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Identity HelpHematite or Goethite?

9th Jul 2014 10:43 UTCJay I. G. Roland

Hello everyone, might someone please give this newcomer, in simple to understand terms, any tips on determining if a specimen is hematite or goethite. I appreciate that the two are related but I would just like to label my specimens correctly.

I have recently visited a series of well known Cornish tin mining sites (St. Just in Penwith district) and picked up what I believed was hematite. Under the 'scope inside vugs of smokey quartz I see I have botroydal forms, spherical/spiky forms and 'bow tie' forms yet when I did a search for such images they tended to be titled 'goethite'.

Is there a simple test one can do or anything in particular to look out for to determine which is which.

All answers gratefully recieved...

Regards,

Jay.

9th Jul 2014 11:34 UTCOwen Melfyn Lewis

Different crystal systems. There are several differences in appearance but AFAIK none are diagnostic in the sense that they must always be present. Goethite does elongate or stringy if inside another transparent crystal host. Hematite does botroidal, platy and 'iron rose'.

9th Jul 2014 12:24 UTCAlf Olav Larsen

Check the streak colour: scratch the mineral on unglazed chinaware (for example the unglazed underside on a cup). Hematite leaves a reddish brown colour while goethite leaves an ocher-yellow colour. Unmistakable.

Regards,

Alf Olav

9th Jul 2014 12:36 UTCColin Robinson

A streak test is usually good enough for most folks. Hematite has a red streak, goethite is yellow-brown.

Not foolproof as these iron oxides are often mixtures.

9th Jul 2014 13:32 UTCRolf Luetcke Expert

Funny, I was just telling this exact problem to a friend two days ago. Above covers it well and I only have one addition I told the friend. Look at a broken surface, esp. on botryoidal forms and look for the inner structure of the mineral. Hematite is solid material with no internal structure visible and Goethite has the inner needle-like structure. So if you look at broken surfaces and can see the inner close packed striated structure Goethite is the mineral. Goethite can also be tiny yellow to brown needles, standing on their own.

Rolf

9th Jul 2014 13:50 UTCErik Vercammen Expert

The difference is not only the crystal system (trigonal for hematite and monoclinic for goethite) but also the composition: hematite is Fe2O3 while goethite is FeOOH.

9th Jul 2014 14:28 UTCOwen Melfyn Lewis

Orthorhombic for goethite, surely? And, IMO, it is the crystal symmetry rather than the chemistry that helps towards visual identification.

9th Jul 2014 14:37 UTCErik Vercammen Expert

Owen,

Thanks for the correction, I had a wrong idea in my head! And you're right about the crystal form being the most helpful, but in the beginning of this thread, there was made the remark that the only difference between hematite and goethite is the crystal system, so I tried to correct that.

9th Jul 2014 16:34 UTCDonald Peck

If you are new to the hobby, and just starting out, then for hematite vs goethite streak is much easier and sufficiently diagnostic. Crystals and crystal structure takes time and experience to learn (but it is worth it).

9th Jul 2014 19:18 UTCDoug Daniels

Looking at the original post, he was describing the HABIT of the mineral....nothing about crystal shape/morphology from which the system can be deduced. He also indicated he was looking at the things under a microscope, so getting a streak may be troublesome - unless he just crushed some of them with a suitable instrument. Even a chemical test (heat a piece in a closed tube and look for water condensate) is probably iffy. So between a rock and a hard place? Wait, he is looking at a rock.... Probably the best idea is Rolf's, if you can look at a broken surface and see the structure.

9th Jul 2014 20:58 UTCAlf Olav Larsen

Even under the microscope you can check the streak colour (actually the colour of the powder) on very small objects. Scratch a steel needle along the mineral in question. Brownish red streak indicates hematite, ocher yellow or brownish yellow indicates goethite.

Alf Olav

10th Jul 2014 10:29 UTCJay I. G. Roland

Hello everyone, thanks for your input. Yes I am aware of the streak test but as Doug correctly states, some of the specimens are simply too small for me to attempt that.

I did however do the test (on the back of a piece of polished cream marble, no tiles at hand) last night on one of the pieces of the host rock and got the pale brown streak of hematite. However, some of the vugs inside some of these 'hematite' samples contain what I described in my original post as being (according to various websites including Mindat) goethite in various forms.

When I type into Google images 'botroydal hematite' or 'botroydal goethite' I seem to get pretty much the same images, and that's without going into detail with the much smaller stuff I am looking at, though the general consensus suggests goethite for most of them.

I was (still am) having difficulty creating decent images but managed the below yesterday which are good enough I think to indicate what I am looking at.

Thanks for looking and advising...

Regards,

Jay.

10th Jul 2014 10:31 UTCJay I. G. Roland

Two more images...the first image is very low mag showing a piece of the host rock on the right, the second image is at much higher mag showing a flower/fan shaped growth of some kind or other.

10th Jul 2014 11:18 UTCDale Foster Manager

Jay Roland Wrote:

-------------------------------------------------------

I was (still am) having difficulty creating decent images but managed the below yesterday which are good enough I think to indicate what I am looking at.

Jay,

The second and third images are Siderite - the form is characteristic from the mines in the immediate area in that part of Kenidjack Valley. Probably a bit of alteration on them as well.

Siderite - Wheal Drea

10th Jul 2014 11:29 UTCJay I. G. Roland

Hello Dale, thanks for your help. I did wonder if the third picture might be siderite (I have samples from the same site that I can positively ID as such) but the texture made me wonder. The siderite I am familiar with has very flat & smooth sides and are almost perfect pyramids. Perhaps these particular ones are 'decayed'?

There is definitely a similarity twixt the crystals of image 2 & 3 only the colour and size differ. If I recall they are actually on the same piece of rock too!

Thanks again Dale,

Regards,

Jay.

10th Jul 2014 11:54 UTCDale Foster Manager

Siderite comes in a multitude of forms and colours, you could create a themed collection of Cornish Siderites that would keep you busy for years if you chose to do so.

16th Feb 2015 03:22 UTCGrass

If you have enough samples, you can try powdering the samples and have them analyzed using XRD just to be sure.

Regards,

Grass

16th Feb 2015 16:51 UTCD. Peck

Jay, powdered siderite will fizz when hot acid is dropped on it.

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