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Identity HelpUnknown amphibole

28th Jan 2017 00:51 UTCReiner Mielke Expert

It looks like an amphibole based on the cleavage and I know there are many possibilities but what would have such a high Ca content that would fit this EDS pattern?

28th Jan 2017 03:36 UTCDonald B Peck Expert

Reiner, (with my ignorance of interpreting EDS) - - Why not actinolite?

28th Jan 2017 05:12 UTCDoug Daniels

Don - don't you mean "actinolite"? (Based on several other threads.....) I too have little experience interpreting EDS, it's on my bucket list of things to do. Good thing I don't actively collect such beasts, although I guess I do have some "tremolite" and "actinolite" specimens in my collection.

28th Jan 2017 05:52 UTCFranz Bernhard Expert

On "my" EDX-system, this would be a typical pattern of a Hedenbergite-Diopside series mineral with some Jadeite-(Aegirine)-component (about 5-10%).

I don´t know the accelarating voltage and response curve of "your" EDX-system and this may well be a typical pattern of Ferroactinolite-Tremolite on this machine!

To you have other EDX-patterns of amphiboles und pyroxens from this machine to compare with?

Franz Bernhard

28th Jan 2017 11:57 UTCReiner Mielke Expert

Mg is too low for actinolite but hedenbergite is a good match and was also suggested by Kerry the EDS operator. The problem is it is from the Miller Property https://www.mindat.org/loc-243198.html and hedenbergite has never been reported from there. Andrew Debnam sent me a few small fragments for testing and under the scope they looked like an amphibole. But then I can't say I know what a fragment of hedenbergite looks like either. I'll have to get a picture of the specimen the fragments came from. Andrew if you are listening can you post a picture for us? I'll post a picture of one of the fragments.

Thank you everyone for your comments.

28th Jan 2017 12:59 UTCHarold Moritz 🌟 Expert

A fellow who analyzes amphiboles for me using TEM-EDS uses the method referenced below. You'll need the various oxide weight per cents, then convert to apfu (with oxide wt% adjusted to allow for anions occupying the W site. Amphiboles typically have 1.5% to 2.5% (OH), values are normallized to 1.75% (not included in total)). I havent read it.

Locock, A. J., 2014, An Excel spreadsheet to classify chemical analyses of amphiboles

following the IMA 2012 recommendations: Computers and Geosciences, v 62, p 1-11

28th Jan 2017 13:20 UTCReiner Mielke Expert

Thanks Harold unfortunately the lack of Fl data limits the usefulness of that and the data from this system is not quantitative enough to be used for this.

28th Jan 2017 14:02 UTCFranz Bernhard Expert

Reiner,

I am not familiar with this type of deposits. However, are there published analytical data of pyroxens and amphiboles from similar occurrences nearby? What is the usual range of compositions of this two minerals in these deposits?

Franz Bernhard

28th Jan 2017 14:31 UTCReiner Mielke Expert

Hello Franz,

There is very little modern analytical data published or available much of which was produced before the amphiboles where reclassified. The most recent work in that Grenville comes from the Bear Lake Diggings (Hawthorne, F.C., Oberti, R. (2006) Short-range order in amphiboles from the Bear Lake Diggings, Ontario. The Canadian Mineralogist, 44, 1171-1179.) Fl is widely distributed in this region and adds another complication to classification.

28th Jan 2017 15:43 UTCFranz Bernhard Expert

Reiner,

no modern work necessary, just some (old) analytical data (wt%) on amphiboles and pyroxenes to get a feeling of the range of compositions. In other words: Are there any Fe⁺²-rich members of pyroxens and amphiboles known in these deposits, that are analytically confirmed? More specifically: Is there any confirmed hedenbergite in these deposits?

Franz Bernhard

28th Jan 2017 15:56 UTCReiner Mielke Expert

No confirmed or unconfirmed hedenbergite has ever been found in these deposits.

28th Jan 2017 16:17 UTCFrank Craig

Well, that would not be a pyroxene or an amphibole on my system (Ca way too high). But, if it is an amphibole, I would call it ferro-actinolite (not enough aluminum to place it in the hornblende group). I would call it hedenbergite if it is a pyroxene. Given the general geology of the area both are certainly in the realm of possibility. But without quantitative data, this wouldn't hold up in court :-)

sorry Reiner

:)-D

28th Jan 2017 17:19 UTCReiner Mielke Expert

Hello Frank,

I was not really expecting a definitive answer and normally I would not even have analyzed such a sample but I was curious as to what the results would look like. I am also inclined to want to call it ferro-hornblende but if it were my specimen I would just call it hornblende.

28th Jan 2017 21:31 UTCRalph S Bottrill 🌟 Manager

Probably hedenbergite, but maybe ferroactinolite or ferroan diopside depending on how the EDS response works on your machine, you cannot rely on the peak heights without calibration. I would do an XRD or at least check the cleavage.

28th Jan 2017 21:54 UTCReiner Mielke Expert

"I would do an XRD or at least check the cleavage." I would just throw the specimen back in the bush and look for something easier to identify. I have no love for amphiboles ( sorry Frank). ;-)

28th Jan 2017 22:16 UTCReiner Mielke Expert

Here is a photo of the end of a cleavage fragment. Looks like a pyroxene to me in which case we would seem to have the first documented hedenbergite from there.

29th Jan 2017 01:55 UTCAndrew Debnam 🌟

here are some pics of the unlovable specimens .........sometimes I feel like just calling them dark and monoclinic. The sample came from the first picture. The other picture is from the same locality.

29th Jan 2017 06:13 UTCFranz Bernhard Expert

Thanks for the photos, they look indeed more "pyroxenish" than "amphibolish".

However, still no documented H. from this locality! It could be smack on the border between diopside and H. Quantitative analyses on a polished grain are necessary. If it is possible, on a complete section through a crystal, some zoning has to be expected.

Franz Bernhard

29th Jan 2017 11:21 UTCReiner Mielke Expert

Looks like diopside-hedenbergite is the best label one is going to get for now.

30th Jan 2017 15:06 UTCAndrew Debnam 🌟

If anybody wants a sample for further analysis I would be happy to provide it

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