Zeophyllite

Chemical formula: Ca<sub>13</sub>Si<sub>10</sub>O<sub>28</sub>(OH)<sub>2</sub>F<sub>8</sub>·6H<sub>2</sub>O

Zeophyllite is a rare calcium silicate, forming characteristic, spherical aggregates with a silky luster and perfect, lamellar cleavage.

## Characteristics Zeophyllite is a hydrated calcium silicate from the phyllosilicate group. It most often occurs as spherical or hemispherical aggregates with a radial-fibrous structure. These spheres, reaching several centimeters in diameter, are composed of densely packed, small, platy crystals. The surface of the aggregates has a characteristic silky luster, and individual lamellae may exhibit a pearly luster. Due to its delicate structure, specimens are very brittle. ## Physical Properties The mineral's Mohs hardness is about 3, making it relatively soft. It has a density of approximately 2.75 g/cm³. It is translucent, and in thicker fragments, opaque. A distinguishing feature is its perfect cleavage in one direction, which causes the aggregates to easily break into small, flexible lamellae, similar to micas. ## Colors and Varieties Zeophyllite is usually white, less often colorless or yellowish. No color or commercial varieties are distinguished. ## History and Name The mineral was first described in 1902 by the Austrian mineralogist and petrographer Antonín Pelikán. The name comes from the Greek words *zeo* (to boil) and *phyllon* (leaf), referring to its behavior when heated (characteristic of zeolites) and its platy-lamellar form and perfect cleavage. ## Uses Zeophyllite has no industrial application. It is solely a mineral of scientific and collector's interest.

Properties

Mohs hardness
3
Luster
Silky to Pearly
Streak
White
Density
2.75
Cleavage
Perfect on {001}
Fracture
Fibrous
Transparency
Translucent to Opaque
Crystal system
Triclinic

Diagnostic features

## Identification The most important diagnostic feature of zeophyllite is its mode of occurrence – white, radial, spherical aggregates with a silky luster. Crucial is the presence of perfect cleavage, which causes even spherical aggregates to break into small, flexible lamellae under needle pressure. It reacts with hydrochloric acid, effervescing and forming a silica gel. ## Distinguishing from Similar Minerals Zeophyllite is sometimes confused with other minerals that form white, radial aggregates: - **Pectolite**: Usually forms similar aggregates, but is much harder (approx. 5 on the Mohs scale) and more compact, not exhibiting such easy, platy divisibility. - **Okenite**: Forms cotton-like aggregates composed of very thin, flexible fibers. Its aggregates are even more delicate and lack an internal platy structure. - **Thomsonite-Ca**: Can form spherical aggregates, but its internal structure is more compact, and the luster is often vitreous, not silky. ## Crystal Forms Zeophyllite crystallizes in the triclinic system. Well-formed, single crystals are extremely rare and appear as small, hexagonal tablets or lamellae. It usually forms spherulitic (spherical) and radial-fibrous aggregates.

Geological environment

## Genesis Zeophyllite is a mineral of low-temperature hydrothermal processes. It forms in voids and fissures (amygdales) in alkaline volcanic rocks, such as phonolites and basalts. It is a product of alteration of other minerals in the presence of solutions rich in calcium and fluorine. ## Mineral Associations It most often co-occurs with zeolite minerals and other hydrothermal minerals. Its typical associations include apophyllite, calcite, thomsonite-Ca, phillipsite, natrolite, and tobermorite. ## Localities The most important and classic zeophyllite localities are in the Czech Republic, from where the type specimen originates (near Velké Březno and Zálezly near Ústí nad Labem). It is also known from the Eifel Mountains in Germany (e.g., from the Schellkopf quarry), from the Khibiny Massif on the Kola Peninsula in Russia, and from several localities in the USA (e.g., Santa Clara County, California).

Rarity

Rare

For collectors

## Quality Criteria The most highly valued zeophyllite specimens are those that exhibit well-formed, large (over 2-3 cm in diameter), perfectly spherical aggregates with an intact surface and an intense, silky luster. Placement on a contrasting, dark rock matrix enhances its attractiveness. Damaged, dull, or incomplete specimens have significantly lower value. ## Popular Localities The historical localities in the Czech Republic (Bohemian Central Uplands massif) and the German Eifel Mountains are considered sources of the best quality collector's specimens. Specimens from these locations are sought after by collectors specializing in rare minerals and aesthetic forms.

Care and storage

## Cleaning Zeophyllite specimens are extremely delicate and brittle. They should only be cleaned mechanically, using a very soft brush to remove dust. For heavier soiling, compressed air can be carefully used from a safe distance. Contact with water is not recommended, as it can penetrate between the lamellae and weaken the aggregate structure. ## What to Avoid Avoid any vibrations and impacts – the mineral easily disintegrates. It is sensitive to acids. Ultrasonic cleaners and any chemical agents should not be used. It should be protected from moisture and sudden temperature changes. ## Storage Zeophyllite requires storage in a separate, padded box, away from harder minerals that could damage it. It is best displayed in a closed display case to protect it from dust and accidental touching.

External references

Sources

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