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Accurate classification of “ferrosaponite”
  
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Joan Viñals




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PostPosted: Apr 10, 2009 18:02    Post subject: Accurate classification of “ferrosaponite”  

Accurate classification of “ferrosaponite” from Pedrera Guixeras, Can Súria, Maçanet de la Selva, La Selva, Girona, España.

In July 2008 a material consisting of nice epimorphs of yellowish-greenish, clay-like substance were characterized by us in the University of Barcelona, as requested by Mr. J. Fabre. The standard methods of XRD coupled by EDS were used. Structural analysis indicated a mineral of the smectite group and chemical composition showed calcium/iron/aluminum silicate. The smectite group is complex because the possibility of multiple solution series as well as the possibility of interlay with other sheet-like clay minerals. Roughly, the group is chemically defined as:
A0.3D2-3[T4O10]Z2.nH2O
were
A: Na,Ca,Li
D: Fe,Mg,Zn....
T: Si,Al
Z: OH,F

Nontronite and Ferrosaponite are two members of this group and the usual formulas are:

Nontronite: Na0.3Fe2(Si,Al)4O10(OH)2·nH2O

Ferrosaponite:Ca0.3(Fe 2+,Mg,Fe 3+)3(Si,Al)4O10(OH)2·nH2O

On the basis of our chemical – presence of Ca and absence of Na- and our structural results the material was classified as “ferrosaponite”.

Recently, a great expert in clay minerals, Dr. Stephan Kaufhold of Technical Mineralogy, GEOZENTRUM (Hannover, Germany), was strongly interested in this material and provided much elaborated structural analysis, including additional IR and also XRD of etylenglycol treated samples. The results confirm a mineral of smectite group and a chemical composition similar to the obtained for us. However, the detailed IR studies are more consistent with a nontronite-type phase, in spite that it doesn’t contained Na.

In the smectite group, the criteria for nomenclature don’t consider the exchangeable cations (Na,Ca, etc) and for this reason the material from “Can Súria” must be classified, at the present, as calcian nontronite. Note that this criteria is different that the approved for zeolites in which the content of exchangeable cations define a species. According also by Dr. Kaufhold, actually one would have to rename these minerals by similar criteria than zeolites, but they have not done yet.

In conclusion, the material must be re-labeled, at the present, as calcian nontronite, but it is possible that in the future the mineralogical name changes to a new species, in accordance of the cation substitution.

Please accept my excuses for the first and not completely accurate characterization.

Joan Viñals
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Jordi Fabre
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PostPosted: Apr 11, 2009 17:11    Post subject: Re: Accurate classification of “ferrosaponite”  

I have already changed all my labels and where them said Ferrosaponite now says "Nontronite calcian".
I am glad to know what is exactly the material from Can Súria as well as to be blessed with an analyzer so prepared as Joan Viñals that instead to live in a kind of "scientific limbo" share with us their scientific work and explain to us in detail the process by which a species becomes correctly identified after further analysis.

Jordi
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PostPosted: Apr 11, 2009 18:34    Post subject: Re: Accurate classification of “ferrosaponite”  

Jordi, I'd suggest "Ca-nontronite" (after the same fashion as clay mineralogists do with the rectorites, etc.) rather than "calcian nontronite", because "calcian..." implies a *variety* of the species nontronite, and this is not in fact just a variety, if Ca is dominant.
Saludos,
Alfredo
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Joan Viñals




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PostPosted: Apr 12, 2009 04:02    Post subject: Re: Accurate classification of “ferrosaponite”  

Alfredo

Thank you for your comments. I agree that in the specific world and literature of clay minerals, names such as “Ca-rectorite” can be used to distinct analogous phases with different exchangeable cations. However, I believe that this “nomenclature” is not officially approved by the IMA/CNMNC. For this reason I don’t recommended their use in general mineralogy and particularly in the collecting world. The name Ca-nontronite could generate additional confusion in this particular affair, because it could resemble an approved species.

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Joan Viñals
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alfredo
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PostPosted: Apr 12, 2009 08:50    Post subject: Re: Accurate classification of “ferrosaponite”  

Hello Joan,
I agree there could be confusion, but "calcian nontronite" would generate even more confusion, because it already has a different meaning: nontronite with part of the Na replaced by Ca, as in "(Na,Ca)..." But there isn't Na, so it is not true to say "calcian nontronite". Ideally, to be completely clear, we should say "Unnamed calcium analogue of nontronite", but that's rather long, so anyone have a shorter suggestion?
Saludos,
Alfredo
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Joan Viñals




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PostPosted: Apr 12, 2009 14:28    Post subject: Re: Accurate classification of “ferrosaponite”  

Alfredo
At the present, the criteria of nomenclature in the smectite group don’t consider the exchangeable cations. So, in this case, the name calcian nontronite only would signify a calcian “variety”. The name “unnamed calcium analogous of nontronite" would presume a new species through a non approved criteria. However, I agree that if a future the criteria was changed, the “unmamed calcium analogous of nontronite" could be the name correct prior to the definitive approval of the specific mineral.
Saludos
Joan Viñals
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marvsT/Nminerals




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PostPosted: Apr 28, 2009 21:25    Post subject: Re: Accurate classification of “ferrosaponite”  

Jordi, I enjoy reading this forum when I can. It contains a lot of interesting information. My question is this: There is a smectite group mineral that occurs with zeolites from India, Oregon and Colorado. It has been called "nontronite", but I do not know if anyone has analyzed any of it. The material described in this topic seems to be very similar but is lighter in color. The zeolite occurrences are visually identical: black and microcrystalline with a greenish streak. Do you know if anyone has ever provided identification or documentation of this? Have you ever seen it? Geodes are presently being found near Nasik, Maharashra, India completely filled with this mineral.
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Ed Huskinson




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PostPosted: Apr 28, 2009 23:58    Post subject: Re: Accurate classification of “ferrosaponite”  

Hey guys!

In the geological community, when one is logging RCR (Reverse Circulation Rotary) drill cuttings or even sometimes core, the term nontronite is used when light green "snotty-looking" clay is encountered, particularly if in volcanic terrane.

It's just a field term, and not all geologists and/or mineral exploration companies use it. There's no time to glycolate, X-ray or XRD samples, but most of us know what it means when the term is encountered on a borehole field log. If/when more detailed study is called for, then the material might be subjected to advanced analysis techniques.

I suppose that I shouldn't be surprised to see an in-depth discussion of nontronite, given the nature of this forum. Again, it's just a field term sometimes used by geologists posting geology onto field sheets.

Thanks,

Ed

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