The Rock - Banded Iron Formation
- Sedimentary Rock Type:
- Related to: Shale
Color: Alternating layers of silver and red
- Texture: Fine grained to fiberous
- Origins: Ancient ocean deposits
- Common Minerals: Magnetite,
quartz, and clays
- Uses: Ore of iron, decorative ornamental stone for
carvings, gravestones, jewelry and tile
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
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.BIF is very old. Some of the oldest sedimentary rocks known to scientists
have banded iron formations among their constituents. They date to as old
as 3 billion years old, but most are aged at around 2.5 billion and some
are as "young" as 1.8 billion. A very young formation of BIF is known to
be only 800 to 600 million years old, but this is an exception to the rule.
It is believed that banded iron formations occurred at these times in Earth's
history due to unique conditions and then never again were the conditions
right for the formation of this most unusual rock. Due to the extreme age
of these formations, almost all BIF formations have undergone some faulting,
fracturing, folding, compaction, veining, intrusions and metamorphism. Although
all BIF formations are probably metamorphosed to some degree, their general
character is still sedimentary.
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
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.
Banded iron formations are found in the continental shield of all continents
of the world. The shield areas of continents contain the oldest precambrian
rocks. At one time these rocks may have been together as one continent,
but later broke apart and became the core of the modern continents as they
exist to day. Although each continent has had its own geologic history that
had its own impact on their shield rocks, it is somewhat amazing that most
BIF rocks are very similar character. This is truly one of the most amazing
rocks found on Earth.