The Rock - Banded Iron Formation
Banded Iron Formation (also known as BIF, or as taconite in North America) is a term that is applied to a very unique sedimentary rock of biochemical origin. These rocks are unique in their make up, unique in their age and unique in their origins. They are found all over the world, but only in certain areas of all the major continents. Every continent has a BIF formation. BIF consists of alternating layers of iron oxides and shale, chert, tiger eye or jasper. The alternating layers are generally only a few centimeters thick although the formations themselves can be massively thick. The iron oxide layers are generally composed of the minerals magnetite and hematite but other rarer iron oxides are also found in these formations. The black to gray to silver colored iron oxide layers contrast with the iron rich chert, jasper and shales which are generally red in color. The tiger eye versions of BIF are the result of low grade metamorphism creating veins of fibrous or asbestos riebeckite. Often these asbestos type crystals are replaced by the mineral quartz. The formerly fibrous nature of the crystals causes the play of light that is known as Tiger's Eye and it is quite attractive and used in jewelry.
Banded Iron Formations are thought to have formed from the precipitation
of iron from the Earth's ancient oceans. Photosynthetic bacteria produced,
for perhaps the first time in the young Earth's oceans, free oxygen which
oxidized the dissolved iron that existed abundantly at the time. Oxidized
iron is not soluble in water and thus it would precipitate out of the oceans
and onto the muddy sea floor. For reasons largely unknown, this was a periodic process resulting
in the alternating bands of iron oxide and shale. The periodic process might
have been due to seasonal fluctuations or storm surges or other hypothesis.
Whatever the reason, there never seemed to be a time when the iron layer
formation or the shale (mud) formation persisted long enough to produce a
layer thicker than 10 centimeters or so. Since the origin of the iron layer is derived from a living organism,
the photosynthetic bacteria, BIF is actually a fossil. Fossils do not need
to be the direct evidence of an organism such as a dinosaur bone or a trilobite.
BIF actually qualifies as a trace fossil. Some of the oldest fossils known
to man just predate banded iron formations. Bacteria are believed to be
the earliest life forms on Earth and eventually the oxygen producing varieties
formed the BIF and helped transform the Earth. BIF is the proof of this transformation when it was under way.
The conditions to form BIF, dissolved iron and episodic oxygenation, existed
early in Earth's history and then once the Earth's oxygen levels stabilized
the conditions for banded iron formation all but ceased to exist. When polished, BIF can be very beautiful. The red jasper or Tiger
Eye makes a wonderful compliment to the sparkling silver gray of the hematite.
The banded layers, sometimes contorted by ages of folding and faulting, make
for surreal landscapes of asymmetric bands. BIF can be used for many ornamental
purposes from bookends to clock faces to gravestones and monuments. Its
only major drawback is its significant weight. Iron is not light. But the
solidification of jasper and compaction of the stone make it very durable
and capable of being processed into relatively thin slabs. This keeps the
weight to usable levels and allows BIF to be used as a popular ornamental
stone. The popular polished Tiger Eye stones that are sold in rock shops
around the world are usually derived from banded iron formations. As an ore of iron, BIF is king. The
hematite and
magnetite concentrations
in BIF are much sought after by mining companies. These two minerals are
the best sources of iron and fortunately there is a lot of BIF to supply
the world's needs for quite a while. Magnetite-rich taconite is preferred, as
the ore is finely powdered and magnetic separation serves to concentrate
the magnetite leaving a fine quartz powder. The magnetite is then
further oxidized to hematite, which is fed into the iron-making process. |
