IRON
Iron is quite rare in its elemental state at the
Earth's surface. In the presence of oxygen, iron turns to rust (iron
oxide). Many meteorites that fall to Earth from the asteroid belt are
composed of native iron (plus impurities). Very few localities on Earth
have native iron of terrestrial origin. These occurrences are
along natural smelting zones, where magma or lava has come in direct contact
with coals or other carbonaceous sedimentary rocks. In such smelting
zones, reducing conditions are created, and metallic iron can crystallize.
Famous localities where this has happened include Disko Island in Greenland and
the Siberian Traps Flood Basalt Province.
Iron has a metallic luster, silvery-gray color, is
somewhat hard (H = 4 to 4.5), is strongly magnetic, has no cleavage, is
malleable, and is moderately heavy for its size. Native iron is always
alloyed with nickel in meteorites. In terrestrial iron occurrences, the
Fe is also alloyed with a little Ni.
Native iron in basalt (cut surface, 1.5 cm across) from the Putoran Plateau
of Siberia. Vesicle-filling iron (silver) in basalt/dolerite (black -
contains labradorite plagioclase feldspar & bytownite plagioclase feldspar
& pyroxene). Available chemical information indicates that the metal
is about 94% Fe, 4% Ni, plus impurities.
The Siberian rock shown above formed during the largest
outpouring of flood basalt in Earth’s history. The Siberian Traps Flood
Basalts erupted through crustal fissures as the large head of a new hotspot
(mantle plume) surfaced. This happened at Permian-Triassic boundary times
(~251 m.y.), and the timing suggests that Siberian Traps volcanism is related
to the Permian-Triassic mass extinction in some way.
The Late Paleozoic sedimentary succession of Siberia
contains coal beds (as do many Late Paleozoic successions on Earth). The
magmas came into direct contact with the coal beds, forming reducing
conditions, and allowed the formation of native iron.