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EducationIron sulphide nodules

26th Nov 2017 21:49 UTCJason Evans

I occasionally see photos of what i refer to as iron sulphide nodules, most recently on a Facebook meteorite group someone has posted what i am convinced is a iron sulphide nodule thinking it is a meteorite.

I told them it's an iron sulphide nodule but then a few other people added their own suggestions and said it was marcasite and I have seen other images of similar nodules also referred to as marcasite, I am pretty sure that pyrite can form similar nodules so why do people call them marcasite, is it not more correct to call them iron sulphide nodules?

This is an example of such nodule which I collected from nearby where it has weathered from chalk.

26th Nov 2017 22:15 UTCJolyon Ralph Founder

You are correct, they can be pyrite, or marcasite, or a mixture. Or possibly pyrite after marcasite too.

And what you see on the surface is mostly goethite.

26th Nov 2017 22:17 UTCBen Grguric Expert

Hi Jason,

In the past these round nodules have traditionally been referred to as marcasite, but probably the bulk of them are pyrite. I've sectioned and polished several examples from Western Australia and examined them under a reflected light microscope. I found they were mostly pyrite, but typically there was always some intergrown marcasite present, and sometimes the core was a grain of sphalerite. The appended sectioned nodule is from Mt Keith, WA and is 4 cm across.

Ben.

26th Nov 2017 22:36 UTCJason Evans

Cheers, it is good to know I am not giving incorrect info to people!

That nodule has a different appearance to the ones I am familiar with which have a radiating needle like habit. Although perhaps they look different if they have been sliced rather than broken apart.

26th Nov 2017 22:41 UTCKelly Nash 🌟 Expert

I was told by a local geology professor (Dr. James Carter UTD,, now retired) that examination of polished cross-sections (in reflected light) of many of the nodules found around the Dallas, Texas area, showed them to be alternating layers of marcasite and pyrite. They are usually called pyrite nodules, if only because sedimentary marcasite is believed to fall apart more quickly, and pyrite often replaces marcasite. I don't know if the replacement goes the other direction, however (marcasite after pyrite?).

27th Nov 2017 08:21 UTCPaul De Bondt Manager

The nodules found in the chalk of Cap-blanc-nez in France and across the channel are ALL pyrite.

The crystals who grow on them can be pyrite or marcasite.

27th Nov 2017 11:45 UTCAlysson Rowan Expert

Marcasite was originally the name applied specifically to the palest of cut iron sulphides used as a gemstone.

The origin of these as "marcasite nodules" is in the gemmological trade:

Marcasite was popular in Victorian England as the basis of the glitz on bejewelled dress accessories - buckles, buttons etc. The so-called marcasite was rather understated in comparison with the gaudy diamond (etc.) encrusted that had been favoured previously.

Much of the cheaper end of the costume jewellery market of the time was based on the nodules dug from the chalk in France and England, the higher quality material was mined in India (white pyrite).

27th Nov 2017 13:32 UTCPaul Brandes 🌟 Manager

So are we looking at some sort of phase transition between pyrite and marcasite (or vice versa) in these nodules? And if so, what's driving the transition(s)?

27th Nov 2017 13:44 UTCErik Vercammen Expert

Speaking for the nodules from the chalk cliffs around the Strait of Dover/Détroit de Calais: they were pyrite from the beginning, so no phase transition.

27th Nov 2017 15:29 UTCRichard Gunter Expert

The smooth surfaced sulphide nodules from the Swan River Formation: https://www.mindat.org/photo-429713.html are Pyrite and have always been so. The Marcasite within the underlying limestone looks quite different.

27th Nov 2017 20:56 UTCReiner Mielke Expert

Does marcasite ever pseudomorph pyrite?

27th Nov 2017 21:08 UTCRichard Gunter Expert

Not in the Manitoba Interlake. The occurrences of primary Marcasite (XRD identified) such as Steep Rock Quarry have no Pyrite and the Mafeking Quarry has Pyrite but no Marcasite. The Marcasite is also associated with Baryte crystals but the Pyrite is not so it may be different fluid compositions between the quarries. The Stony Mountain Quarry, north of Winnipeg, has Pyrite aggregates and no Baryte. All of the quarries have secondary Calcite.

27th Nov 2017 22:26 UTCMark Andrews

These iron sulphide nodules occur around Austin, Texas. I have collected some and a few friends have collected some as well. My wife is actually the undisputed champion of collecting them around our house. From childhood, I had always assumed they were marcasite. A few years ago one of my coworkers performed xrd on a few samples found around Austin. All three of the samples he xrd'd were identified as pyrite.

27th Nov 2017 23:30 UTCBen Grguric Expert

Marcasite is essentially metastable with respect to pyrite. Around 430 degrees C marcasite irreversibly transforms to pyrite though most of the nodules found in sedimentary settings would not have experienced these temperatures.

Although marcasite is metastable it would appear low pH (acid) conditions tend to favour the formation of marcasite over pyrite. pH could fluctuate during nodule formation and result in some marcasite-rich and some pyrite-rich layers.

Don't know of any examples of marcasite pseudomorphing pyrite. Ramdohr doesn't mention it either. Marcasite, commonly in lamellar intergrowth with pyrite is often seen pseudomorphing pyrrhotite. This happens in the supergene zone of many pyrrhotite-bearing ore deposits.

Although you can use XRD to identify the main species in nodules, I'm not confident you can rule out the presence of small amounts of intergrown marcasite with this technique alone. The most sensitive method would be to examine the nodule as a polished section with a reflected light microscope. Marcasite is very distinctive due to its anisotropy.

29th Nov 2017 15:01 UTCErik Vercammen Expert

I remeber to have seen a picture of a stalactite with alternating pyrite and marcassite layers.

29th Nov 2017 16:00 UTCReiner Mielke Expert

Thank you Ben.

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