Carletonite

Chemical formula: KNa<sub>4</sub>Ca<sub>4</sub>Si<sub>8</sub>O<sub>18</sub>(CO<sub>3</sub>)<sub>4</sub>(F,OH)·H<sub>2</sub>O

Carletonite is a rare silicate with a characteristic, intensely blue color and perfect cleavage, found exclusively in one location worldwide.

## Characteristics Carletonite is a complex layered silicate, valued for its unique and attractive appearance. Its most characteristic feature is its intense, azure or blue color, which can transition to lighter or darker shades, or even be colorless. It typically forms massive aggregates, in which its square cross-sections and perfect cleavage are visible. Crystals are extremely rare and appear as cubes or short prismatic habits. This mineral is opaque or at most translucent in thin fragments. ## Physical Properties Carletonite is characterized by a hardness ranging from 4-4.5 on the Mohs scale, making it a relatively soft mineral. It has a vitreous luster, and on cleavage planes, it exhibits a pearly luster. Its density is approximately 2.45 g/cm³. A distinguishing feature is its perfect cleavage in one direction, which causes it to easily split into thin plates. ## Colors and Varieties The predominant and most desired color of carletonite is an intense blue, from light sky blue to deep indigo. Colorless, white, or pinkish specimens are also found. Some fragments may exhibit pleochroism, meaning a change in hue depending on the observation direction (from colorless to blue). No named varieties of carletonite are distinguished. ## History and Name The mineral was discovered in 1963 at the Poudrette quarry on Mont Saint-Hilaire in Quebec, Canada. Its scientific description was prepared by George Y. Chao from Carleton University in Ottawa in 1971. The name "carletonite" comes from the name of this university, honoring its contribution to mineralogical research. ## Uses Due to its extreme rarity and occurrence in only one location worldwide, carletonite has no industrial applications. However, it is a highly valued and sought-after collector's stone. It is sometimes cut into cabochons for connoisseurs of rare gemstones, but its softness and perfect cleavage make this very difficult.

Properties

Mohs hardness
4-4.5
Luster
Vitreous, Pearly
Streak
White
Density
2.45
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Tetragonal

Diagnostic features

## Identification The most important diagnostic features of carletonite are its unique, intensely blue color, perfect cleavage in one direction, and characteristic square cross-sections in massive aggregates. It reacts with hydrochloric acid, releasing carbon dioxide bubbles. ## Distinguishing from Similar Minerals Carletonite is sometimes confused with sodalite, which also occurs at Mont Saint-Hilaire. However, sodalite usually has a greasy luster (carletonite has a vitreous/pearly luster), lacks distinct cleavage, and crystallizes in the isometric system. Blue-colored quartz is much harder (hardness 7) and has a conchoidal fracture. Lazurite is usually fine-grained and often contains pyrite inclusions, which are absent in carletonite. ## Crystal Forms Well-formed crystals are extremely rare and usually appear as short, tetragonal prisms or cubes. Most often, this mineral occurs in the form of massive, platy, or granular aggregates, embedded in the host rock.

Geological environment

## Genesis Carletonite is a hydrothermal mineral. It forms in the late stages of crystallization within intrusions of alkaline rocks, particularly in xenoliths of hornfels and marble that have been metamorphosed by nepheline syenite. ## Mineral Associations This mineral co-occurs with a number of other rare minerals typical of the Mont Saint-Hilaire environment. The most common associations include: microcline, nepheline, arfvedsonite, aegirine, apophyllite, pectolite, leucosphenite, natrolite, and albite. ## Localities The only confirmed locality of carletonite in the world is the Mont Saint-Hilaire alkaline complex (specifically the Poudrette quarry) in the Montérégie region, Quebec, Canada. This is its type locality and, to date, the only one.

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized carletonite specimens are characterized by a deep, uniform, azure color. Large, massive blocks of intense color, suitable for cutting cabochons, fetch very high prices. Specimens with well-visible, though rare, crystals are sought after by collectors specializing in systematic minerals. Aesthetic associations with other minerals, such as white pectolite, also enhance their appeal. ## Market Prices Carletonite is one of the more expensive collector's minerals. Prices for small fragments (1-2 cm) of good color start at several hundred dollars. Larger, rich specimens can cost from several thousand to even tens of thousands of dollars, depending on quality, size, and aesthetics. Cut cabochons are extremely rare and expensive, and their price per carat is very high. ## Popular Localities All carletonite specimens available on the collector's market come from a single location - the Poudrette quarry on Mont Saint-Hilaire in Quebec, Canada. This is one of the most famous mineralogical localities in the world, known for over 400 different minerals, including many unique ones.

Care and storage

## Cleaning Carletonite should be cleaned with extreme caution only. It is best to use a soft brush to remove dust. If necessary, it can be quickly rinsed in distilled water and immediately thoroughly dried with a soft cloth. Ultrasonic and steam cleaners should be avoided. ## What to Avoid The mineral is sensitive to acids, which can damage it. Due to its low hardness, it should be protected from scratching by harder minerals (e.g., quartz). Avoid sudden temperature changes and prolonged exposure to strong sunlight, which theoretically could cause its intense color to fade. ## Storage Carletonite specimens are best stored in separate, padded boxes or display cases to avoid contact with other minerals and the risk of scratching. Due to its perfect cleavage, it is brittle and susceptible to mechanical damage, so it should be handled gently.

External references

Sources

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