FMF - Friends of Minerals Forum, discussion and message board

[bugrock]

Banded Iron Formations (BIF) are largely Precambrian deposits and one source I found indicates there were three occurrences (time intervals) during the Precambrian and there are more than 20 sites known in the world.

The rock type 'jaspilite' is known from the BIF's and has alternating bands of hematite/magnetite and jasper. But at least some of these formations also have yellow to orange layers of asbestos containing material with the 'tiger eye' pattern. Some of this material is an attractive lapidary source, such as the 'tiger iron' known from western Australia.

By the way goggle images for 'tiger iron' and you will find many images of this type of rock along with a few of a famous golfer swinging a club or illustrating his physique!

Anyway, to the questions:

Do all BIF include layers with the asbestos containing 'tiger eye'? I do not recall it from the Lake Superior region deposits but it is well known from western Australia and also recently learned it occurs in the Ukraine.

What is the origin of the 'tiger eye' ? The bands I've seen look peculiar for simple sedimentary layering, the asbestos fibers appear to run at a right angle or at least an oblique angle with respect to the bedding bands. Did the asbestos form within certain layer types subsequent to the sedimentary event, perhaps in response to metamorphic processes? If the fibers deposited during sedimentation of the layers would think the fiber orientation would be random unless perhaps there is some other electrochemical process that keeps the fibers in parallel arrangement.

If someone could point me to a good review regarding the BIF I am interested in learning more.

Thanks

[David Von Bargen]

Look for binghamite from Minnesota.
BIFs usually are subjected to metamorphism during which the asbestos grows. Parallel alignment of crystals in metamorphic environments is due to the stress fields present during the metamorphic event.

[alfredo]

Think of the unusually high content of microscopic asbestos fibers in Great Lakes waters. (Nothing to panic about, by the way; I don't think it's associated with stomach cancer.)