aggregates. It also occurs in granular masses. It is usually colorless or white, less commonly adopting pale pink or yellowish hues. Specimens are most often transparent to translucent, with a distinct, vitreous luster, which can be pearly on fracture surfaces. ## Physical Properties The hardness of yugawaralite on the Mohs scale is 4.5, classifying it as a relatively soft mineral. Its density ranges from 2.17-2.25 g/cm³. It exhibits perfect cleavage in one direction, which is visible on some specimens. The luster is vitreous, and on cleavage planes, it is pearly. ## Colors and Varieties This mineral is most often colorless or white. Rarely, specimens with a delicate pink, yellowish, or gray tint are found, likely due to minor impurities. No named color varieties or commercial types are distinguished. ## History and Name Yugawaralite was first described in 1952 by Japanese mineralogists Kin-ichi Sakurai and Akira Hayashi. Its name comes from its discovery locality - the Yugawara hot springs on the Izu Peninsula in Kanagawa Prefecture, Japan. It was approved as a new mineral species by the International Mineralogical Association (IMA). ## Uses Yugawaralite has no industrial application. It is valued solely as a collector's stone due to its rarity, attractive crystal forms, and classic occurrence localities.

Properties

Mohs hardness
4.5
Luster
Vitreous, Pearly on cleavage surfaces
Streak
White
Density
2.17-2.25
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification The key diagnostic feature of yugawaralite is its characteristic crystal habit - flattened, tabular, often forming fan-shaped aggregates. Vitreous to pearly luster, low hardness (4.5), and occurrence in vesicles of volcanic rocks are also important clues. It reacts to acids. ## Distinguishing from Similar Minerals Yugawaralite can be confused with other zeolites with a tabular habit, such as heulandite or stilbite. Heulandite usually has a more coffin-shaped crystal habit and a distinct pearly luster on cleavage planes. Stilbite more often forms sheaf-like aggregates. Final distinction often requires advanced studies, e.g., X-ray diffraction (XRD) analysis. ## Crystal Forms Crystals are tabular, flattened along one of the crystallographic axes. They often occur as fan-shaped, radial, or almost spherical aggregates that line the interiors of vesicles and rock fissures. Single, well-formed crystals are less common.

Geological environment

## Genesis Yugawaralite is a low-temperature hydrothermal mineral. It crystallizes in amygdaloidal vesicles and fissures in andesites, basalts, and volcanic tuffs. It forms as a result of hot solutions interacting with volcanic rocks, often in areas of geothermal activity. ## Mineral Associations It most commonly co-occurs with other zeolites, such as laumontite, stilbite, heulandite, chabazite, as well as with quartz (including chalcedony and amethyst), calcite, and prehnite. These minerals together form characteristic parageneses in geodes and fissures. ## Localities The most important and classic localities for yugawaralite are in Japan, at its discovery site (Yugawara, Kanagawa Prefecture). Significant specimens also come from India, particularly from quarries around Pune and Nashik in Maharashtra state. Occurrences have also been reported in the USA (Alaska, Oregon), Iceland, and Italy.

Rarity

Rare

For collectors

## Quality Criteria The most prized yugawaralite specimens are those with well-formed, sharp, transparent crystals with strong luster. Groups of crystals forming aesthetic, fan-shaped or radial aggregates are highly valued. A contrasting association with other minerals, such as green prehnite or purple amethyst, significantly enhances the specimen's appeal. Crystal size and lack of damage are key factors influencing value. ## Popular Localities Specimens from India, from the Pune and Nashik regions, are considered among the best in the world, often distinguished by their size and crystal perfection. Classic, though often smaller, specimens from the type locality in Yugawara, Japan, hold historical significance.

Care and storage

## Cleaning Yugawaralite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Due to its relatively low hardness, ultrasonic cleaners and steam cleaning should be avoided. ## What to Avoid The mineral is sensitive to acids, which can damage it. Contact with household and laboratory chemicals should be avoided. As a zeolite, it can undergo dehydration (loss of water) at elevated temperatures, leading to its destruction. It should be protected from high temperatures and sudden temperature changes. ## Storage Specimens should be stored in individual, padded boxes or display cases to prevent scratching by harder minerals. It is recommended to keep it in stable humidity and thermal conditions, away from direct sunlight and heat sources.

External references

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