Telluric iron

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Iron, native iron or telluric iron
Iron-136213.jpg
Sawed slab of basalt with bright, metallic native iron inclusions from Uivfaq, Disko Island (size: 7.8 x 3.5 x 0.6 cm)
General
Category Native element mineral
Formula
(repeating unit)
Fe
Strunz classification 1.AE.05
Dana classification 1.1.17.1
Crystal symmetry Isometric hexoctahedral
H-M symbol: (4/m32/m)
Space group: I m3m
Unit cell a = 2.8664 Å; Z=2
Identification
Color Steel-gray to iron-black, white in polished section
Crystal habit Massive, as interstitial blebs, rare as crystals
Crystal system Cubic
Twinning On {111} and on {112}
Cleavage {001}; with parting on {112}
Fracture Hackly
Tenacity Malleable
Mohs scale hardness 4
Luster Metallic
Diaphaneity Opaque
Specific gravity 7.3–7.87
References [1][2][3]

Telluric iron, also called native iron, is iron that originated on Earth, but 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.

Material properties[edit]

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. 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.

Type 1[edit]

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 to 25 micrometers thick; forming the pearlite.

Type 1 is found as very large boulders, typically ranging from a few tons to tens of tons. The metal could not be cold worked by the ancient Inuit people, (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.

Type 2[edit]

Type 2 telluric iron also contains around 0.05 to 4% nickel, but typically less than 0.7% carbon. Type 2 is a malleable nickel-iron which responds well to cold working. The carbon and nickel content have a great effect on the final hardness of the cold-worked piece.

Type 2 is found as small grains mixed within basalt rock. The grains are usually 1 to 5 millimeters in diameter. The grains are usually found individually, separated by the basalt, although they are sometimes sintered together to form larger aggregates. The larger pieces also contain small amounts of cohenite, ilmenite, pearlite and troilite. Type 2 was used by the Inuit to make items such as knives and ulus. The basalt was usually crushed in order to release the pea-sized grains, which were them hammered into discs about the size of coins. These flat discs were usually inserted into bone handles so that they slightly overlapped each other, forming an edge that resembled a combination of a knife and a saw.[4][5]

History[edit]

Aside from a very small deposit of telluric iron in Kassel, Germany, which has now been depleted, and a few other minor deposits from around the world, the only known major deposits exist in and nearby the area of Disko Bay, in Greenland. Found in the volcanic plains of basalt rock, the material was used by the local Inuit to make cutting edges for tools like knives and ulus. The Inuit were the only people to make practical use of telluric iron.

In the late 1840s, Adolf Erik Nordenskiöld discovered large boulders of iron near the Disko Bay area of Greenland. Knowing that the Inuit had made tools from the Cape York meteorite, Nordenskiöld assumed that the metal was of meteoric origin, since both contain significant amounts of nickel and both had Widmanstatten structures. The existence of telluric iron was doubted by most scientists at the time, and few had reason to question Nordenskiöld's finding. In 1871, on his second expedititon to Greenland, Nordenskiöld collected three large samples of telluric iron, still believing them to be meteoric, and brought them back to Europe for further study. These samples can be found currently in Sweden, Finland and Denmark. A 25 ton block now rests outside of the Riksmuseum in Stockholm, a 6.6 ton block outside the Geological Museum in Copenhagen, and a 3 ton block can be found in Kaisaniemi Park in Helsinki.

Accompanying Nordenskiöld in 1871 was K. J. V. Steenstrup. Due to circumstances like the shape of the boulders, which often had sharp corners or jagged edges that are not characteristic of meteorites (which ablate considerably during atmospheric entry), Steenstrup disagreed with Nordenskiöld about the origin of the boulders, and set out on an expedition of his own in 1878. In 1879, Steenstrup first identified the type 2 iron, showing that it also contained Widmanstatten structures. Steenstrup later wrote about his finding,

In the autumn of 1879, I made a discovery in connection with this matter, for in an old grave at Ekaluit ... I found 9 pieces of basalt containing round balls and irregular pieces of metallic iron. These pieces were lying together with bone knives, similar to those brought home by [Sir John] Ross, as well as with the usual stone tools ... whereas the 9 pieces of basalt with the iron balls were evidently the material for the bone knives. This iron is soft and keeps well in the air, from which reason it is fit for use in the manner described by Ross. The rock in which the iron appears is a typical, large-grained felspar-basalt, and the discovery has a double significance, firstly, because it is the first time we have seen the material out of which the Esquimaux [Eskimo] made artificial knives, and secondly, because it showed that they have used telluric iron for that purpose."

After the discovery in the grave, Steenstrup found many large outcrops of ferriferous basalt, containing the type 2 iron. Since the type 2 was located within volcanic basalt, Steenstrup was able to show that the iron was of terrestrial, or telluric, origin. In his treatise, Steenstrup added,

This peculiar layer of basalt is filled from top to bottom with iron-grains of all sizes from a fraction of a millimeter to a length of 18 mm. with a breadth of 14 mm., which is the greatest I have found.... When polished, this iron shows beautiful Widmannstatten figures.... Metallic nickel-iron with Widmannstatten figures has now been proved to be also a telluric mineral, and the presence of nickel together with a certain crystalline structure are consequently not sufficient to give the character of meteorites to loose iron blocks.

Steenstrup’s findings were later confirmed by meteorite expert J. Lawrence Smith in 1879, and then by Joh Lorenzen in 1882. The extremely rare telluric iron has been studied ever since.[4][5][6]

Occurrence[edit]

Native iron from basalt quarry at Bühl, Weimar, Kassel, North Hesse, Hesse, Germany (size: 6.6 x 5.9 x 1.8 cm)

In addition to the Disko Island deposit native iron has been reported from Fortune Bay, Mellemfjord, Asuk, and other locations along Greenland's west coast. Other locations include:[1]

Native nickel-iron alloys with Ni3Fe to Ni2Fe occur as placer deposits derived from ultramafic rocks. Awaruite was described in 1885 from New Zealand.

References[edit]

  1. ^ a b Handbook of Mineralogy
  2. ^ Native iron on Mindat
  3. ^ Iron on Webmineral
  4. ^ a b Iron and steel in ancient times By Vagn Fabritius Buchwald - Det Kongelige Danske Videnskabernes Selskab 2005 Page 35-37
  5. ^ a b Meteoritic Iron, Telluric Iron and Wrought Iron in Greenland By Vagn Fabritius Buchwald and Gert Mosdal - Kommissionen for videnskabelige Undersogelser i Gronland 1985 Page 19-23
  6. ^ The Mineralogical magazine and journal of the Mineralogical Society, Volume 6 By Mineralogical Society (Great Britain), KJV Steenstrup, J. Lorenzen - Wessrs, Williams, and Straham 1882 Page 1--38
  7. ^ a b c d e f g Palache, Charles, Harry Berman and Clifford Frondel, Dana's System of Mineralogy, Vol. 1, pp.114-118, Wiley, 1944 ed.
  8. ^ Hatrurim Fm on Mindat.org