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Improving Mindat.orgPyrope from Australia?

20th Feb 2017 00:09 UTCReiner Mielke Expert

https://www.mindat.org/photo-615643.html looks like almandine to me.

20th Feb 2017 02:20 UTCStuart Mills Manager

How can you tell?

20th Feb 2017 03:26 UTCJolyon Ralph Founder

Noble (1983), Catalogue of Minerals of South Australia says Pyrope, Reiner.

Unless you have information contrary to this...

20th Feb 2017 13:33 UTCReiner Mielke Expert

I am basing my remark on how well developed the crystal is. All the verified pyrope specimens I have seen are very crude crystals or anhedral masses.

20th Feb 2017 14:05 UTCJamison K. Brizendine 🌟 Expert

I agree with Reiner simply based on the geology of the area.

Almandine garnets form in folded metamorphic rocks and pyropes form in ultrabasic igneous rocks. So unless the specimen came from an ultrabasic igneous rock (which is doubtful) and the local geology (Flinders Ranges) show folded metamorphic and sedimentary rocks, then it would be probable that this specimen is an Almandine.

Jolyon do you have the citation available from Noble (1983)?

20th Feb 2017 15:38 UTCHarold Moritz 🌟 Expert

There are plenty of pyropes from kimberlites and their ilk. But outside of these, or peridotites or eclogites, pyrope is extremely unlikely. From metaseds, almandine greatly predominates, with spessartine a distant second, based on analyses I've done from New England rocks.

Arent the diamond-bearing kimberlites in Western Australia? Not an expert but I dont think there are any in South Australia.

20th Feb 2017 17:53 UTCUwe Kolitsch Manager

I agree with Reiner. Not everything in Noble (1983) is correct.

Unfortunately, the uploader (Judy Rowe) died last year.

Photo now user-only; referral to this thread added to photo caption.

21st Feb 2017 05:28 UTCAlfredo Petrov Manager

"Pyrope" seems to be in common use for crystals that are only pyrope-bearing and not pyrope-dominant. I've checked specimens of "pyrope" from other localities that turned out to be only around 30% pyrope molecule and were almandine-dominant. Seems to be a common practice, especially in the gem trade.

21st Feb 2017 10:07 UTCAndy Stucki (2)

Considering the metamorphic grade the rocks underwent in the Flinders Ranges (low to high temperature, but only low pressure to medium pressure), pyrope is pretty much impossible to be the dominating garnet component in these specimens. From these rocks it can be expected that the garnet as it is pictured may be almandine with more or less significant content of spessartite and grossular.

So I expect the the classifictaion by Noble (1983) to be at fault.

It's not entirely the host rock that matters (it does have to be Mg-rich, however) but the pressure the rocks underwent for garnet to be mostly pyrope. And pressure has to be considerable (usually requires depths in excess of 50 miles/80 km).

(And I totally agree with Alfredo that calling a garnet "pyrope" in the trade may not mean that it is all or mostly pyrope)

Andy

21st Feb 2017 10:19 UTCKeith Compton 🌟 Manager

Hi

I thought that the garnets from this area were mainly a mixture of spessartine/almandine.

I did find one article that in part alludes to such garnets being spessartine/almandine in the northern Flinders Ranges (although it should be noted that they are not for this specific locality):

Pierre-Alain Wulser (2009) Uranium metallogeny in the North Flinders Ranges region of South Australia, thesis for the Faculty of Science, The University of Adelaide.

Cheers

Keith

23rd Feb 2017 22:13 UTCRalph S Bottrill 🌟 Manager

Pyrope does occur in the southern Flinders Ranges, as small grains in and around several kimberlite pipes, but I am not aware of any in this particular area. Besides the size, colour and form would be unusual for such occurrences. High grade metamorphic rocks do occur in the Northern Flinders, eg corundum and hercynite bearing rocks near Arkaroola, but no eclogites or other likely pyrope hosts AFAIK. But this site (not well located) is west of the ranges and probably in the Curnamona-Willyama complex, which hosts similar garnets near Broken Hill. In fact it looks extraordinarily like the manganoan almandines from the Thakaringa area, near Broken Hill.

24th Feb 2017 16:01 UTCUwe Kolitsch Manager

Changed to garnet, and info added to caption.

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