Sheldrickite
Chemical formula: NaCa<sub>3</sub>(CO<sub>3</sub>)<sub>2</sub>F<sub>3</sub>·H<sub>2</sub>O
Sheldrickite is a very rare mineral from the carbonate group, forming characteristic, spherical aggregates of acicular crystals.
Description
## Characteristics Sheldrickite is a hydrated fluoride and carbonate of sodium and calcium, occurring extremely rarely. It forms very small, acicular or prismatic crystals, which almost always occur as radial or spherical aggregates, reaching up to several millimeters in diameter. Individual crystals are usually colorless or white and transparent, and their clusters have a silky or vitreous luster. Due to its habit and small size, sheldrickite specimens are most often appreciated by collectors specializing in microminerals. ## Physical properties This mineral is characterized by relatively low hardness, approximately 3.5 on the Mohs scale, which makes it susceptible to scratching. It has a vitreous luster, and on fracture surfaces, it can be slightly silky. It is transparent to translucent. Its calculated density is approximately 2.85 g/cm³. ## Colors and varieties Sheldrickite is a colorless or white mineral. No colored varieties or trade names are known. ## History and name The mineral is named in honor of Professor George M. Sheldrick (born 1942), a British chemist and crystallographer, author of widely used computer programs for solving crystal structures. The mineral was approved by the International Mineralogical Association (IMA) in 1999, and its discovery was made at the Poudrette quarry in Mont Saint-Hilaire, Quebec, Canada. ## Applications Sheldrickite has no industrial application. It is solely an object of scientific and collecting interest as a very rare mineral species with a specific composition and habit.
Diagnostic features
## Identification The key diagnostic feature of sheldrickite is its unique form of occurrence - small, white or colorless spherical aggregates composed of radially arranged, acicular crystals. The locality (Mont Saint-Hilaire) is also a strong indicator. Crystals exhibit hemimorphism (they have different terminations at opposite ends of the polar axis), which is a feature observable under high magnification. ## Distinguishing from similar minerals Sheldrickite can be confused with other white, acicular minerals found in Mont Saint-Hilaire, such as dawsonite, natrolite, or some zeolites. Dawsonite is also a carbonate and reacts with acid, but it forms fibrous aggregates, not such regular spherulites. Natrolite forms slender, prismatic crystals with a square cross-section. Certain differentiation often requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal forms Sheldrickite crystallizes in the orthorhombic system. It forms acicular to prismatic, hemimorphic crystals, which almost always aggregate into radial, spherical, or hemispherical groups.
Geological environment
## Genesis Sheldrickite is a hydrothermal mineral, crystallizing in the late stage of magmatic evolution. It forms in vugs (miaroles) and fissures of nepheline-syenite pegmatites, which are part of the complex alkaline massif of Mont Saint-Hilaire. ## Mineral associations This mineral coexists with many other, often rare minerals typical of this locality. The most common associations include microcline, aegirine, analcime, catapleiite, dawsonite, natrolite, and ancylite-(Ce). ## Localities The only confirmed locality of sheldrickite in the world is its type locality - the Poudrette quarry, Mont Saint-Hilaire, Montérégie region, Quebec, Canada. This is one of the most famous mineralogical sites in the world, known for over 400 different mineral species.
Rarity
Very rare
Collector aspects
## Quality criteria The collecting appeal of sheldrickite is determined by several factors. Specimens with well-formed, sharp, and clearly defined spherical aggregates are most highly valued. The size of these aggregates is important - the larger they are, the more valuable the specimen. The contrast with the rock matrix and the presence of other rare associated minerals are also significant. Specimens rich in numerous, undamaged spherical clusters achieve the highest value. ## Popular localities As this mineral is known from only one place in the world, all collector specimens come from the Poudrette quarry in Mont Saint-Hilaire, Canada. This locality is currently inaccessible to collectors, which further increases the rarity and value of material acquired in the past.
Care and storage
## Cleaning Sheldrickite specimens are very delicate and should be cleaned with the utmost care. The safest method is to use a soft brush to remove dust. Compressed air can be used from a safe distance. Due to the presence of water in the structure and possible reactivity, prolonged contact with water should be avoided, and if necessary, only distilled water should be used, and the specimen should be dried immediately. ## What to avoid Contact with acids, which will cause rapid dissolution of the mineral with the release of carbon dioxide, must be absolutely avoided. The mineral is sensitive to high temperatures, which can lead to its dehydration and destruction. It should be protected from impacts and scratching due to its low hardness and perfect cleavage. ## Storage It is recommended to store sheldrickite specimens in closed "micromount" boxes, which protect them from dust, moisture, and mechanical damage. Avoid exposure to direct sunlight and sudden temperature changes.