FMF - Friends of Minerals Forum, discussion and message board

[JesseKoz]

First post here and hoping I can get some help identifying a mineral specimen I found some years back. I know previously from going through some books it appeared to most likely be hematite, but I wanted to see if I can get some more knowledgeable opinions. It always held my interest due to the fascinating mix of habits displayed.

Provided below in the locality section is a 250k geological map of the area from aus geoscience, with the locality being 40' North, 15' East. This is only 10-15 km away from the Mia Mia Volcanics subgroup.

Locality:
Northern Territory, Central Australia
Geological 250k map - 40' North, 15' East (Fig 7.)

Color:
The specimen displays a black to brown / reddish brown color, with red staining on sections of the surface. I would imagine the red is from a high iron content within the specimen.

Luster:
Dull metallic luster

Habit:
The top surface of the specimen displays globular habits and has sections of almost glass-like smoothness. The sides of the specimen show prominent striations running perpendicular to the top surface. Some of the striations appear to almost radiate out from a point, but this may just be due to uneven fracturing. One single face on the side of the specimen displays 90-degree angles and is much smoother (Fig 5.), with the striations not as prominent and running in a 45-degree angle. The 90-degree edge makes me think it may be rhombohedral, from reading online I see that Hematite can display both this and radiating forms.

Streak:
I don't own a streak plate, but using the unglazed edge from the bottom of a porcelain bowl to perform a streak test produced a dark red streak.

[alfredo]

Yes, it certainly does look like hematite. And some of those protruding knobbly hemispheres look like the "kidney ore" or "glaskopf" varieties of hematite: https://www.mindat.org/min-29758.html
Nice find.

[JesseKoz]

Thanks for the quick reply alfredo and for confirming my hunch. I had come across the varieties of hematite when doing some reading which was one of the things that first pointed me towards hematite. Very interesting form.

Just wondering, is the amount of globular habit on the surface enough to call this 'kidney ore', 'glaskopf' or 'botryoidal' variety? My assumption was it might not display enough of this habit to fall under those labels.

[Aniol Soler]

You can try scratching a bit of the surface or take a tiny piece somewhere and make it into a powder. If the powder comes out yellowish-brownish its goethite, if it's kind of red its hematite.

For what you say and the aspect looks as hematite yes.

There's a post on Spanish forum about that with bit more of info ;)

https://www.foro-minerales.com/forum/viewtopic.php?t=1149

Cheers

[alfredo]

The IMA (International Mineralogical Association) only makes official definitions for species, not varieties, so all those varietal names end up with rather fuzzy definitions and you are likely to find collectors disagreeing about where their boundaries are. Sometimes the classification even seems to depend on whether one is 'buying' or 'selling' the specimen! ;))

So just follow your own heart in the matter of varietal names, and you can rest assured that, whatever you decide to call it, someone here will disagree with you. Pay them no attention ;))

[JesseKoz]

@Aniol Soler Thanks for the suggestion and further info, I managed to get a small chip powered and confirmed it is a dark red.

@alfredo Thank you, interesting information! I think I will just label it hematite. Most important thing for me is it being a cool example of earth's geological processes, and that it sure is.

[SteveB]

Common to most collected specimens you don't usually end up with a “pure” textbook specimen and of course people want to claim they have a rarer variant or something completely unrelated. For example, it's common to find a rough quartz piece which on close examination shows the glint of a crystalline surface somewhere, but it's just quartz, not a crystal. Yours I would label, if it was mine, as hematite showing some botryoidal development. I don’t know if there is a more formal way to say this. It's clearly not a complete homogenous Kidney ore example so labelling it as such I think if to misrepresent it. Maybe the correct phrasing is “hematite exhibiting some botryoidalism”? That kidney ore texture is certainly prominent on the specimen, and I would suggest it be noted in the labelling, but it can’t be used to label the entire specimen as its not representative of the entire specimen.
Do you have a regular magnet (not one of these dodgy ebay “super rare earth magnets” just the typical magnets we played with in high school science? Does it attract to this specimen weakly, strongly or not at all? I’m wondering if this specimen may be a mineral phase like Maghemite in the iron oxide group that produces magnetite and hematite? I think my label suggestion above though is as close as can be suggested, certainly for your personal collection.

[LeadMineGal]

Looks like a nice thick slice of petrified Porterhouse steak to me!
..Just a little humor to make you smile today...

[Firmo Espinar]

I would say... yes, it´s hematite.