Mcguinnessite

Chemical formula: Cu²⁺MgCO₃(OH)₂

Mcguinnessite is a rare, secondary copper and magnesium mineral, forming characteristic blue-green coatings and rosette aggregates.

## Characteristics Mcguinnessite is a hydrated copper and magnesium carbonate, belonging to the rosasite group. It occurs as very small, platy or acicular crystals, which most often form spherical or rosette aggregates, as well as thin coatings and incrustations on the host rock. Due to the small size of the crystals, its features are difficult to observe with the naked eye. ## Physical Properties This mineral is characterized by low hardness, measuring 2.5 on the Mohs scale, which makes it very brittle and susceptible to mechanical damage. It has a density of approximately 3.02 g/cm³. It is translucent, and its luster is described as silky or dull, especially in the case of compact aggregates. ## Colors and Varieties Mcguinnessite ranges in color from light blue to blue-green. Color zoning is observed in it – parts richer in copper (crystal cores) tend to be darker, more blue, while magnesium-rich zones (outer parts) are lighter, pale blue-green, even almost white. ## History and Name The mineral was first described in 1981 by Donald C. Erd, R.C. Erd, William S. Wise, and Charles W. Chesterman. Its name commemorates the American amateur mineralogist and collector, Albert L. McGuinness (1904-1979), who first found this mineral. ## Uses Mcguinnessite has no industrial application. It is solely a mineral of scientific and collector's interest, sought after for its rarity and visually attractive forms of occurrence.

Properties

Mohs hardness
2.5
Luster
Silky
Streak
White
Density
3.02
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Mcguinnessite is most easily recognized by its characteristic form of occurrence – as small, spherical or rosette aggregates with a silky luster. Its light blue or blue-green color is also a diagnostic feature. It reacts vigorously with acids, which is typical for carbonates. ## Distinguishing from Similar Minerals Mcguinnessite can be confused with other secondary copper minerals, such as aurichalcite, rosasite, or malachite. It usually differs from aurichalcite by a more blue hue and the absence of pearly luster. Distinguishing it from rosasite, to which it is closely related, can be very difficult and often requires advanced chemical analysis (mcguinnessite contains magnesium, while rosasite contains zinc). Malachite has an intensely green color, whereas mcguinnessite is blue-green or blue. ## Crystal Forms It forms very fine, acicular or platy crystals, which combine into spherical, hemispherical, or rosette aggregates. It also occurs as thin coatings and incrustations.

Geological environment

## Genesis Mcguinnessite is a secondary mineral, forming in the oxidation zone (weathering) of copper ore deposits. It forms as a result of surface waters acting on primary minerals in ultramafic rocks, such as serpentinites, which provide the necessary magnesium. ## Mineral Associations It most often co-occurs with other secondary copper and magnesium minerals. Typical associated minerals include chrysocolla, malachite, azurite, brochantite, as well as minerals from the serpentine group (e.g., antigorite, chrysotile) and pokrovskite. ## Localities The most important and type locality is the Copper King Mine in Mendocino County, California (USA), from which the best-known specimens originate. It has also been reported in several other localities worldwide, including Japan (Ohmine mine, Okayama Prefecture) and Italy (Liguria), but these occurrences are of much lesser significance.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized mcguinnessite specimens are characterized by well-formed, spherical or rosette aggregates of intense, blue-green color. Contrast with the host rock color and the absence of damage to delicate structures are important. The size of the aggregates and the abundance of occurrence on the specimen significantly increase its collector's value. ## Popular Localities Specimens from the type locality – the Copper King Mine in California, USA – are by far the most sought after by collectors. Material from this location is considered the global standard for this mineral.

Care and storage

## Cleaning Mcguinnessite specimens should be cleaned with the utmost care, exclusively using compressed air to remove dust. Due to its reactivity with acids and low hardness, the use of water, and especially any chemical agents, is discouraged. ## What to Avoid Contact with acids must be absolutely avoided, as they cause its rapid dissolution with the release of carbon dioxide. The mineral is very soft and brittle, so it must be protected from impacts, scratching, and vibrations. It should not be exposed to moisture or extreme temperatures. ## Storage It is recommended to store specimens in closed, stable containers (e.g., "membrane box" type or in boxes with soft lining) to protect them from mechanical damage, dust, and humidity changes. Direct exposure to sunlight should be avoided.

External references

Sources

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