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Have you ever wondered about the vibrant green mineral called Annabergite? This rare gem, named after Annaberg in Saxony, Germany, is a hydrous nickel arsenate with the chemical formula Ni3(AsO4)2·8H2O. Known for its bright apple-green color, Annabergite forms in the oxidation zones of nickel-cobalt deposits. It shares a close relationship with erythrite, its cobalt counterpart, and often appears alongside minerals like calcite, quartz, and silver. With a hardness of 1.5 to 2.5 on the Mohs scale, it’s relatively soft and exhibits perfect cleavage on the {010} plane. Annabergite’s unique properties and striking appearance make it a fascinating subject for mineral enthusiasts and geologists alike.
Key Takeaways:
- Annabergite is a rare, apple-green mineral with a unique chemical composition. It forms in the oxidation zone of nickel-cobalt deposits and is often found alongside minerals like calcite and limonite.
- Notable occurrences of annabergite include locations in Canada, Greece, the USA, Spain, and Germany. It is often associated with minerals like gersdorffite, nickeline, and quartz, making it a fascinating subject for earth-science enthusiasts.
What is Annabergite?
Annabergite is a rare mineral that has intrigued scientists and collectors alike. Its unique properties and vibrant colors make it a fascinating subject of study. Let's dive into some key facts about this mineral.
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Classification and Subclassification: Annabergite is classified as a phosphate, arsenate, vanadate mineral. It belongs to the subclass of hydrated arsenates.
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Chemical Composition: The chemical formula for annabergite is Ni3(AsO4)2·8H2O, indicating it is a hydrous nickel arsenate.
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Crystal System and Class: Annabergite crystallizes in the monoclinic system and belongs to the prismatic crystal class with the H-M symbol (2/m).
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Space Group and Unit Cell Dimensions: Its space group is I2/m. The unit cell dimensions are a = 10.179(2) Å, b = 13.309(3) Å, c = 4.725(1) Å, and β = 105(1)°.
Physical Properties of Annabergite
Understanding the physical properties of annabergite helps in identifying and studying the mineral. Here are some of its key characteristics.
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Hardness and Density: Annabergite has a hardness of 1.5 to 2.5 on the Mohs scale and a density of approximately 3.07 g/cm³.
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Cleavage and Fracture: The mineral exhibits perfect cleavage on the {010} plane and irregular fracture.
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Optical Properties: Annabergite is biaxial with refractive indices nα = 1.622, nβ = 1.658, and nγ = 1.687. It shows a birefringence of δ = 0.065.
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Color and Luster: Typically, annabergite displays a bright apple-green color. It can also appear pale green, pale rose, pale pink, white, or gray. Its luster ranges from vitreous to adamantine.
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Diaphaneity and Streak: The mineral is transparent to translucent and has a pale green to white streak.
Occurrence and Formation
Annabergite forms in specific geological environments and is often found in certain types of deposits. Here’s how it occurs and forms.
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Occurrence and Formation: Annabergite is a secondary mineral forming in the oxidation zone of nickel-cobalt deposits. It results from the oxidation of skutterudite and cobaltite.
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Relationship with Other Minerals: Annabergite is closely related to erythrite, its cobalt analogue. Both minerals are isostructural and form a continuous series where nickel and cobalt ions can substitute freely for each other.
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Synonyms and Name Origin: Named after Annaberg in Saxony, Germany, where it was discovered. It was previously known as "nickel bloom".
Detailed Physical Properties
Exploring the detailed physical properties of annabergite reveals more about its structure and appearance.
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Crystal Habit: Annabergite typically occurs as fibrous veinlets, crystalline crusts, or earthy masses. Large crystals are extremely rare.
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Cleavage: Perfect on {010}, indistinct on {100} and {102}.
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Tenacity: Sectile, meaning it can be scratched by other minerals.
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Luster: Subadamantine, pearly on cleavages, may be dull or earthy.
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Streak: Pale green to white.
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Diaphaneity: Transparent to translucent.
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Specific Gravity: Approximately 3.07 g/cm³.
Notable Occurrences
Annabergite has been discovered in various locations around the world. Here are some notable places where it can be found.
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Cobalt, Ontario, Canada: Known for its rich deposits of annabergite.
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Lavrion, Greece: Another significant location for finding this mineral.
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Humboldt, Nevada, USA: A notable site in the United States.
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Sierra Cabrera, Spain: Famous for its annabergite deposits.
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Germany: The country where annabergite was first discovered.
Associated Minerals
Annabergite is often found alongside other minerals, which can help in its identification.
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Calcite (CaCO3): Commonly associated with annabergite.
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Gersdorffite (NiAsS): Another mineral found with annabergite.
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Limonite: Often occurs in the same deposits.
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Nickeline (NiAs): Frequently found with annabergite.
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Silver (Ag): Can be associated with annabergite.
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Dolomite (CaMg(CO3)2): Another mineral found in the same locations.
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Quartz (SiO2): Commonly associated with annabergite.
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Smithsonite (ZnCO3): Often found alongside annabergite.
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Erythrite (Co3(AsO4)2·8H2O): Its cobalt analogue.
Varieties and Substitutions
Annabergite has some interesting varieties and substitutions that make it unique.
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Dudgeonite: A variety where some nickel is replaced by calcium, named after P. Dudgeon who found it in Creetown, Kirkcudbrightshire.
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Cabrerite: A variety where some nickel is replaced by magnesium, named for Sierra Cabrera in Spain.
Annabergite: A Mineral Worth Knowing
Annabergite's unique properties and vibrant colors make it a standout in the mineral world. With its bright apple-green hue, perfect cleavage, and fascinating formation process, it captures the interest of geologists and collectors alike. Found in various locations globally, from Canada to Greece, this mineral often forms in the oxidation zones of nickel-cobalt deposits. Its close relationship with erythrite and its historical significance, dating back to 1758, add to its allure. Whether you're intrigued by its chemical composition, physical properties, or geological environment, annabergite offers a wealth of information and beauty. Understanding these facts not only helps in identifying this rare mineral but also deepens appreciation for the natural processes that create such wonders. So next time you come across a bright green mineral, you might just be looking at a piece of annabergite.
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