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Ferrisicklerite formula

Posted by Vik Vanrusselt  
Vik Vanrusselt March 10, 2012 05:03PM
The formula for Ferrisicklerite is now (Fe3+,LiMn2+)PO4

Shouldn't there be a comma between the Li and the Mn2+ ?

Formula written differently here:

I'm a beginner in chemistry, how come there are different ways to write formulae?

Alfredo Petrov March 10, 2012 06:42PM
Vik, I know nothing about the actual structure of this mineral but, looking only from the point of view of electrical charge balance, Mindat's formula (yes, without the comma) balances, and the one in Bideaux's book that Rruff links to does not balance. Mindat's version, without the comma, just means that any Fe(III) that gets replaced by Mn(II) and Li, must have equal amounts of Li and Mn(II) (with combined charge of 3+) to keep the charge balance. Now if any Mn(III) or other elements enter the structure then we have a different story; I'm judging only by the way the formula is currently written.

Edited 2 time(s). Last edit at 03/10/2012 06:49PM by Alfredo Petrov.
David Von Bargen March 10, 2012 08:09PM
If you do a search for ferrisicklerite on the Am Min crystal structure database

you will see two metal sites M1 and M2

M1 has an occupancy of 0.33 Li1+ atoms (rest are unoccupied/vacancies)
M2 has 0.66 of Fe3+ and 0.33 of Mn2+ (etc)

Structurally the formula should be written
Alfredo Petrov March 10, 2012 11:37PM
David, is the ratio Fe:MnLi fixed at 2:1 (in which case it would make sense to replace our current formula with the structural one you show above), or variable (in case we can leave it with comma)?
Pedro Alves March 11, 2012 12:10AM
something like: Li1-x (Fe3+x ,Mn2+1-x)Σ=1,00PO4
Where Li=Mn2+ and Fe>Mn

Pedro Alves
Van King March 11, 2012 12:13AM
Ferrisicklerite and sicklerite are names for leached/oxidized alterations in triphylite and lithiophilite, respectively. Lithium is leached and iron and manganese are oxidized in a couple sequence. The theoretical end-members, heterosite and purpurite, have no Lithium and only iron and manganese in 3+ valence state. Usually, less than half of the Lithium and iron plus manganese were affected and ferrisicklerite and sicklerite were varieties of triphylite and lithiophilite, respectively. Because these species are now varieties, there has been little concern whether the alterations were slight or extensive. The structure of the RUFF specimen indicates it should have been called heterosite.

Best Wishes, Van King
Marco E. Ciriotti March 11, 2012 12:45PM
Paper on the structural study of the lithiophilite-sicklerite series is in the pipeline of the Canadian Mineralogist.

▪ Hatert, F., Ottolini, L., Wouters, J., Fontan, F. (2012): Crystal chemistry of olivine-type phosphates: a structural study of the lithiophilite–sicklerite series. Canadian Mineralogist, 50, (in press).

Edited 1 time(s). Last edit at 03/11/2012 12:47PM by Marco E. Ciriotti.
Uwe Kolitsch March 12, 2012 09:04AM
Formulae of ferrisicklerite and sicklerite changed to conform to those given in the IMA list.
Alfredo Petrov March 12, 2012 12:59PM
Now the sicklerite formula is different on the Sicklerite page and Ferrisicklerite-Sicklerite Series pages, and is the same as Lithiophilite.
Van King March 12, 2012 02:01PM
Yes. Sicklerite should be (Li1-x, <>x)({Mn2+}1-x,{Mn3+}x)(PO4) to indicate the coupled leaching/oxidation progress.

Best Wishes, Van King

Edited 2 time(s). Last edit at 03/12/2012 02:03PM by Van King.
Uwe Kolitsch March 12, 2012 03:54PM
We could use Van's formula and the IMA one as the simplified formula.
Pedro Alves March 12, 2012 04:25PM
and for the Ferrisicklerite case?

Pedro Alves March 12, 2012 04:47PM
Hello again,
just some questions:

is this formulae correct/applicable (for ferrisicklerite)?
Li1-x (Fe3+x ,Mn2+1-x)Σ=1,00PO4
as an attempt to balance the charges.
If yes, the Mn content is directly related with Li. And, the more Li is leached, the lower Mn we have. Resulting in ,when total Li leached, Heterosite.
In the most part of the cases, i believe, Mn is not leached (only Li). The 'loss' of Li affects the balance of the charges, which is solved by the Mn2+ oxidation to 3+. Is that correct?

Comments and corrections are welcome.


Edited 3 time(s). Last edit at 03/12/2012 05:04PM by Pedro Alves.
Van King March 12, 2012 08:24PM
I've reviewed the literature or triphylite-lithiophilite alteration as well as having made my own study using quantitative wet chemical analyses. Neither Fe nor Mn seems to be leached from triphylite or lithiophilite in the alteration of triphylite-lithiophilite to heterosite-purpurite. The alteration of Fe compared with Mn is another story as it requires an extra 0.5 electron volt potential to raise the oxidation state of Mn (2+ -> 3+) compared with iron. The general formula would then be more complicated with mixed valences of both Fe and Mn as alteration may be greater on the exterior of a phosphate nodule compared with the "center". It is not necessary to show a summation when the subscripts are already 1. The important implication that has not been widely reported is that if triphylite or lithiophilite is corroded as well as oxidized, not the same as leached of Li and oxidized in situ, the resulting secondary phosphates may be more Mn-rich than their parent. I made a study of Fe:Mn:Mg:Zn ratios in triphylite at Newry, Maine to see if there were a change in the elemental ratio within a zone within the pegmatite. The results were that there was no change in the ratios, at least within 1 mole %, and the experiment supported Moore's (1974) assertion that elemental ratios did not change within a granite pegmatite zone. The two transects were made from the upper to lower contact (4 m +/-) as well as laterally 30 meters. The generalized formula of sicklerite and ferrisicklerite would resemble each other except that the former would show Mn while the latter would show Fe and an intermediate composition would be represented by the presence of both Fe + Mn. There are a few locations where there are very different Fe:Mn ratios sometimes with a change of species name, because the difference was large. I would believe that the different ratios were for specimens from different zones within the granite pegmatite.

Best Wishes, Van King
Jeff Weissman April 04, 2012 06:56PM
Mindat is listing both sicklerite and ferrisicklerite as approved minerals with grandfather status. On the purpurite and heterosite pages it is implied that they are varieties, so what is the resolution to this contradiction?

Van, by the way, I didn't know that purpurite and heterosite where "theoretical" :-D, re your message of March 10, 2012 07:13PM
Richard Gunter April 04, 2012 07:56PM
As additional information to Van King's work on the triphylite from Newry, the Tanco Mine has an interesting suite of lithiophillite compositions, which change markedly within individual zones of the pegmatite. The plotted lithiophilite percentages, from the 1998 IMA Field Trip Guidebook B6, has lithiophilite (mol %): in the Border zone of 50% to 70%; in the Wall zone of 55% to 70%; in the Aplitic Albite zone of 60% to 65%; in the Lower Intermediate zone of 65% to 85%; in the Upper Intermediate zone of 75% to 95%; in the Central Intermediate zone of 75% to 90% and in the Lepidolite zone of 92% to 100%. The lithiophilite is not altered at Tanco so there are no daughter phases, even though the accompanying ambygonite alters to montebrasite coated by secondary apatite.
Jeff Weissman June 29, 2012 06:06AM
Still no action on this -

Mindat is listing both sicklerite and ferrisicklerite as approved minerals with grandfather status. On the purpurite and heterosite pages it is implied that they are varieties, so what is the resolution to this contradiction?
Peter Haas July 01, 2012 04:48PM
Some remarks:

If you really prefer using variables in subscripts, please note that something like (Ax, B1-x) is wrong, because (1-x) B cannot replace x A - except x = b/(a+b) (where a and b are the charges of A and B, respectively), in which case the use of a variable does obviously not make any sense, or the charge is balanced by coupled substitution, in which case you'll have to obscure the formula further by introducing even more variables.

In other words: if you are using variable subscripts, you have to omit the comma: (AxB1-x) is correct. Note however, that this has exactly the same meaning as (A, B).

Now, if you still want to keep the variables, please also note:
(1) There is no need to keep the brackets in an expression such as (AxB1-x). In order to avoid confusion, the brackets should be omitted - unless you want to indicate that A and B are occupying a specific site, in which case they are better replaced by curly brackets though.
(2) You have to define x unambiguously, as this is not self-explaining. This condition (here, 0 <= x < 0.5) is an integral part of the formula.

Now, let's have a closer look at Rui's last proposal. Note that I have added the missing condition for x and also left out the comma, for the reasons explained above:

Li1-x(Fe3+xMn2+1-x) (PO4), 0 <= x < 0.5

This can be modified by applying simple mathematical logics (in fact, the following transformations don't have anything to do with chemistry):

First, the brackets can be omitted:

Li1-xFe3+xMn2+1-x (PO4), 0 <= x < 0.5

Note that I keep the bracket for the anion. This is to make clear that "PO4" is a single anion. Now, some of you might want to add that this is obvious anyway. Note however, that it is much less obvious when the mineral contains amphoteric elements, i.e. elements that can (and often do) occur both as cations and as constituents of oxoanions: in those cases, the bracket must be used in order to make clear what is meant. Instead of using the brackets when we (the specialists) think they are needed and leaving them out when we (the specialists) think they are not, we have decided to use them consistently.

Second, lithium and manganese have the same subscript and can be combined. Note that I need brackets now in order to indicate that the subscript applies for two cations (those within the brackets) and not only one (again, this is a mathematical convention, not a chemical one):

Fe3+x(LiMn2+)1-x (PO4), 0 <= x < 0.5

All those notations have exactly the same meaning. However, I still prefer to keep formulas as simple as possible and this implies avoiding variables in subscripts when they are actually not needed. If you could follow the discussion down to this point, you will easily notice that the formula written above has the same meaning as the one I add below:


We really should stop questioning our notations of chemical formulas every time we are asked a question about the meaning of a particular notation or about seeming discrepancies between our formulas and those found on different websites. Understanding chemical formulas of complicated minerals requires a chemical knowledge that exceeds what is taught on high schools. There is no way to simplify chemical formulas so as to be understood by anybody - pretty much in the same way as there is no way to guarantee that a phrase in a foreign language will be understood by anybody, no matter how simple it is written.

Consequently, there will always be questions from people who just don't understand the formula, no matter which notation we preferrably adopt, and ask us because they want to learn. Trying to adopt a simplfied notation in a fruitless attempt to avoid
such questions in the future will only raise more questions in the long run. More importantly however, it will also not resolve the respective person's problem; this still has to be taken care of by an appropriate explanation (which, in some cases, will be rather lengthy). There also will always be people who do not understand the formula and do not want to learn, but just spot a difference between our formula and the formula on another website and think that this alone qualifies as an error. Although I generally dislike smarties, this behaviour has to be encouraged as well, since there is always a finite chance that they did really find an error.

This said, we still need to answer one of Vik's questions:

It is the composition of a mineral (or, more generally, of a chemical compound) that is defined, but not the formula itself - a slight but important difference. Formulas are an important tool in a chemist's daily work and as such, they have to be flexible (in fact, if we have to be strict, we're mostly using systematic compound names, not formulas). The only requirement for a formula is that it must properly reflect the composition, but there are no definitions whatsoever concerning the level of detail or the notation. Even the IUPAC has surprisingly few recommendations on the formulas of inorganic compounds. They generally recommend to use sum formulas (i.e. formulas with a zero level of structural detail), because they are easily standardized (on the other hand, they also are horribly impractical for more complicated compounds). They also recommend (very amusing, actually) to write the individual elements in alphabetical order. There are many IUPAC rules for organic compounds (where they are sorely needed, not only to ged rid of the flood of superfluous trivial names), but they are entirely irrelevant for minerals.
Jeff Weissman July 16, 2013 04:07PM
Help me out here

sicklerite and ferrisicklerite are listed on their respective pages as Grandfathered

yet, on the purpurite page is given "Intermediate alteration products were once thought to be valid minerals, but they were merely part of a continuous process of alteration: Ferrisicklerite and Sicklerite"

Is there an IMA decision on this, otherwise, need to correct the purpurite and heterosite pages
Alfredo Petrov July 16, 2013 06:24PM
I'm not familiar with the specifics of this case, but I'm going to get into my usual pedantic rant here: As a generality: the term "Grandfathered" is not (contrary to what most systematk collectors seem to think) a synonym for "Valid", neither does use of the term "grandfathered" confer validity on anything, it merely means the species was described before the existence of the IMA (1959), and that the IMA took no action on it, and that it was at that time "generally accepted" to be a valid species. The term "grandfathered" is grammatically a most inopportune word, because it implies that some sort of action was taken, when in reality no action was taken. I wish mineralogy would banish this stupid and unscientific word from its lexicon.

One implication of this is that a "grandfathered" mineral ceases to be considered a "valid" species when any significant number of people express doubts about its validity, because then it's no longer "generally accepted" and so no longer fulfills the criteria for "grandfathering". Some mineralogical list compilers believe that all doubtful minerals should be kept on the valid species list until a formal discreditation is published. Zoologists, luckily, don't follow the same policy, or else there'd still be a section on unicorns in zoology texts. :-D
Jeff Weissman July 16, 2013 08:48PM
Alfredo, In general I'd agree with you on Grandfather status not being equated to valid. However, Grandfathered does mean generally accepted to be valid unless proven otherwise.

In the case of sicklerite, the mineral does appear to be generally accepted as valid, in light of recent papers, for example see a discussion on the crystal-chemical structure of sicklerite that Marco mentions near the start of this thread , now published. Certainly the authors of this paper are knowledgeable in what makes a mineral valid, and did not suggest that sicklerite is not valid - hence by default sicklerite (and also by analogy ferrisicklerite) should be considered Grandfathered and generally accepted as valid, as these names continue to appear in and be used in the peer-reviewed literature as "valid" minerals. That said, perhaps a proposal should be made to invalidate these two as intermediates, until then, some consistency needs to be had in the MinDat database.

Hence my argument and suggestion that the entries in the purpurite and heterosite pages be modified, at least for internal consistency. I will do so accordingly.

Edited 1 time(s). Last edit at 07/16/2013 08:54PM by Jeff Weissman.
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