safflorite

Why hasn't Safflorite been subdivided into different species based on the Co/Fe ratio? There are 5 different varieties found in the cobalt camp and the compositional variation is at least as great as the Arsenopyrite-Glaucodot-Alloclasite series.

I am somewhat outdated with respect to this, but Arsenopyrite is Monoclinic and Glaucodot is Orthorhombic so why would they be divided according to Co:Fe ratio? Is it that the ratio is directly related to the structure? At what %Co does it become orthorhombic? Is Alloclasite then the Monoclinic equivalent of Glaucodot ( chemically the same but structurally different)?
At any rate Safflorite and Lollingite are both orthorhombic so is the dividing line at 1:1 for Co:Fe? What about the Nickel? what effect does that have on this division?

Well, it seems you're not the only "outdated" one, Reiner; now I'm confused myself. I see we do list glaucodot as orthorhombic here on Mindat, although we link it to the "Arsenopyrite Group" (which usually implies some similarity of structure). But when I click on Arsenopyrite Group, glaucodot is not one of the species listed as a member! And when I consult other references, glaucodot appears listed with the same monoclinic space group as arsenopyrite. So I give up... until someone else, with better knowledge of structures, steps in to clarify. (NB: Note that mineral Groups are still rather incomplete on Mindat; it's one of the projects currently being worked on.)

With regard to Safflorite-Lollingite, you are right - the dividing line depends on the Co:Fe. Whichever one dominates (in number of atoms per formula unit, of course, not analyzed weights), decides the name.

Alfredo:

The paper you want is H. Yang and R.T. Downs (2008), Crystal structure of glaucodot, (Co,Fe)AsS, and its relationships to marcasite and arsenopyrite, American Mineralogist 93, 1183-1186.

The structures of arsenopyrite, allocalsite and glaucodot are based on the small, simple orthorhombic cell of marcasite. However, the [S2] dumbbell anion of marcasite is replaced by an asymmetric [AsS] in arsenopyrite etc, and the As-S pairs can order in various different orientation patterns, which produce bigger unit cells with the a and/or c repeats increaesd by integer multiples.

The alloclasite cell is the same size as that of marcasite, subject to size effects involving Co/Fe and As/S. Call this a 1a x 1c structure.

Arsenopyrite is differently ordered, and is 2a x 2c.
Glaucodot has a 3a x 1c ordering pattern.

Alloclasite has Co > Fe, and hence is a polymorph of cobaltite. It is not an exact analogue of arsenopyrite, because of the different As-S vs S-As ordering patterns.

Glaucodot has Co and Fe nearly equal; Yang and Downs suggest that the ideal formula is actually (Co0.5Fe0.5)AsS, or, given cation ordering, Co(Co0.5Fe0.5)Fe(AsS)3. Hence, it can be a differently-ordered polymorph of either arsenopyrite or allocalsite/cobaltite. They confirm that it is orthorhombic Pn2(1)m with a = 14.16Å, b = 5.65 Å, c = 3.32Å approx.

Thanks, Andrew, for the detailed response. I've added the Yang & Downs reference to the respective species pages.

Thanks for the reference but I am not sure in simple terms how it all relates. Is Glaucodot then a polymorph of Arsenopyrite? or an entirely separate species? How much overlap if any, is there in terms of Co:Fe ratio between Arsenopyrite and Glaucodot? What is the maximum amount of Co that the Glaucodot structure can accommodate before it becomes Cobaltite? I assume then that Alloclasite is a totally separate species and not an end member of a series, is that correct? Can we ignore Ni in all this?

Reiner:

1. Polymorphs (unless the structure difference is a very minor polytypic variation) *are* distinct species. Graphite and diamond are polymorphs of each other, as are calcite/aragonite/vaterite. Glaucodot is a completely distinct species from arsenopyrite or alloclasite.

2. Species are distinguished on the basis of chemistry and structure, not composition gaps. So even if there were a complete solid solution between FeAsS and CoAsS in a single structure type, the compositions with Fe > Co would have one species name, and the compositions with Co > Fe would have another, and both could be valid species. In practice, different structures seem to occupy distinct regions in this composition space: the arsenopyrite structure is stable with Fe >> Co, alloclasite with Co >> Fe, and glaucodot in the middle. Since there is some evidence of Co-Fe ordering in glaucodot, it should probably be an exception to the simple "change species name when the predominant cation changes" principle, but this is not absolutely confirmed yet, as far as I know.

3. Technically, glaucodot with Fe > Co is a polymorph of (a hypothetical Co-rich) arsenopyrite, while glaucodot with Co > Fe is a polymorph of (a hypothetical Fe-rich) alloclasite.

4. The details of exactly what the composition ranges of these minerals are, whether there is overlap, etc, have not, to my knowledge, been mapped out yet. More research required.

Hope that helps!

Hello Andrew,

That helps me a lot, thank you, but it leaves me wondering about the group designation. Mindat says that glaucodot is a member of the arsenopyrite group? Maybe I am not clear on what constitutes a group, but if monoclinic arsenopyrite can be in the same group as orthorhombic glaucodot then wouldn't monoclinic alloclasite be in the same group?
I find the idea that alloclasite is a polymorph of Cobaltite interesting. Analysis of "Cobaltite" from the Cobalt camp, in addition to large variations in Co & Fe can have large amounts of Ni but the amount of Ni in Alloclasite is very limited, wouldn't a true polymorph have to have the same compositional range?

Reiner

Only for the dominant element. Minor substitutions can differ between polymorphs.

Glaucodot is not a member of the Arsenopyrite group.

A few days ago, Jim, we had it erroneously linked to the arsenopyrite group, but someone correced that already. (Lots of other groups still need reworking though.)

I corrected it. The groups definitely need work, but I'd hold off a while before doing anything.

It is my understanding that cobaltite is the end member of a series with gersdorffite but that may have changed since I learned that. Nevertheless analyses done on Cobaltite and cobaltian gersdorffite from Cobalt, Ontario showed a continuous composition ranging from CoAsS to (Co 0.24 Ni 0.67 Fe 0.09)AsS. (CM,1971,VII,Pt.1, pg.171). On the other hand the maximum amount of Ni found in alloclasite was about 0.13. Can you still call Ni a minor substitution?

EDIT!!!! - This post was NOT made by ME - Reiner used my computer!! I deny all knowledge!



Edited 1 time(s). Last edit at 10/23/2009 12:11PM by Maggie Wilson.