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Posted by Is Ras  
Is Ras June 02, 2010 05:06PM
Hi all,
I´m currently working as a junior geologist on an iron ore exploration project in South America. Recently, the team have been debating the importance and significance of paying close attention to mineral form of Fe particularly with regards to the initial surface meters. Our primary Fe-mineral is magnetite, found within a body of host rocks that include volcanogenic felsic rocks and andesites (termed so for their composition rather than their origin). Hematite is found in small quantities, particularly at the surface. But our team is struggling to diffrentiate between oxidation and weathering (I have argued that in this case the two are more or less one and the same, as the weathering we have is a chemical process which can also be defined as oxidation). What I would like to know is, how do we know if the occurence of martite indicates oxidation or weathering, and how do we differentiate this with the occurence of hematite at the surface (where it appears to also be a replacement mineral)? The red clays and soils that we have at the surface are surely hematite due to weathering of already present magnetite?

I welcome all opinions, it may seem trivial but it is a hot topic and one posing much confusion currently.
Alfredo Petrov July 21, 2010 12:29PM
You might get some clues from the oxygen isotope composition of the martite/hematite. Google "oxygen isotope" + hematite - There have been lots of studies.
Reiner Mielke July 21, 2010 03:46PM
Chemical weathering can and usually involves oxidation. I have no doubt that "deep" weathering could produce martite given enough time and under the right conditions. However I doubt that martite could be formed at surface due to chemical weathering. Limonite is usually the product of such weathering but hematite is also possible (I am assuming you know the difference between martite and hematite). As Alfredo suggested oxygen isotopes should be able to distinguish martite formed at low temp and pressure ( weathering conditions) from that formed hydrothermally. However I doubt that you could distinguish martite formed under the same TP conditions but at different depths.
Mark Willoughby July 22, 2010 06:24AM
Howdy Isabel,

Martite is a psuedomorph of Hematite after Magnetite, not a weathering product. Therefore it is a chemical replacement of the Magnetite by Hematite, produced at depth and under pressure. It may be either a straight replacement process or an oxidation like process, but without depth and pressure the shape and form of the Magnetite would be lost. If the physical shape changed then it would be a replacement product not a psuedomorph and would not be called Martite.

Oxidisation of either Magnetite or Hematite can and does cause 'reddish' coloured sands, soils and clays; although so does the weathering of biotite from granite, which is actually more common.

Reiner is right about the Limonite too, Goethite is another possible result of Hematite or Magnetite oxidising. Although both are more of a complex chemical reaction than just basic oxidation, basic oxidation would just leave rust, or red soils.

You are right that oxidisation is a chemical weathering process; however oxidisation under pressure and depth is not a weathering process and is not quite the same.

And just as a note the felsic volcanic rock andesite was named for its occurrence in the volcanoes of the Andes Mountains, where it was first studied. The name is now just used for that type of rock anywhere it is found.

Martite does not suggest weathering, which we can gather from it being a psuedomorph. Other than the isotopes as Alfredo suggested, you will probably not be able to work out whether the surface Hematite is a product of oxidation/surface weathering or deeper oxidation during placement.

Do you know if you have a BIF type deposit or a simple placement deposit?
Knowing that, will give you some of the answers.

Cheers Mark.
Marco E. Ciriotti July 23, 2010 09:09AM
Perhaps useful.
Just published:

Hongyang Zhu, Yanzhang Ma, Haibin Yang, Cheng Ji, Dongbin Hou, Lingyun Guo (2010): Pressure induced phase transition of nanocrystalline and bulk maghemite (γ-Fe2O3) to hematite (α-Fe2O3). Journal of Physics and Chemistry of Solids, 71, 1183-1186.
Ralph Bottrill August 14, 2010 07:06AM
Hematite usually requires moderate temperatures for formation; it can be found in some gossans, but is perhaps just residual rather than supergene (but occurs in some re-buried palaeogossans). I would suggest that the red weathering product of the magnetite may be lepidocrocite? Fine grained goethite and maghemite (both found in magnetite deposits I have worked on) are usually brown rather than red. You need an XRD analysis.

jacques jedwab August 14, 2010 08:16AM
The whole problem is difficult, for different reasons:

-Ferromagnetism is misleading, since some "hematites" (from Itabira BIF, notably) are attracted by the hand magnet. In the field, thunder strikes magnetize hematite outcrops.

-XRD is (or may be) misleading, since there is only one peak difference between martite and hematite.

I should suggest transmitted light microscopy of local, well determined and finely powdered materials. Polished sections under oil immersion, reflected light should also be helpful.

Ralph Bottrill August 14, 2010 10:08AM
Agreed RL & TL microscopy would be very useful but XRD is still important to determine hydrous iron oxides etc

Peter Haas October 23, 2010 01:49AM
From a chemical point of view, an oxidation reation (Fe2+ -> Fe3+) takes places. The mechanism is essentially the same, no matter whether it occurred at the surface (superficial weathering) or secluded inside the rocks. "Superficial weathering" and "in-situ oxidation" may be more appropriate terms to describe the two modes of formation, unlike "oxidation" and "weathering" which suggest that weathering is not an oxidation
Dr.S.Niranjan Kumar October 19, 2011 09:50AM
Let us first understand that, weathering and oxidation are two different aspects. Oxidation is a chemical reaction while the weathering process may or may not involve oxidation. Regarding the martite - it is a replacement phenomena involving magnetite, which may occur both at surface or at deeper levels subject to the correct oxidation conditions and makes the ore grade rich. If magnetite weathers at surface then the products comprise of limonite, goethite and haemeatite/ maghaematite. I think it is clear.
Erskine. January 09, 2012 08:30AM
I have enjoyed the intellectual debate so far and would like to found out more from great minds like you all. I am also a fresh mind in an iron mine in Western Australia. I have come accross most of the iron minerals under discussion but what i found challenging is how to physically distinguish between them in the pit.

I would like to know what causes the sparkling crystals in martite? Is it usually common with all martite formation? How does one visually differentiate between martite, magnetite, geothite, and limonite as all these can occur in oxidised environment.

Your comments would be much appreciated.
Russell Shapiro January 29, 2012 09:07PM

This has been an interesting discussion to follow. I work on alteration due to both silica-leaching and oxidation enrichment (high-grad ore) and contact metamorphism. I am very keen on the hematite-magnetite-"martite" etc. pathways. I can add very little to this discussion except that in my experience, the iron oxides are very small (~1 um diameter) and disseminated. Under SEM, one can make out the magnetite shape of martite. Transmitted light with a 100x oil objective gets you close enough to make out subhedral mineral shapes. But, I rely on reflected light to distinguish hematite (including martite) from magnetite. Also, in my studies, it is most common under burial diagenesis and prehnite-pumpellyite conditions. Greenschist and higher leads to reduction and growth of magnetite.

(attached image of 1 um diameter bacterial filament from Biwabik Iron Range; transmitted plane polarized light)
open | download - filament.jpg (91.3 KB)
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