Hydrodresserite

Chemical formula: BaAl<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>4</sub>·3H<sub>2</sub>O

A rare barium and aluminum carbonate, forming spherical aggregates of white, acicular crystals, found in alkaline syenite intrusions.

## Characteristics Hydrous dresserite is a rare hydrated barium and aluminum carbonate. It forms characteristic, small spherical or hemispherical aggregates, reaching up to 4 mm in diameter. These are composed of fine, acicular or bladed crystals with a radial arrangement. Individual crystals are usually microscopic in size. Due to their delicate structure, specimens are very brittle. ## Physical Properties This mineral is characterized by low hardness, approximately 3.5-4 on the Mohs scale. It has a silky or pearly luster on the surfaces of aggregates, while individual crystals exhibit a vitreous luster. It is translucent to transparent in the case of single crystals. Its density is approximately 2.85 g/cm³. ## Colors and Varieties Hydrous dresserite is usually colorless or white. No colored varieties or trade names are distinguished for it. ## History and Name The mineral was discovered in the Francon Quarry in Montreal (Quebec, Canada) and described in 1977 by J.L. Jambor, D.G. Fong, and A.C. Roberts. Its name honors Dr. John Alexander Dresser (1866-1954), a Canadian geologist who made significant contributions to the study of the geology of the Mont Saint-Hilaire intrusion and other alkaline rocks in Quebec. The name also refers to its chemical composition (hydroxyl group and water). ## Uses Due to its rarity, small size, and delicacy, hydrous dresserite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and micromounts.

Properties

Mohs hardness
3.5-4
Luster
Silky
Streak
White
Density
2.85
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Triclinic

Diagnostic features

## Identification A characteristic feature of hydrous dresserite is its small, white, spherical aggregates with a radial, fibrous internal structure. It reacts violently with hydrochloric acid, which is a key diagnostic feature for carbonates. Its occurrence is limited to very specific geological environments. ## Distinguishing from Similar Minerals It can be confused with other rare carbonates from the same locality, such as dresserite, strontiodresserite, or weloganite. Dresserite has a very similar appearance, but hydrous dresserite contains hydroxyl groups and water, which can only be confirmed by advanced analytical methods (e.g., XRD, EDS). It is distinguished from calcite and aragonite by its lower hardness and specific mode of occurrence. ## Crystal Forms Crystals are in the form of elongated, flattened blades or needles. They almost always occur as radial or spherulitic aggregates.

Geological environment

## Genesis Hydrous dresserite is a secondary mineral, formed in the late stages of hydrothermal processes within vugs and fractures in nepheline syenites and carbonatite dikes, which are part of complex alkaline intrusions. ## Mineral Associations This mineral co-occurs with other rare minerals found in the Francon Quarry, such as weloganite, dresserite, strontiodresserite, quartz, dawsonite, as well as calcite and analcime. ## Localities The most important and classic locality for hydrous dresserite is the Francon Quarry in Montreal, Quebec, Canada. This is the type locality for this mineral. It has also been identified in the Poudrette intrusive complex (Mont Saint-Hilaire) in Quebec and in the Khibiny Massif on the Kola Peninsula in Russia.

Rarity

Very rare

For collectors

## Quality Criteria The most prized hydrous dresserite specimens are those with well-formed, spherical aggregates with a distinct, radial structure, deposited on a contrasting rock matrix. Due to the microscopic nature of the crystals, the aesthetics of the entire aggregate and its intact condition are crucial. The size of the spherulite (the largest reach several millimeters) and its association with other rare minerals from the given locality significantly increase the specimen's value. ## Popular Localities Undoubtedly, the most sought-after and classic specimens come from the type locality – the now-closed Francon Quarry in Montreal (Canada). Specimens from Mont Saint-Hilaire are also valued by collectors specializing in minerals from this famous locality.

Care and storage

## Cleaning Hydrous dresserite specimens are extremely delicate and brittle. Mechanical cleaning, including the use of brushes, should be avoided. If absolutely necessary, the specimen can be carefully rinsed in distilled water and then left to air dry completely at room temperature, without the use of heat sources. ## What to Avoid The mineral is sensitive to acids, which cause it to effervesce violently and decompose. Contact with all chemicals should be avoided. It is also sensitive to high temperatures, which can lead to dehydration and destruction of its structure. It should not be exposed to shocks and impacts. ## Storage It is recommended to store specimens exclusively in specialized "micromount" boxes, which protect them from mechanical damage, dust, and sudden changes in humidity. Avoid exposure to direct sunlight and store in stable room conditions.

Sources

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