|
 While
not always recognized as such, Banded Iron Formations (BIFs)
are another form of stromatolites. BIFs are massive, laterally
extensive and globally distributed chemical sediment deposits
that consist primarily of Fe-bearing minerals (iron oxides)
and silica. Iron can occur naturally in two states. Reduced,
or ferric iron is soluble in water. Archaean oceans were rich
in ferric iron that was released from Earth's interior. In
the presence of oxygen, however, the iron becomes oxidized
and precipitates out as a solid. Earth's BIFs are the result
of oxygen released by photosynthetic organisms combining with
dissolved iron in Earth's oceans to form insoluble iron oxides
-- this is known as "the great rusting of the earth".
Banding results from cyclicality in oxygen production. Earth
started out with vast amounts of iron dissolved in the world's
seas. BIFs in the geologic record date from about 3.8 billion
years ago in Greenland to about 1.8 billion years ago with
a maximum abundance at some 2.5 billion years ago, with another
large reoccurrence in Neoproterozoic time (from about 0.8 and
0.6 billion years ago. Interestingly, it has been estimated
that the amount of oxygen locked in earth's BIFs is some 10
times the amount contained in the atmosphere.
Some
of the most famous banded iron comes from the Western Australia,
and locally goes by the monikers Tiger Iron and Tiger Eye.
With fine polishing, the beauty is stunning testament to one
of
the most
important
events in geological history -- the Earth needed to first rust,
so that the atmosphere could then be oxygenated, setting the
stage for the appearance and evolution of all eukaryotic life.
Among the so-called Tiger Irons, that from the Marra Mamba
Formation is the most treasured, and is often used in the jewelry
trade. At 2.7 billion years of age in the Archaean, it is also
among the oldest. This Marra Mamba Tiger Eye comes from only
two small deposits in the Hammersley Ranges of Western Australia. |
