Lalondeite

Chemical formula: (Na,Ca)<sub>6</sub>(Ca,Na)<sub>3</sub>Si<sub>16</sub>O<sub>38</sub>(F,OH)<sub>2</sub>·3H<sub>2</sub>O

An extremely rare sodium calcium silicate, forming colorless, tabular crystals in rosettes, known exclusively from Mont Saint-Hilaire, Canada.

## Characteristics Lalondeite is a very rare, complex, hydrated sodium calcium fluoride silicate. It forms characteristic, thin, tabular crystals that are colorless or white. These crystals often arrange themselves into rosette-like or fan-shaped aggregates, reaching up to 1 mm in size. The mineral is transparent and exhibits a vitreous luster on crystal faces. ## Physical Properties The mineral's luster is vitreous, and its crystals are fully transparent. The calculated density is 2.46 g/cm³. Due to its structure and chemical composition, it is a brittle and delicate mineral. Hardness has not been precisely determined, but it can be assumed to be low. ## Colors and Varieties Lalondeite occurs as a colorless or white mineral. No colored varieties or trade names are known. ## History and Name The mineral is named in honor of Dr. André E. Lalonde (born 1955), a professor of mineralogy at the University of Ottawa, in recognition of his contributions to the study of the mineralogy of the famous Mont Saint-Hilaire locality in Quebec, Canada. It was described as a new mineral species from this very location. ## Uses Due to its extreme rarity and microscopic crystal sizes, lalondeite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for specialized collectors of rare minerals and micromounts.

Properties

Luster
Vitreous
Streak
White
Density
2.46
Cleavage
Perfect on {010}, good on {100} and {001}
Transparency
Transparent
Crystal system
Triclinic

Diagnostic features

## Identification A characteristic feature of lalondeite is its rosette-shaped aggregates, composed of very thin, colorless, tabular crystals. A key diagnostic clue is its origin from Mont Saint-Hilaire and its co-occurrence with other rare alkaline minerals. Positive identification is only possible using advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS). ## Distinguishing from Similar Minerals Lalondeite can be confused with other colorless or white silicates found at Mont Saint-Hilaire, such as catapleiite, natrolite, or colorless forms of serandite. It is distinguished by its specific crystal habit – thin plates forming rosettes – however, visual differentiation may be impossible without specialized equipment. ## Crystal Forms Crystals are thin, tabular, flattened parallel to {010}. They typically form fan-shaped, divergent, or fully developed, rosette-like aggregates.

Geological environment

## Genesis Lalondeite forms in a very specific geological environment. It occurs in pegmatitic segregations within nepheline syenite xenoliths. These xenoliths are embedded in an intrusive, alkaline gabbro-syenite complex. ## Mineral Associations This mineral co-occurs with a rich assemblage of other rare alkaline minerals, such as aegirine, albite, analcime, catapleiite, eudialyte, microcline, natrolite, nepheline, serandite, and sodalite. ## Localities The only confirmed locality for lalondeite in the world is Mont Saint-Hilaire, Montérégie region, Quebec province, Canada. This is the type locality for this mineral.

Rarity

Extremely rare

For collectors

## Quality Criteria The collector's value of lalondeite specimens depends on several factors. Well-formed, sharp, and undamaged rosettes of the largest possible diameter are most highly prized. Transparency, vitreous luster, and aesthetic placement on the rock matrix, especially in association with other rare, color-contrasting minerals, are also important. ## Popular Localities As this mineral is known from only one location in the world, all collectible specimens come from Mont Saint-Hilaire, Canada. This locality is a mecca for mineral collectors and is famous for the occurrence of hundreds of unique species.

Care and storage

## Cleaning Lalondeite specimens are extremely delicate and brittle. For dust removal, it is best to use compressed air or a very soft brush (e.g., squirrel hair). If necessary, very brief rinsing in distilled water is permissible, followed by thorough drying. Ultrasonic cleaners should be strictly avoided. ## What to Avoid The mineral is sensitive to mechanical shocks due to its perfect cleavage. Contact with acids and other chemicals should be avoided. Heating can lead to dehydration and destruction of the crystal structure. It should be protected from sudden temperature changes. ## Storage It is recommended to store specimens exclusively in specialized "micromount" boxes, which protect them from mechanical damage, dust, and moisture. Exposure should be away from heat sources and direct sunlight.

Sources

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