Macdonaldite

Chemical formula: BaCa<sub>4</sub>Si<sub>16</sub>O<sub>36</sub>(OH)<sub>2</sub>·10H<sub>2</sub>O

Macdonaldite is a rare, colorless or white mineral from the silicate group, forming radial aggregates of tiny, acicular crystals.

## Characteristics Macdonaldite is a hydrated barium calcium silicate, typically occurring as tiny, acicular or bladed crystals. These crystals often form radial, spherical, or hemispherical aggregates, which can reach several millimeters in diameter. Due to the small size of the crystals, individual features are difficult to observe with the naked eye. The mineral is usually colorless to white and has a vitreous or pearly luster, especially visible on the surfaces of aggregates. ## Physical Properties Macdonaldite has a Mohs hardness of approximately 3.5 to 4. It is relatively light, with a density of about 2.21 g/cm³. The crystals are brittle and exhibit perfect cleavage in one direction. The mineral is typically translucent. ## Colors and Varieties This mineral is usually colorless or white. No named color varieties or commercial varieties are distinguished. ## History and Name Macdonaldite was discovered in 1964 in samples from the Dallas Gem Mine quarry in San Benito County, California, USA. It was named in honor of Gordon Andrew Macdonald (1911–1978), an American volcanologist and professor at the University of California, in recognition of his contributions to geological research, including the rocks in which the new mineral was found. It was described by J. T. Alfors, M. C. Stinson, and R. A. Matthews in 1965. ## Uses Macdonaldite has no industrial significance. Its occurrence is limited, and specimens are valued exclusively by collectors of rare minerals and micromounts.

Properties

Mohs hardness
3.5-4
Luster
Vitreous, Pearly
Streak
White
Density
2.21
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification A characteristic feature of macdonaldite is its radial aggregates of very fine, acicular or bladed crystals, white in color with a pearly or vitreous luster. It occurs in a specific geological environment, in veins cutting glaucophane schists. ## Distinguishing from Similar Minerals It can be confused with other white zeolite-like minerals or silicates that form radial aggregates. Identification often requires advanced methods, such as X-ray diffraction (XRD), due to the small crystal size and visual similarity to other secondary minerals. ## Crystal Forms It forms tiny, acicular or bladed crystals, which almost always occur as radial, spherulitic aggregates. Single, well-formed crystals are extremely rare and microscopic in size.

Geological environment

## Genesis Macdonaldite is a secondary mineral, formed by low-temperature hydrothermal processes. It forms in veins and fractures cutting metamorphic rocks, such as glaucophane schists. Its formation is associated with the circulation of barium- and calcium-rich solutions within these rocks. ## Mineral Associations This mineral co-occurs with other rare minerals, such as sanbornite, quartz, celsian, cymrite, pabulite, fresnoite, and titanite. ## Localities The most important and historical occurrence of macdonaldite is its type locality – the Dallas Gem Mine quarry (also known as the Sanbornite-deposit) in San Benito County, California, USA. This is the only confirmed and well-documented locality for this mineral in the world.

Rarity

Very rare

For collectors

## Quality Criteria The most desirable specimens for collectors are those with well-formed, distinct radial aggregates, contrasting with the dark host rock (schist). The size and completeness of the spherulitic aggregates, as well as their abundance on the matrix, significantly increase the value of the specimen. Purity (lack of damage) is crucial due to the mineral's delicacy. ## Popular Localities The only source of collector specimens is the historical type locality in San Benito County, California. All specimens available on the market come from this single location, which makes them particularly interesting for collectors specializing in minerals from specific localities ("locality collectors").

Care and storage

## Cleaning Due to its brittleness and the delicate structure of its aggregates, mechanical cleaning is not recommended. If necessary, compressed air can be used to remove loose dust particles. Contact with water, especially deionized water, may be risky due to potential reactivity. ## What to Avoid Avoid contact with acids, which can damage the mineral. Due to its brittleness, it should be protected from impacts, shocks, and scratching. It should not be heated or exposed to sudden temperature changes. ## Storage Macdonaldite specimens, especially those in the form of delicate aggregates, are best stored in closed "micromount" boxes, which protect them from dust and mechanical damage. Avoid storing in areas with high humidity.

External references

Sources

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