Oneillite

Chemical formula: Na₁₅Ca₃Mn²⁺₃Fe²⁺₃Zr⁴⁺₃Nb⁵⁺(Si₂₅O₇₃)(O,OH,H₂O)₃(OH,Cl)₂

Oneillite is a very rare, yellowish-brown mineral from the eudialyte group, discovered in the famous Poudrette quarry at Mont Saint-Hilaire, Canada.

## Characteristics Oneillite is a complex silicate with a very complicated chemical composition, belonging to the eudialyte group. It occurs as small, granular aggregates or poorly formed crystals up to 1 cm in size. Its characteristic feature is its yellowish-brown color. It is a transparent to translucent mineral with a vitreous luster. ## Physical Properties Its Mohs hardness is 5-6, placing it in the same category as apatite or orthoclase. The mineral's density is relatively high, approximately 3.20 g/cm³. The fracture is uneven. ## Colors and Varieties Oneillite is known exclusively in yellowish-brown hues. No colored or commercial varieties have been distinguished. ## History and Name The mineral was first described in 1998 by John D. Grice and George Y. Chao. Its name honors John J. O'Neill (1886-1966), Dean of Engineering at McGill University, who was the first to conduct detailed geological studies of the Mont Saint-Hilaire complex. ## Applications Due to its extreme rarity, oneillite has no industrial applications. It is solely an object of interest for specialized mineral collectors and scientific institutions.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: White
Density: 3.20
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Identification of oneillite is primarily based on its unique locality (Mont Saint-Hilaire), yellowish-brown color, vitreous luster, and co-occurrence with other minerals from the eudialyte group. Field identification is practically impossible without knowledge of the geological context. Final confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals Oneillite can be confused with other minerals from the eudialyte group, such as eudialyte itself, which often have a red or pink color but can also take on brown shades. Distinguishing them requires specialized laboratory tests due to subtle differences in chemical composition and crystal structure. ## Crystal Forms Oneillite crystals are poorly formed and rarely exceed 1 cm. They typically form granular or massive aggregates within the host rock.

Geological environment

## Genesis Oneillite forms in the late stages of crystallization within nepheline syenites and their associated pegmatites. It is an igneous mineral, crystallizing from highly differentiated, rare-earth and alkali-rich residual melts. Its type locality, Mont Saint-Hilaire, is a classic example of such an alkaline intrusion. ## Mineral Associations This mineral co-occurs with other rare minerals typical of the nepheline syenite environment. At the type locality, it has been found in association with aegirine, albite, microcline, nepheline, sodalite, and other members of the eudialyte group. ## Localities The only confirmed and described occurrence of oneillite in the world is its type locality: the Poudrette quarry (and associated outcrops) at Mont Saint-Hilaire, in the Montérégie region, Quebec, Canada. This is one of the most famous mineralogical localities in the world, known for the discovery of dozens of new minerals.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a oneillite specimen is determined primarily by its quantity and degree of development within the host rock. The most desirable specimens are those with visible, distinctly colored grains or crystal fragments, contrasting with the surrounding rock (matrix). Due to its rarity, every authentic specimen is valuable, and its value increases with the richness of mineral associations. ## Popular Localities The only source of oneillite specimens is the Mont Saint-Hilaire complex in Canada. Specimens from historical finds in the Poudrette quarry are the most prized and represent classics for this mineral.

Care and storage

## Cleaning Oneillite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, a damp cloth with distilled water can be used, and the specimen should then be immediately dried. Ultrasonic cleaners should be avoided. ## What to Avoid The mineral should be protected from contact with acids and other strong chemicals. It should not be subjected to sudden temperature changes. Prolonged exposure to direct sunlight may not be advisable, although there is a lack of detailed research on this topic. ## Storage Oneillite, as a very rare and fragile mineral, should be stored in a separate, padded display box to avoid scratches and mechanical damage. It should be protected from dust and moisture.