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LocalitiesSnowbird Mine, Cedar Creek Mining District, Mineral County, Montana, USA

20th Jan 2017 21:02 UTCReiner Mielke Expert

Considering how rare ankerite is, has the ankerite from here been confirmed? http://www.mindat.org/loc-3879.html In other words is it Fe dominant or just ferroan dolomite? I don't have access to the references. Thanks

20th Jan 2017 21:46 UTCRichard Gunter Expert

Hi Reiner:

Ray Lasmanis (1977) says: "X-ray diffraction and atomic absorption analysis show that 20 percent of the magnesium positions are taken by iron, thus defining the mineral as ankerite." I do not know whether the definition of Ankerite has changed since 1977 but that is his data.

20th Jan 2017 22:05 UTCAlfredo Petrov Manager

That would not be ankerite by modern definition, just ferroan dolomite (like the majority of the world's so-called "ankerites").

This is actually a good illustration of a recurring problem: There are an increasing number of minerals that have, in actual practice, two definitions, a strict species definition, and a more fuzzy variant as used by field geologists, petrologists, and old label writers. Ankerite, piemontite, turquoise, native platinum... and probably many more. Most of the specimens with those labels do not fit the current definition. Redefining a species doesn't make the older more fuzzy usage go away. We just have to get used to the idea that not everyone is going to use species names with the same strict current definitions. So, concretely, the rock in question in this thread needs to be changed for Mindat's strictly mineralogical purposes to "ferroan dolomite", but that does not mean that the previous designation as "ankerite" was an error.

20th Jan 2017 23:14 UTCRichard Gunter Expert

Hi Alfredo:

There are several other examples from the literature. The Ankerite associated with my Scheelite from Nova Scotia was determined by XRD in a 1974 B.Sc. thesis. The thesis does not give any chemistry for the carbonate. Is this true Ankerite anymore? Can you determine modern Ankerite from Ferroan Dolomite by XRD?

21st Jan 2017 00:52 UTCReiner Mielke Expert

"Can you determine modern Ankerite from Ferroan Dolomite by XRD?" I would say no. You would need to determine the Fe/Mg ratio by chemical analysis.

21st Jan 2017 07:06 UTCJohn Mason Expert

I just use ferroan dolomite as a field term - on the basis that XRD time is limited so other material always takes priority.

21st Jan 2017 11:23 UTCReiner Mielke Expert

So is anyone going to add dolomite to the list of minerals found there and remove ankerite?

21st Jan 2017 19:21 UTCDoug Daniels

Just to play devil's advocate - can you be sure that it is actually dolomite (ferroan, iron-bearing, or whatever)? What about listing questionables, such as ankerite, as say "ankerite (r)", where the "r" would stand for "reported, not verified". Yeh I know, one more complication, and how many species could be added to a locality with such a designation?

21st Jan 2017 19:33 UTCUwe Kolitsch Manager

Did anyone check the other two cited refs. ...?

"Econ Geol (2004) 99:1727-1744; Econ Geol (1985) 80:394-409"

21st Jan 2017 20:33 UTCReiner Mielke Expert

"can you be sure that it is actually dolomite (ferroan, iron-bearing, or whatever)?" Well the analysis says it is:"X-ray diffraction and atomic absorption analysis show that 20 percent of the magnesium positions are taken by iron" so dolomite needs to be added to the locality regardless of the other references.

21st Jan 2017 21:36 UTCRalph S Bottrill 🌟 Manager

Ferroan dolomite has been added and ankerite marked unconfirmed until we check the othersrefs. I made it an XRD confirmation as we dont have AAS as an analytical method, it could be added to the method list, though its use is uncommon nowdays.

21st Jan 2017 22:08 UTCAlfredo Petrov Manager

I haven't read the Econ Geol refs (anyone here have access to them?), but even if they list "Ankerite", without an analysis, it probably still means Fe-bearing dolomite in modern terminology.

Ankerite, sensu stricto, is quite uncommon. The only ones I've ever seen myself are the pale green ones from the Cerro Sapo carbonatite dyke; all brownish "ankerites" in my experience turned out to be Fe-bearing dolomite. (Real brownish ankerites apparently do exist, I just never found any myself.)

21st Jan 2017 22:25 UTCRalph S Bottrill 🌟 Manager

I can check the Econ Geol refs but I suspect Alfredo is correct, most geologists use it either as a field term based largely on colour, or the old definition (>10%FeCO3).

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