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• 2,69 Milliarden oder 2690 Millionen Jahre altes Eisenerz: typisch für ein Erz ist der schimmernde Metallglanz sowie die recht hohe Dichte von 5,1 bis 5,3 Gramm pro Kubikzentimeter. Zum Vergleich: Der schwere Basalt liegt bei etwa 3,0 g/cm³.
  

• Specularite from the Precambrian of Minnesota, USA.
  

• The Soudan Iron-Formation is a Neoarchean-aged banded iron formation (BIF) unit in the Vermilion Greenstone Belt of northeastern Minnesota, USA. Stratigraphically, the Soudan is considered to be the middle member of the Ely Greenstone. Metamorphosed pillow basalt successions occur above and below the iron-rich interval:
  

• upper member of the Ely Greenstone - Soudan Iron-Formation member - lower member of the Ely Greenstone
  

• Radiometric dating of Ely Greenstone rocks (see Jahn & Murthy, 1975) indicates that the Soudan Iron-Formation is about 2.69 billion years old.
  

• The Soudan Iron-Formation was deposited in a deep-water environment and is closely associated with several types of volcanic rocks formed in an ancient island arc setting. As such, the unit is classified as an Algoma-type iron-formation. Superior-type iron-formations were deposited in relatively shallow water, continental shelf settings.
  

• The Soudan Iron-Formation is exposed in & around the town of Soudan, Minnesota at roadcuts, glacially-eroded outcrop knobs, surface mines, and underground mines. The best exposures are at Soudan Underground Mine State Park. The Soudan Mine targeted high-grade iron ore (specularite) for many decades and closed in the early 1960s. The site is now accessible to the public and underground tours are offered.
  

• The rock shown above is specularite, which is a high-grade iron ore and the mining target at the Soudan Mine. Specularite is a finely-crystalline to coarsely-crystalline metamorphic rock dominated by silvery-gray specular hematite (Fe2O3 - iron oxide). "Specular" means "mirror" - it refers to the shiny, intensely lustrous appearance of each specular hematite crystal flake. Specularite occurs as small to large, irregular-shaped masses within the jaspilite facies of the Soudan Iron-Formation. Studies have shown that the specularite formed by metamorphic alteration of jaspilite (= quartz-rich, banded iron formation), resulting in the leaching away of quartz/silica.
  

• The reddish coloration is a thin, scattered, surficial coating of hematite dust.
  

• Stratigraphy: Soudan Iron-Formation member, Ely Greenstone, Vermilion Greenstone Belt, Neoarchean, ~2.69 Ga
  

• Locality: Soudan Mine, Soudan Underground Mine State Park, Soudan, northeastern Minnesota, USA
  

• Source: Specularite (high-grade iron ore) (Soudan Iron-Formation, Neoarchean, ~2.69 Ga; Soudan Mine, Soudan, Minnesota, USA) 9
  

Source

• Source: https://commons.wikimedia.org/wiki/File:Specularite_(high-grade_iron_ore)_(Soudan_Iron-Formation,_Neoarchean,_~2.69_Ga;_Soudan_Mine,_Soudan,_Minnesota,_USA)_9_(19026398982).jpg
  

• Created: June 10th, 2015
  

• Author: James St. John
  

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