Gonnardite

Chemical formula: (Na,Ca)₂(Si,Al)₅O₁₀·₃H₂O

Gonnardite is a rare mineral from the zeolite group, forming delicate, acicular or fibrous aggregates, usually white or yellowish in color.

## Characteristics Gonnardite is a hydrated sodium and calcium silicate, belonging to the natrolite subgroup within the zeolites. It forms characteristic, delicate aggregates with a radial, spherulitic, or fibrous structure. Individual "needles" in the aggregates are usually very thin and brittle. It rarely occurs as visible, single crystals, and its appearance is typical of many other secondary zeolites, which makes identification difficult without advanced analysis. ## Physical Properties This mineral is characterized by a hardness ranging from 4-5 on the Mohs scale, making it relatively soft. Its density is approximately 2.25 g/cm³. Gonnardite's luster is most often vitreous or silky on the surfaces of fibrous aggregates. It is translucent. ## Colors and Varieties Gonnardite is most commonly white. It can also take on yellowish to salmon-red hues. There are no named color or commercial varieties. ## History and Name The mineral's name comes from Ferdinand Gonnard (1833-1923), a French engineer and mineralogist, who first described this mineral in 1896. The type locality from which the described material originated is La Chaux de Bergonne in Auvergne (France). ## Uses Gonnardite has no industrial significance. Its only use is its collector's value as a rare representative of the zeolite group.

Properties

Mohs hardness: 4-5
Luster: Vitreous
Streak: White
Density: 2.25
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Identifying gonnardite is difficult and often impossible without chemical analysis (EDS) or X-ray analysis (XRD). It can be preliminarily recognized by its mode of occurrence – it forms radial, spherical, or fibrous aggregates in cavities of volcanic rocks. Its brittleness and delicate structure are characteristic. ## Distinguishing from Similar Minerals Gonnardite is visually almost identical to other zeolites from the natrolite subgroup, such as natrolite, mesolite, or scolecite, which form similar acicular and radial aggregates. Natrolite has crystals with a square cross-section, while gonnardite crystallizes in the orthorhombic system, however, this feature is rarely visible in typical aggregates. Final differentiation requires specialized studies. ## Crystal Forms It most often occurs as spherulites (spherical aggregates with an internal radial structure) and fibrous and acicular aggregates. Single, well-formed crystals are extremely rare.

Geological environment

## Genesis Gonnardite is a secondary mineral, formed by low-temperature hydrothermal processes. It crystallizes in vesicles and fissures of volcanic rocks, such as basalts, phonolites, or tephrites, where it precipitates from post-magmatic solutions. ## Mineral Associations It often co-occurs with other zeolites, such as chabazite, phillipsite, natrolite, thomsonite, as well as with calcite and apophyllite. ## Localities Key localities worldwide include Tvedalen in Norway; Aci Castello and Aci Trezza in Sicily (Italy); many locations in the Czech Republic (around Ústí nad Labem); the Kerch Peninsula in Crimea. It has also been found at the type locality in Gignat, France, and in basalts in County Antrim, Northern Ireland.

Rarity

Rare

For collectors

## Quality Criteria The most prized gonnardite specimens are those that form well-defined, spherical aggregates (spherulites) with a silky luster, contrasting with the color of the host rock. The size of the spherulites and the absence of damage to the delicate fibers are important. Specimens from rare localities or with unusual colors (e.g., salmon) are also more highly valued. ## Popular Localities Localities in Italy (e.g., Aci Castello) and Norway (Tvedalen) are considered classic and provide high-quality specimens.

Care and storage

## Cleaning Gonnardite specimens are very delicate and brittle. They should only be cleaned with compressed air to remove dust. Contact with water, especially with detergents, is inadvisable due to the risk of damaging the fibrous aggregates. ## What to Avoid Avoid ultrasonic cleaners, all chemicals, acids, and detergents. The mineral is sensitive to shocks and impacts – acicular crystals break off easily. It should not be heated, as, being a zeolite, it contains water in its structure, which it can easily lose. ## Storage It is recommended to store specimens in closed, padded boxes or display cases to protect them from dust and mechanical damage. Avoid areas with high humidity or exposure to direct sunlight.