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Mineralogical ClassificationLepidolite

14th Apr 2006 01:22 UTCJolyon Ralph Founder

What's current status of Lepidolite, how should it be categorized in mindat.org ?

Jolyon

14th Apr 2006 01:44 UTCJim Ferraiolo

According to the "Nomenclature of Micas", Rieder, et al. (Can Min.1998), lepidolite is a series name defined as "trioctahedral micas on, or close to, the trilithionite-polylithionite join, light micas with substantial lithium".

14th Apr 2006 08:45 UTCAnonymous User

Ok, so I am still a bit confused as to the nomenclature. Is lepidolite, a variety of the trilithionite-polylithionite series, or a synonym for the trilithionite-polylithionite series, or a distinct species series of the mica group (~lepidolite = trilithionite-lepidolite-polylithionite series), in which case this latter arrangement would indicate that lepidolite will be a recognized species in the not so near future? Not likely, so it seems most probable that Lepidolite should be classified as a synonym for the trilithionite-polylithionite series.

Also on the topic of Micas, would not the muscovite species variety Lithian Muscovite equate to a muscovite-trilithionite-polylithionite series, and likewise permissible would be muscovite-manganoan trilithionite-polylithionite series.

Not to get off topic, but another related question is why don't we have a classification for Mica Group, or Feldspar Group in the database, even though the species descriptions refer to the groups by name? There is yet a Tourmaline Group, so I am wondering if there are others group classifications in need of input, or what the criteria is for group name inclusions??

:-O

14th Apr 2006 12:52 UTCJim Ferraiolo

Lepidolite is 'redefined' a series name for trilithionite-polylithionite. It is not a synonym for the series.

Muscovite is a dioctahedral mica, while polylithionite, trilithionite,annite, etc. are trioctahedral micas. I 'm not sure if a series is possible between a dioctaghedral and a trioctahedral mica. This division is based on the number of octahedral cations (Li,Fe²⁺,Fe³⁺,Mg,Mn²⁺,Mn³⁺,Al,Ti,Cr,V,Zn) per formula unit. Dioctahedral micas has less than 2.5pfu. Trioctahedral micas have 2.5 cations per formula unit.

14th Apr 2006 16:06 UTCJim Ferraiolo

Scott,

Lepidolite is 'redefined' as a series name. It is no longer considered an individual species, nor is it a synonym for the series. It is the series name

Muscovite is a dioctahedral mica, whereas the lepidolites are trioctahedral micas. I don't think (but I am not positive)that a series between the two is possible. Dioctahedral micas have less than 2.5 octahedral cations per formula unit. Trioctahedral micas have 2.5 octahedral cations pfu. BTW, octahedral cations in micas are Li, Fe²⁺, Fe³⁺, Mg, Al, Ti, also Mn²⁺, Mn³⁺, Zn, Cr, V.

14th Apr 2006 16:12 UTCAlan Plante

The Mica Group subcommittee report of the CNMMN (cited by Jim above) was not very clear about series relationships - left a number of questions begging. Reading between the lines, it appears that the "lepidolite series" encompasses polylithionite and trilithionite *plus* undefined phases within the "lepidolite realm" as they definied it (see Jim's note.) So "lepidolite" is a bit broader in definition than just the polylithionite-trilithionite-? series.

Probably the best way to understand why "lepidolite" has been retained is to read what the report says about this:

"This report also includes series names intended to designate incompletely investigated micas that are to be used by field geologists or petrographers. Such names (e.g. 'biotite') are defined only in some series, thus in fact sanctioning a practise that is already common. Assigning a name to an incompletely investigated layer silicate may be risky, and it should be preceded by at least optical examination. Once such material has been studied in detail, end-member names should be preferred, with or without modifiers and suffixes. Series names are not to be associated with varietal modifiers."

So basically they decided to let terms such as "biotite" and "lepidolite" remain for use in cases where detailed study has not determined the actual species present - which is frequently the situation in the field and petrography (as opposed to mineralogy) lab. A side-effect of this is that rockhounds can continue to use the terms for specimens that they cannot identify down to the species level - so rather than "push the envelope" on a visually IDed specimen they can label it "Lepidolite Series" - which is more scientifically honest than flipping a coin and saying it is either trilithionite or polylithionite (which it might not even be in any case...)

It is definitely no longer considered a species name - nor is it ever likely to be again. Ditto "biotite", "glauconite", "illite", "phengite" and "zinnwaldite" - now all series names per the Mica subcommittee report cited, and now all subject to the *caveates* in the paragraph quoted.

KOR!

Alan

14th Apr 2006 22:10 UTCAnonymous User

Great, thanks Alan for some clarification on the term Lepidolite. I must confess it is nice to have a loose term for identification, esp. in the field, or in front of a computer - for those of us without access to a laboratory. So is it acceptable to call a mica, which is obviously not the more mafic biotite, but which has not been properly analyzed - a muscovite, or would the group term 'mica' be the correct usage. Lastly, are we going to have a classification in mindat.org for the Mica Group?

14th Apr 2006 23:38 UTCJim Ferraiolo

Thanks, Alan.

15th Apr 2006 04:27 UTCAndrew G. Christy Manager

Slight correction to Alan P's post: ""Trilithionite" is one of the exceptions to the rule - it is not an end member name, yet it has been allowed to stand as a species name".

By the definition of 'end-member' in Frank Hawthorne's Canadian Mineralogist paper of 2002, trilithionite IS an end-member.

It is not always possible to produce a charge-balanced (electrically neutral) formula by having only one type of species at each type of site in a crystal structure. The Hawthorne definition allows precisely ONE type of site to contain two differently-charged species in order to make up an intermediate charge total. The numbers of the two species do not have to be integers in whatever formula unit is chosen.

K(Li1.5 Al1.5)(OH)2 is fine as a trioctahedral end-member.

There are many, many similar examples out there, particularly among complex oxides and silicates.

15th Apr 2006 16:33 UTCAlan Plante

Hi Andrew

Thank you. - I am not familiar with the Hawthorne paper, which came out well after Reider, et.al. in '98. I was going by what Reider and company published - which was that trilithionite and wonesite were allowed species names for non-endmember compositions. If this is no longer the case, for one or both, the word needs to get out to a broader audience - such as us hicks in Stickville! :~}

Cheers!

Alan

15th Apr 2006 16:35 UTCAlan Plante

Hi Jim

I guess we were both typing at the same time. Your post was not up when I started, and was there after I submitted my reply.

I chuckled to myself: "It figures, Jim did in a few words what I took a bushel to get to." :~}

Cheers!

Alan

15th Apr 2006 22:50 UTCJim Ferraiolo

Yep, Alan, that's me. Short, but not necessarily precise. If you want a copy of Hawthorne's end-member paper, let me know.

16th Apr 2006 13:17 UTCAlfredo Petrov Manager

A long time ago on another thread I pointed out that "arsenian renierite" must be an end member (and thus deserve a new name) for the same reason. There wasn't any acceptance for that idea at the time, but I think Andrew's post on trilithionite is a similar situation.

16th Apr 2006 15:07 UTCAlan Plante

Thanks, Jim - I' appreciate receiving a copy of Hawthorne's paper. - Alan

23rd Apr 2006 17:33 UTCAlan Plante

Hi Andy

Jim F. kindly forwarded me a copy of Frank's paper. I now see exactly what you mean about charge-balancing allowing for dual site occupancy - as opposed to there being a substitutuion list. You - and Frank - are, of course, right.

Too bad Frank wasn't on the Mica Subcommittee to keep them on the striaght and narrow! :~} (And, yes, the whole long list of mineral species composition statements could do with some proverbial fine-tooth chemical combing...)

KOR!

Alan

23rd Apr 2006 23:35 UTCJim Ferraiolo

Alan,

glad you enjoyed the paper. Unfortunately, Frank can't be everywhere, but his paper should be applied to all discussions, and end-member compositions be considered by the Commission.

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