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Ankerite with no proof
Posted by Reiner Mielke
Vik Vanrusselt February 11, 2018 04:57PMHello,
I got the ID from the label.
Nevertheless, I have changed the ID to Dolomite.
1) How would one test this specimen to distinguish ankerite from dolomite?
2) How come https://www.mindat.org/photo-855026.html is ankerite "without proof" when my specimen isn't?
Edited 3 time(s). Last edit at 02/11/2018 05:26PM by Vik Vanrusselt.
Reiner Mielke February 11, 2018 05:45PMHello Vik,
EDS would tell you. Unfortunately there are lots of other unproven ankerites not to mention hundreds of other minerals. I have brought this up before and management does not want to go back and change things as this would require a lot of work. Real ankerite is very rare and one has to start somewhere. However if you think it is ankerite then it would be worth testing.
I'm still looking for a piece of ankerite for my collection even though the local gold mines all report that they have ankerite. There is even a townsite and mine called Ankerite but all they have is ferroan dolomite. It is common among geologists to call any iron bearing dolomite ankerite even though it does not contain enough iron to be a true ankerite. Which reminds me I have to send some samples off next week to see if they are ankerite. "Fingers crossed."
Franz Bernhard February 11, 2018 06:40PMHello Reiner,
Styrian Erzberg has plenty of real ankerite (At. Fe > At. Mg). Should be easy to get a sample from Austrian collectors.
Hope you are lucky with your tests. You know, if At. Fe is close to At. Mg, qualitative EDS will not work, quantitative analyses will be necessary.
Uwe Kolitsch February 11, 2018 08:07PMBeran, A. (1979): Die Stellung der Ankeritgesteine im Rahmen der Genese von Sideritlagerstätten der östlichen Grauwackenzone. Tscherm. Mineral. Petrogr. Mitt. 26, 217-233.
has electron microprobe analyses showing that ankerite (Fe>Mg) occurs.
Page updated. (The ref. was also missing in the list.]
PS Chris: have sent you the PDF.
Edited 1 time(s). Last edit at 02/11/2018 08:12PM by Uwe Kolitsch.
Mark Heintzelman February 11, 2018 10:22PMA link to the previous discussions on this: https://www.mindat.org/forum.php?read,7,353729,353960#msg-353960
Yes, by it's modern definition in attempts to add consistent rules in determining boundaries in solid solution series, ankerite has now become a virtually theoretical mineral species, very rarely found in nature. Despite not wishing to pursue the efforts involved, all current ankerite localities and images in the galleries really do merit being moved to Ankerite-Dolomite series until proven by a full analysis, for sake of accuracy and due to it's actual rarity in nature.
I certainly appreciate Renier taking up this quest for accuracy . . . death by a thousand cuts, if it must be done that way. (thanks).
ANKERITE-DOLOMITE (Styrian Erzberg, Eisenerz, Styria, Austria) 10.8 x 5.2 x 3.0 cm
Vik Vanrusselt February 11, 2018 10:29PMThe "problem" I have with the "ankerite" photos from Styrian Erzberg on Mindat (https://www.mindat.org/gallery.php?loc=263&min=239)
is that only 1 (one!) of them actually has a SEM-EDS result in the caption, namely this one: https://www.mindat.org/photo-722902.html
Therefore, I concur with Mark Heinzelman's comment to rename all "ankerite" photos on Mindat to "ankerite-dolomite series", unless sufficient analysis results are available.
Ralph Bottrill February 12, 2018 03:04AMI would agree with that move, then put the onus on people to prove it’s existence in particular locations and specimens. Better to be accurate and vague than precise and wrong. The trouble is that the IMA let a long-standing name for a Fe-bearing variety of dolomite be used for a theoretical Fe-dominant end-member (already known as ferrodolomite). It’s maybe too late to change now, but the message certainly didn’t get through to most geologists, collectors and mineral curators who mostly still use the old definition.
Ankerite is not incredibly rare, I commonly find when analysing dolomites that some crystal zones have Fe>Mg, but Mg overall nearly always dominates so it’s a stretch to call the whole specimens Ankerite.
Franz Bernhard February 12, 2018 06:19AMJust a few data from the small deposit at Prinzenkogel, Styria, Austria,
to back up Ralphs statement.
BSE-images of polished sections, field of view 0.46 mm and 0.31 mm, respectively.
Both from sample AN801.
And a triangular Fe-Mg-Mn diagramm of these minerals.
Franz Bernhard February 12, 2018 06:44AM"And now to something completely different":
A diagram with carbonate rock analyses from the Styrian Erzberg in a paper from 1973, Beran´s work is already mentioned.
The green line is At. Fe : At. Mg 1:1
The red line is ideal stoichiometric dolomit-ankerite series.
It shows clearly that there are "dolomite rocks" and "ankerite rocks" at the Styrian Erzberg.
But this does nothing say about the nice, nearly white rhombohederal crystals...
Christian Auer February 12, 2018 06:56AMThank you Franz. Unfortunately colour is no good ID reason for those pale yellow-white rhombs from the Styrian Erzberg. I once ID 8 different species. Three were pure dolomite, one ankerite and the rest mixing up within a single xtl.
By the way, what is the white rim on your first BSE picture, galena?
Alfredo Petrov February 12, 2018 07:42AMLet's not disparage people who use the name Ankerite for material that is really Fe-rich dolomite in modern terminology. Yes, on Mindat we need to stick to modern species definitions, but keep in mind that these definitions have changed over time and so an old label that says "Ankerite" should not be treated as if it were written by some careless nincompoop - It just needs "modernisation".
The reality is that both collectors and geologists use a lot of "field terms" that do not match modern species definitions, and I see nothing particularly heinous about that, as long as we keep in mind that that happens, and that the vast majority of specimens will never be analyzed, at least with the current state of technology. So yes, let's change all unanalyzed "ankerites" to Ankerite-Dolomite Series, even those from the Ankerite type locality (where Fe-rich dolomite seems to be at least as abundant as true ankerite), but at the same time I'd like to stress that labels saying "Ankerite", when they really mean Fe-rich dolomite, are not "errors", and we don't need to crticise the label writers, they just need redefinition.
Pavel Kartashov February 12, 2018 10:26AMRalph Bottrill Wrote:
> Ankerite is not incredibly rare, I commonly find
> when analysing dolomites that some crystal zones
> have Fe>Mg, but Mg overall nearly always dominates
> so it’s a stretch to call the whole specimens
I can't to agree with Ralph here - too much samples labeled as "ankerites" shows low or negligible iron contents even in most enriched by it zones. They often aren't even "ferrodolomites" at all. Another usual case is when dark oxidized carbonate is composed by mixture of dolomite (low ferroan) and mixture of iron oxides. This is typical for localities of Nagol'nyi Kryazh - prolific sourse of "ankerite" samples in former USSR. However I saw similar "ankerites" and from Erzgebirge.
What about a diagram Mg-Fe-Mn, it is good and visual, BUT in case with ankerite it always necessarily must to be accompanied by Mg-Fe-Ca one, because of we have perfect solid solutions in system Mg[CO3]-Fe[CO3]-Mn[CO3] much more abundant and widespreaded in nature than Fe-rich minerals of dolomite-ankerite series. And part of "ankerites" from his Mg-Fe-Mn diagram quite able to stay on bottom side of tetrahedron Mg-Fe-Mn-Ca with Ca at top. ;-)
So Franz should to show us one more diagram for his samples AN1018 &AN1019 - for example Mg-(Fe+Mn)-Ca one.
From other hand, his the second diagram shows almost complete absence dolomie points on it, what is in contradiction with his the first diagram. :-\??
Edited 1 time(s). Last edit at 02/12/2018 10:31AM by Pavel Kartashov.
Franz Bernhard February 12, 2018 12:12PMChristian,
the rims around the siderite are "limonite".
the ankerites-dolomites from Kaltenegg and Wagnerhöhe are nearly stoichiometric Ca(Mg,Fe,Mn)(CO3)2, for this reason I didn´t made a diagram with Ca. Looking into the original data set, the ankerite-dolomite poorest in Ca has the composition:
Ca0.952(Fe0.460, Mg0.448, Mn0.140)(CO3)2
(concering all four quantitatively analyzed samples).
The second diagram from Styrian Erzberg has nothing do to with the first diagram from Prinzenkogel (Kaltenegg, Wagnerhöhe), its a totally different deposit.
Edited 1 time(s). Last edit at 02/12/2018 12:42PM by Franz Bernhard.
Franz Bernhard February 12, 2018 01:23PMJust to show context, the polished hand specimen AN801 from Kaltenegg, with galena-bearing carbonate-quartz-veinlets in gneiss. Size of specimen ca. 15 cm, position of polished sections indicated. (Sorry, no better photo available.)
Franz Bernhard February 12, 2018 02:11PMReiner,
sorry, photo and data not good enough for upload...
No, I have no confirmed euhedral dolomite from Styrian Erzberg, but I have checked only very few samples. But have a look at the post of Christian:
"Unfortunately colour is no good ID reason for those pale yellow-white rhombs from the Styrian Erzberg. I once ID 8 different species. Three were pure dolomite, one ankerite and the rest mixing up within a single xtl."
The story goes on...!
Pavel Kartashov February 12, 2018 02:41PMDear Franz,
please try to make better photo from this or similar sample. It is very important for the explanation to dumb collectors, that real fresh ankerite is colorless or white without brownish hue. This brownish coloration is often the "main diagnostic property" for ankerite determination. I simply tired to explain again and again this simple fact.
Alfredo Petrov February 12, 2018 03:36PMAnd let's add that any analyses by SEM-EDS can't be considered "confirmed" either way, as SEM-EDS does not always do a good job with light elements like Mg, especially on non-flat surfaces, etc., so even some of the so-called "confirmed" ankerites aren't really confirmed. ;((
David Von Bargen February 12, 2018 03:48PMIt really is handled by the note on the ankerite page.
Note: Most (unanalysed) "ankerite" specimens are actually Fe-bearing (ferroan) dolomite (Mg>Fe), because of nomenclature changes. The majority of alleged "ankerite" photos here actually depict Fe-rich dolomite (except for a relatively few analytically confirmed ankerites). Even the old so-called "type locality ankerite" from Erzberg, Styria, would not quite fit the modern definition, with analysis showing (Mg 0.520, Fe 0.457, Mn 0.021) apfu
Reiner Mielke February 12, 2018 03:53PMHello Alfredo,
What you are proposing I think is too high a bar. I think we need to distinguish between absolutely confirmed and those that have supporting evidence confirming. If not then you would have to declare most of the minerals and photos in mindat as being unconfirmed. Maybe we need a partially confirmed category?
Jeff Krueger February 12, 2018 06:16PMThis is a really interesting and illuminating conversation.
Regarding the recatagorizing/renaming of unconfirmed specimens, would it at least be possible/make sense for the select few photos/specimens in the Ankerite Gallery>Photos of Ankerite>Ankerite page be confirmed specimens? Right now the gallery contains many beautiful and, I suspect, unconfirmed photos of brown, Fe-bearing (ferroan) dolomite (Mg>Fe). This might be misleading.
I mention this because I often refer to the gallery images as a reference tool. By having mis-identified specimens in the gallery the problem of mis-attribution may be perpetuated.
Ralph Bottrill February 12, 2018 09:56PMDavid
I'm not sure that note is sufficient to counter the huge number of ankerite locations and photos added, and doubt most casual users would check?
I agree if we could automatically replace all ankerites with "dolomite-ankerite" it would be a lot more accurate, then we just need to find all the analytically confirmed ones.
I never meant to criticise anyone for using the name, its going to take a generation at least for geologists, average collectors and other people to stop using the name, and nobody expects all curators or collectors to relabel all their 100-year old, non-analysed specimens.
Pavel, maybe its rare in Russia, but I have confirmed it in a number of locations in Tasmania, using microprobe, or calibrated SEM-EDAX on polished sections, and not due to incusions. If you get assemblages with coexisting dolomite and siderite you can usually find some, but rarely in big homogeneous crystals.
Great data thanks Franz, it would be great to have this formally added to Mindat!
Maybe we need a partially confirmed category?
Interesting thought Reiner though some would suggested it is either confirmed or not and what would constitute partial confirmation: hardness, colour, habit? Re colours, I have seen kutnahorite grading into true ankerite (and dolomite) and the colour is a very pale pinkish cream.
Reiner Mielke February 12, 2018 11:14PMI think partially confirmed would have to be restricted to analytical methods such as XRD, RAMAN, EDS, R.I., S.G. etc.specifying which were used. I would not allow tests such as hardness, blow pipe, colour, cleavage, habit etc. It would have to be decided by mindat what would qualify.
Krzysztof Andrzejewski February 13, 2018 12:29AMRE: Vik Vanrusselt : 1) How would one test this specimen to distinguish ankerite from dolomite?
- Alizarin red test
used by Huegi (1945) Friedman (1959) Warne (1962), Evamy (1963) Dickson (1965); I succesfully used version of Evamy, details:
Evamy B. D. - The application of a chemical staining technique to a study of dedolomitization.. Sedimentology 1963 vol. 2 no. 2.
Very useful solution in field research, very simply and cheap, could be quickly made in any chemistry laboratory. One drop on the mineral specimen and
calcite/aragonite without Fe2 + - turn red
calcite with a small amount of Fe2 + - turn pinkish purple
Fe-calcite - purple
dolomite without Fe2 + - clear
Fe dolomite - turn light blue
ankerite - turn dark blue
time of reaction - aprox. 1 minute and you have results. At home you keep bottle in cold & dark in kitchen fridge. Tested many times during the field trips to Nowa Ruda/Slupiec in Sudetes Mountains, Lower Silesia (Neurode/Schlegel in Schlesien), Poland, where ankerite and calcite are main carbonate minerals in hydrothermal veins in carboniferous series.
P.S. Colour of ankerite is absolutely not diagnostic.
Edited 1 time(s). Last edit at 02/13/2018 12:31AM by Krzysztof Andrzejewski.
Reiner Mielke February 13, 2018 01:19AMThe alizarin test is qualitative and could only distinguish close to end member compositions. It may be useful if you confirmed ankerite at a particular occurrence using EDS first and then correlated it with a consistent alizarin color for rapid bulk analysis like in rock sections. However for confirming an unknown composition it would not be a reliable method.
Ted Hadley February 13, 2018 02:40AMI collected a bunch of "ankerite" material from San Benito County, CA, USA, about 25 years ago. I proved it all to be ferroan dolomite with SG alone. If one is careful, the results are quite reliable. That was my introduction to the confusing world of the mineral Ankerite.
Martin Rich February 13, 2018 03:18AMThe same problem here at the Mitterberg disdrict, Salzburg. Most collectors named such white rhombohedrons as ankerite. Paar, W. (1983) analysed it as ferroan dolomite with 25 % formular units of CaFe(CO3)2. Seems right in the most cases, but it is not impossible that ankerite sensu stricto could occurre at this locality.
@ Christian: I have some smaller samples like this one in the photo below. If you have time and interest, I can send you some for analysis.
Alfredo Petrov February 13, 2018 05:17AM"What you are proposing I think is too high a bar. I think we need to distinguish between absolutely confirmed and those that have supporting evidence confirming. If not then you would have to declare most of the minerals and photos in mindat as being unconfirmed. Maybe we need a partially confirmed category?"
No, Reiner, I'm not trying to establish a high bar; quite the opposite - I'm appealing for more tolerance in naming when a mineral name also has a parallel "fuzzier" meaning (as in the cases of ankerite, piemontite, freibergite, numerous tourmalines and amphiboles, etc etc), or is traditionally used as a "field term", which is why I was pointing out how extremely difficult it would be to insist on 100% confirmation. Let's face reality: the vast majority of mineral specimens are never going to be analyzed. Even the ones that are "analyzed" are often insufficiently analyzed to determine the species (especially difficult ones like tourmalines and amphiboles). In such cases I think it's sufficient just to put a warning notice on the species page (as Mindat has already long had on the Ankerite page) explaining the complications involved. Admittedly just my personal bias; other Mindat members will probably want to be much stricter.
"Maybe we need a partially confirmed category?" That is pretty much the category that the majority of rare species photos in Mindat already fall under.
As for the comments about colour in ankerite, the only true ankerites I'm personally familiar with, the ones from the Cerro Sapo carbonatite in Bolivia, are light green! I see no reason why ankerites should be brownish (as most "ankerites" in collections are), unless partially weathered and oxidised. Pure Fe2+ salts are generally colorless.
Krzysztof Andrzejewski February 13, 2018 09:15AMRE: Reiner Mielke Wrote: The alizarin test is qualitative and could only distinguish close to end member compositions.
- - so to distinguish TRUE dolomite from TRUE ankerite for example :) :) :)
In Poland every mineralogy student must have good ability to distinguish colors & shades but in this case, ankerite color is so deep (dark) blue, that with no doubt you can confirm this mineral. ONLY in other cases - when the sample turn light blue, you can start this discussion as above, so I would spare myself energy and exclude first all what is possible without wasting money.
About colours of ankerite - in Nowa Ruda I have found mostly milky ones rhombohedrons, turning yellowish or even brownish after lying for a while on dumps, but also sometimes very nice greenish (fresh after recovering) botryoidal forms. And with very similar mineralisation to Kladno in Czech Republic like on the mindat links added by Vik Vanrusselt above.
Before Silesia (polish - Slask, german - Schlesien, czech - Slezsko) became Polish, German mineralogists made chemical gravimetric analysis of the ankerites of this area:
Eisenhuth 1902 ( Walbrzych/Waldenburg)
MgO - 15.01%
FeO - 9.69%
CaO - 29.69%
CO2 - 44.84%
Hoehne 1944 ( Walbrzych/Waldenburg - Graf Hochberg Grube)
MgO - 11.58%
FeO - 12.20%
MnO - 2.80%
CaO - 28.60%
CO2 - 44.55%
Hoehne 1944 ( Walbrzych/Waldenburg)
MgO - 12.80%
FeO - 12.42%
MnO - 1.00%
CaO - 28.50%
CO2 - 44.60%
Hoehne 1944 (Wolibórz/Volpersdorf - Nowa Ruda/Neurode area)
MgO - 12.60%
FeO - 8.63%
MnO - 0.90%
CaO - 29.92%
CO2 - 43.10%
As attachment a map accroding to M.Ptak
Krzysztof Andrzejewski February 13, 2018 02:06PMFranz Bernhard Wrote:: none of these minerals is ankerite by modern definition.
the sample No2 ( Graf Hochberg Grube) FeO - 12.20% MgO - 11.58% too?? So now I'm lost totally....
Alfredo Petrov February 13, 2018 03:15PMKrzysztof, Mg is a much lighter element than Fe, so a mineral with "MgO - 11.58%; FeO - 12.20%" has a lot more atoms of Mg than Fe and is therefore dolomite, not ankerite. Don't worry, this same error seems to have been made with lots of so-called "ankerites". And exactly the same error has been made many times by people checking chromite-magnesiochromite.
Jeff Weissman February 13, 2018 05:00PMA composition that is borderline ankerite/dolomite - Ca(Mg0.5Fe0.5)(CO3)2, has 28.02 wt% CaO, 10.07 wt. % MgO, and 17.95 wt. % FeO, with the balance being CO2. The weight of FeO has to be more than 1.783x the weight of MgO for the mineral to be ankerite.
Jeff Weissman February 13, 2018 05:09PMI corrected Doan's mine - https://www.mindat.org/loc-53016.html
Updated entry for "ankerite" from Wheatley Mines - https://www.mindat.org/loc-4075.html
Updated entry for "ankerite" from Phoenixville Tunnel - https://www.mindat.org/loc-51015.html
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