❓Your Mineral Identification Questions answered here

[Dihusky]

MartyZ popped over and we re-photographed one of the 'cocoons' to provide a close up of the crystals, plus a different pebble with a crystal. Added a small stone which is very pleochroic as Goldierocks described earlier, three different angles of the same stone.

There was a piece of Tourmaline in the mix of stones MartyZ brought over, very distinguishable and totally opaque to any light. The Tourmaline came from the same creek wash and though looked the same dark colour, was quite clearly a different mineral when examined in more detail.

To get much more detail I'd need to find a microscope as this is about the limit of what my camera can do without getting extension tubes/bellows etc

 

[user 4386]

Fair enough and very interesting RI plus SG will tell you if accurate. Assume it is not two types of tourmaline together (common). However looks like it is probably something different.

 

[Lefty]

The mystery grows ever deeper..........

 

[Lefty]

Those "cocoons" look so much like the ones I have, though the cocooned stones are black spinel and pyrope garnet.

 

[Lefty]

Does epidote fall within the hardness/SG ranges of these specimens?

 

[Wally69]

Have you looked at fayalite, I think it fits the measured parameters.

It is a like peridot but instead of Mg it contains Fe, giving a brown tone instead of green.

 

[user 4386]

Fayalite and tourmaline are unlikely to occur together geologically, nor spinel and garnet (although garnet can). Epidote and tourmaline are common together.

 

[Lefty]

Fayalite and tourmaline are unlikely to occur together geologically, nor spinel and garnet (although garnet can). Epidote and tourmaline are common together.

Had the specimens sent to a gemmologist who confirmed them as garnet (of an overwhelmingy pyrope %) and black spinel. Not sure about the volcanic rock they are embedded in together but the specimen is in mum's garden and is very similar to Marty's (the host rock, not the embedded crystals). Will see if I can get a photo next time I'm over there. Had a smaller piece with garnet and black spinel in it together but don't know where that one went. Think it was the one we sent for testing.

 

[user 4386]

Fayalite and tourmaline are unlikely to occur together geologically, nor spinel and garnet (although garnet can). Epidote and tourmaline are common together.

What I meant was that tourmaline is unlikely to occur with fayalite, spinel or garnet (although it does rarely occur with garnet). Not that spinel and garnet cannot occur together (also common)

 

[Lefty]

Fayalite and tourmaline are unlikely to occur together geologically, nor spinel and garnet (although garnet can). Epidote and tourmaline are common together.

What I meant was that tourmaline is unlikely to occur with fayalite, spinel or garnet (although it does rarely occur with garnet). Not that spinel and garnet cannot occur together (also common)

Ah ok, cool.

 

[Dihusky]

Next piece of the jigsaw, the RI is 1.68 - 1.7 and the crystal is Bi-axial, Kym from Symmetry who tested it thinks it might be a variety of Enstatite but was very unsure and is also intrigued.

 

[user 4386]

Cleavage is possibly a bit average for enstatite. Epidote group would be a bit softer....(otherwise I might have said allanite). RI and SG too low for any of the tourmalines. Only leaves analysis or XRay

 

[Martyz]

Thanks Goldierocks. I am communicating with the QUT Central Analytical Research facility in Brisbane who are able to perform X-ray analysis. Ive been told that perhaps if they can get one of their PhD students interested then it might save me having to fork out a few hundred dollars. Thanks to Dihusky I have been able to provide them with all the test results and images that have been available to this forum so there is already enough information to illustrate that the mineral is not something immediately obvious.
Ill post any conclusive results as soon as I have them and hopefully Dihusky will be able to bring one these little mysteries to life on his cutting wheel.
Thanks to all the members for your help.
Marty :Y:

 

[user 4386]

Thanks Goldierocks. I am communicating with the QUT Central Analytical Research facility in Brisbane who are able to perform X-ray analysis. Ive been told that perhaps if they can get one of their PhD students interested then it might save me having to fork out a few hundred dollars. Thanks to Dihusky I have been able to provide them with all the test results and images that have been available to this forum so there is already enough information to illustrate that the mineral is not something immediately obvious.
Ill post any conclusive results as soon as I have them and hopefully Dihusky will be able to bring one these little mysteries to life on his cutting wheel.
Thanks to all the members for your help.
Marty :Y:

That's the way - it is easy with XRay or SEM

 

[Martyz]

Hi everyone,
I have been in contact with Dr Andrew Christie of the Qld Museum / University of Qld regarding the yellow mystery rock and here is most of his assessment based on the same information provided to the forum.
You seem to have a mix of xenoliths/xenocrysts (fragments caught up in and embedded in basalt) with alteration rims around them, and also some weathered-out fragments.

It is possible that more than one material is involved (see below), but.

The test data sound very like magnesium-rich olivine, which would not be unusual in an alkali basalt.

For 90%Forsterite/10%Fayalite composition, Id expect:

Mohs H = 7
Streak white
Bulk Colour pale yellow-green
SG = 3.34 or so
RI = 1.66-1.69.
Biaxial (due to orthorhombic symmetry).

The alteration rim around some in the basalt would also be typical, since olivine can react a bit with the host lava.

...The combination of minerals to which you are referring is, if I understand you right, just an olivine solid solution. Nature is a messy geochemical soup, so few minerals form as pure chemical compounds. Magnesium and iron atoms can swap randomly for each other in the crystal structure of olivine, and the relative proportions of the two depend mainly on environmental factors such as the composition of the molten silicate that they crystallised from and the temperature/pressure at which they did so. Most olivine from the upper mantle or from basaltic lavas is close to 90% of the magnesium end-member, 10% of the iron end-member, and has properties (colour, density, optics, etc) pro rata intermediate between pure forsterite and pure fayalite.

Without more expensive tests being conducted then some of the distinct qualities of this crystal can be attributed to the pro rata intermediate properties described above with Olivine.
So, unless there is any reason to question this being an Olivine mineral then I think we can let it rest... although Im still open to all opinions.
Thanks,
Marty

 

[user 4386]

Xenocrysts with reaction rims seems likely. It would be unusual to get olivines of differing composition in a single sample - and I would expect the olivine to be quite magnesian.

 

[Martyz]

Hi Goldierocks,
Do you think that Dr Christies conclusions are for the most part plausible or do you still think there is enough reason to seek further testing?
If Olivine is the logical candidate then what could I expect further tests to reveal?
I ask this because there seems to be little interest in the notion of performing further tests by the people Im dealing with as much as I would like to.
Marty

 

[user 4386]

Personally I would not bother. But obviously it is up to you.

 

[Martyz]

Righto, thanks Goldierocks.
So.... Im now now going to ditch my scientist cap and dub the unnamed rock Ziviscarlite Olivine.... until someone tells me otherwise.

 

[LONEWOLF]

That was a tricky one Martyz...

LW.....