Telluric iron, also called native iron, is iron that originated on Earth, and is found in a metallic form rather than as an ore. Telluric iron is extremely rare, with only one known major deposit in the world, located in Greenland.
Introduction With the exception of its molten core, nearly all elemental iron on Earth is found as iron ores. All metallic iron was thought to have been transformed into iron oxides during the Great Oxidation Event, beginning roughly 2 billion years ago, among other theories. Until the late 1800s, iron as a native metal was only a matter of speculation, outside of isolated Greenland. The only known terrestrial iron in metallic form was found as meteorites, which were deposited onto the Earth from outer space. Telluric iron is so named after the Latin word Tellus, meaning "Earth" (the planet, as opposed to terra meaning "earth": the land, ground or soil), combined with the suffix -ic meaning "of" or "born from", differentiating it from meteorites. Telluric iron resembles meteoric iron, in that it contains both a significant amount of nickel and Widmanstatten structures. However, telluric iron typically contains only around 3% nickel, which is too low for meteorites, of which none have been found with less than 5%. There are two types of telluric iron: Both type 1 and type 2 contain comparable amounts of nickel and other impurities. The main difference between the two is the carbon content, which greatly affects the hardness, workability, and melting point of the metal.
Material properties Telluric iron is metallic iron that formed within the Earth's mantle and crust. Although minor deposits of telluric iron have been found around the world, the west shores of Greenland hold the only known major deposits. However, these deposits may vary drastically in shape and composition, even in the same region, as well as drastic variations between different regions such as Uivfaq, Asuk, Blaafjeld, and Mellemfjord. The common factor is that all Greenlandic deposits tend to be found in dikes (lava-filled fractures in the bedrock) or extrusions where molten rock was able to flow out onto the surface. Another commonality is that all deposits are found in association with graphite-rich feldspar, likely contributing to the high carbon-content and low oxide presence in the metal, although it is unknown if the metal managed to escape being oxidized with the rest of Earth's iron, or if it began as beds of ore and coal that subducted and then were naturally smelted in the lava due to the reducing environment provided by the carbon-rich, graphitic feldspar. Telluric iron in Greenland is unique, in that it can be found in nearly all phases of iron-carbon alloys, and with drastically varying crystalline structures. In some rock it is found mixed with basalt as very small grains with sharp corners and irregular shapes, whereas in others the small, grain-sized droplets in the molten magma were able to coalesce into larger, pea-sized droplets that crystallized with a mostly spherical or oblong shape. Still in others the dike or extrusion may be made almost entirely out of very high-carbon cast-iron, which could more easily coalesce within the magma and flow into cracks due to its lower viscosity and melting point. This cast iron is often crusted with or contains inclusions of basalt, as it extruded out of the ground as very large, globular masses within the lava, out of which large boulders formed due to natural erosion of the surrounding basalt. Telluric iron is largely divided into two groups, depending on the carbon content. Type 1 is a cast-iron typically containing over 2.0% carbon, while type 2 ranges somewhere between wrought iron and a eutectoid steel. Both types tend to handle weathering in the elements very well, but tend to decompose and crumble very quickly in the dry, controlled atmosphere of a museum, although type 2 is far more prone to this kind of damage.
Type 1 Type 1 telluric iron contains a significant amount of carbon. Type 1 is a white nickel cast-iron, containing 1.7 to 4% carbon and 0.05 to 4% nickel, which is very hard and brittle and does not respond well to cold working. The structure of type 1 consists mainly of pearlite and cementite or cohenite, with inclusions of troilite and silicate. The individual ferrite grains are typically about a millimeter in size. Although the composition of the grains may vary, even within the same grain, they are mostly composed of fairly pure nickel-ferrite. The ferrite grains are connected with cementite laminations; typically 5–25 micrometers thick; forming the pearlite. Type 1 is found as massive extrusions or very large boulders, typically ranging from a few tons to tens of tons. The metal could not be cold worked by the ancient Inuit (the local inhabitants of Greenland), and proves extremely difficult to machine even with modern tools. Machining of type 1 is possibly best accomplished with a carborundum wheel and water cooling. However type 1 was possibly used as hammer and anvil stones by the Inuit. When sawed in half, boulders of type 1 tend to have a thick shell of cast-iron on the outside that can barely be broken with pneumatic jackhammers, but inside a much more brittle construction of iron grains in an almost powdery form, sintered together to form a porous, sponge-iron type of material that pulverizes at the strike of a hammer.
… excerpt ends here. Continue reading the full article.
