Montroyalite

Chemical formula: Sr₄Al₈(CO₃)₃(OH)₂₆·₁₀H₂O

Montroyalite is a rare hydrated strontium aluminum hydroxycarbonate, forming white, spherical aggregates and rosettes in syenitic hornfels.

## Characteristics Montroyalite is a rare mineral from the carbonate group, chemically classified as a hydrated strontium aluminum hydroxycarbonate. It occurs as small, white, spherical aggregates or rosettes, reaching up to 2 mm in diameter. Individual crystals forming the aggregates are platy or lath-like, which gives the clusters a delicate, radial internal structure. Due to its composition and occurrence, it is a scientifically interesting mineral. ## Physical Properties This mineral is characterized by low hardness, measuring 3.5 on the Mohs scale. It is relatively light, with a specific gravity of 2.677 g/cm³. It is translucent and has a white streak. Its fracture is uneven to splintery. ## Colors and Varieties Montroyalite occurs exclusively in white. No color varieties or commercial varieties have been distinguished. ## History and Name The mineral's name comes from its discovery locality – the Francon Quarry, located on Mont Royal in Montreal, Quebec, Canada. It was first described and recognized as a new mineral species in 1985. ## Uses Montroyalite has no industrial applications. Its significance is limited to the scientific and collecting spheres, as a rare and well-defined mineralogical species.

Properties

Mohs hardness: 3.5
Streak: White
Density: 2.677
Fracture: Irregular/Uneven,Splintery
Transparency: Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Montroyalite is identified by its characteristic appearance – it forms small, white, spherical or rosette-like aggregates with a radial internal structure. Key to identification is the occurrence locality (Francon Quarry) and paragenesis with other rare alkaline minerals. ## Distinguishing from Similar Minerals It can be confused with other white secondary minerals forming spherical clusters, such as calcite, aragonite, or some zeolites (e.g., thomsonite). Differentiation requires chemical analysis (presence of strontium and aluminum) or crystallographic studies. In the context of the type locality, its appearance and co-occurrence with other minerals from this paragenesis are usually sufficient for preliminary identification. ## Crystal Forms Montroyalite crystals are very small, in the form of thin plates or laths. They combine into radial, spherical aggregates (spherulites) and small rosettes.

Geological environment

## Genesis Montroyalite is a low-temperature hydrothermal mineral. It forms in vugs and fissures in hornfelses resulting from contact metamorphism of calcareous-silicate xenoliths within alkaline syenitic intrusions. ## Mineral Associations This mineral co-occurs with other rare minerals characteristic of alkaline environments, especially from the type locality. These include weloganite, dresserite, dawsonite, strontianite, quartz, and analcime, among others. ## Localities The only confirmed and thoroughly described occurrence of montroyalite in the world is its type locality – the Francon Quarry in Montreal (Quebec, Canada). This is a site that has yielded many rare and unique alkaline minerals.

Rarity

Very rare

For collectors

## Quality Criteria As montroyalite is a micromineral, the value of a specimen is primarily determined by the quality and definition of the spherical aggregates. The most prized samples are those with well-formed, sharp, undamaged spherulites or rosettes, clearly contrasting with the host rock. Association with other rare minerals from the Francon Quarry is also important, as it enhances the scientific and aesthetic value of the specimen. ## Popular Localities The only source of collector specimens is the historical type locality in Canada – the Francon Quarry. It is currently inactive and inaccessible, meaning no new specimens enter the market. All available samples come from old collections.

Care and storage

## Cleaning Montroyalite specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to its low hardness and possible reactivity, contact with water and any chemical agents should be avoided. ## What to Avoid Ultrasonic cleaners, strong streams of water, acids, detergents, and solvents should be absolutely avoided. The mineral is soft and susceptible to scratches and mechanical damage. It should be protected from high temperatures and sudden temperature changes. ## Storage Collector specimens of montroyalite, as microminerals, are best stored in closed, padded "micromount" boxes, which protect them from dust, light, and mechanical damage. Display should be away from direct sunlight and heat sources.

External references

https://www.mindat.org/min-2770.html