Along with everything else that is going on we have been doing some background work on some phosphate minerals spurred on by a request by seller Modris Baum (LMB Minerals) and something he had discovered on a Brazilian hureaulite specimen.
Modris has been doing some investigation on the matrix of his specimen to try and determine the minerals it comprises of.
His investigation had lead him to a conclusion that the matrix was manganese rich rockbridgeite and with some interesting replacement of vivianite within - (could the vivianite be phosphoferrite? - ed).
A request was made to add manganoan rockbridgeite to the database, however when I checked Mindat this mineral term is a synonym of rockbridgeite. This usually means "not a different mineral", this despite Modris having a paper describing it, namely Redhammer et al, Acta Crystallographica C62, i24-i28 (2006).
So here we had a contradiction and it did seem strange, so I asked Anatoly Kasatkin for his opinion on this. Here is Anatoly's reply:
| As you know, rockbridgeite contains Fe in both 2+ and 3+ valence states occupying different structural positions. There is also a mineral frondelite which is Mn2+ analogue of rockbridgeite.
So, talking about Mn-rockbridgeite main question is the valence state of Mn. I've read the paper of Redhammer et al.(2006) you are refering to, and the authors, while they solved the problem of Fe2+/Fe3+ ratio through Mossbauer spectroscopy, clearly note the following regarding Mn: "One remaining point of question is the valence state of Mn, which cannot be fixed with the methods available to us. On the basis of structure formula calculations, we propose Mn to be in the trivalent state".
It means that if all Mn in quantities measured by authors (0.62 apfu) is 3+, we have Mn-rich rockbridgeite. However, if we suggest all Mn to be 2+ we'll have Fe-rich frondelite.This is what the authors confirm saying : "The sample studied here corresponds to an Mn-rich rockbridgeite (or iron-rich frondelite)".
As you see, these minerals having Fe and Mn in different valence states are extremely hard to be correctly IDed even with structural analysis.
Also, I should remark about correct terminology.
FERROAN and MANGANOAN always = Fe2+ and Mn2+
FERRIC and MANGANIC = Fe3+ and Mn3+
As you see, even the authors of the above paper (2006) made a mistake - if they suggest all Mn to be trivalent, the correct name of the paper would be "Manganic rockbridgeite:..."
Concluding, manganoan rockbridgeite is definitely NOT a separate species, it is a Mn-bearing rockbridgeite having Fe2+>Mn2+ in its structural position. If you have Mn2+>Fe2+ in the same position, you get frondelite. Frondelite and rockbridgeite form a solid-solution series in many localities. |
Thanks to Anatoly, to solve our problem was now quite easy, we added a new series to the database, frondelite-rockbridgeite.
You can read more detailed information about the determination work done by Modris in his item listing, referenced below. It is very interesting and very thorough.
The "phosphate rich" week is ending with the launch of Christian Rewitzer's Hagendorf special auction.
Christian has provided us with a very special selection of 54 beautifully photographed specimens, some rare and some from the newly identified group of phosphates derived from schoonerite etc., some from his private collection
Hagendorf Mine was closed in 1984 and nowadays it is getting harder to obtain even the more common species from this famous location
Like Modris (and a number of other sellers) Christian has added id analysis information to many of his listings - available under the "analysis" tab in the item description.
