Argentopyrite crystal structure

Referenza:
• Yang, H., Pinch, W.W., Downs, R.T. (2009): Crystal structure of argentopyrite, AgFe2S3, and its relationship with cubanite. American Mineralogist, 94, 1727-1730.

Abstract:
The structure of argentopyrite, AgFe2S3, was determined for the first time with single-crystal X-ray diffraction. In contrast to the previously reported orthorhombic symmetry, our data show that argentopyrite is monoclinic with space group P1121/n (non-standard setting) and unit-cell parameters a = 6.6902(2), b = 11.4497(4), c = 6.4525(2) Å, beta = 90.2420(8)°, and V = 494.26(3) Å3. Similar to cubanite (CuFe2S3), the structure of argentopyrite is also based on approximately hexagonal close-packed S atoms, with cations ordered over one half of the tetrahedral sites, forming corner-shared AgS4 and FeS4 tetrahedral sheets parallel to (001). The two structures differ chiefly in the linkage between the two adjacent tetrahedral sheets and the ordering patterns of cations within a tetrahedral sheet. Topologically, the structure of argentopyrite can be obtained by a displacement of a tetrahedral sheet in the cubanite structure along the (a/2 + b/6) direction relative to the sheet beneath, giving rise to a cluster of four edge-shared FeS4 tetrahedra in argentopyrite, as compared to two in cubanite. There are two distinct Fe sites (Fe1 and Fe2) in argentopyrite, rather than only one, as in other MFe2S3 sulfide minerals (M = monovalent cations). Together with published Mössbauer data, we suggest that there exists some degree of Fe2+-Fe3+ order-disorder in argentopyrite, with Fe2+ favoring the more distorted Fe2 tetrahedral site. Argentopyrite appears to possess all the features proposed by Putnis (1977) for a high-temperature ordered form of cubanite.

In addition to the trilling, twins and blocky singles of cubanite, there is also an acicular habit. Mark Mauthner probed one such xl from Sandon and to everyone's surprise it was Argentopyrite at one end and cubanite at the other end. This abstract leads me to believe there is a series between High Cubanite and Argentopyrite and makes me wonder if the acicular cubanite is in fact High Cubanite? Does any one have a PDF of this paper they could send me? Thanks.
rwmw@telus.net



Edited 2 time(s). Last edit at 11/06/2009 07:51PM by Rob Woodside.

Bob sent me their paper today The second to last sentence is, "Additional research is needed to verify whether the HTO phase of cubanite really possesses the argentopyrite-type structure." I bet it does!!!

Rob,

is the acicular habit of cubanite the potential HTO phase. I have found it in St. Adnreasberg (all "acicular millerites" I have analyzed from there were cubanite) and at St. Marie. The xx are easily large enough for single X study...

Frank

Hi Frank,
Yes that is my belief, but as Yang and Downs say, "additional research is required." I think Mark published his Argentopyrite/cubanite observation in his paper on Sandon (in MR?) and I don't know what has become of his specimen. In addition to your material, the ROM has some from Greyrock, B.C. (as does Lloyd Twaites, the source of my former specimen) and some from St Marie aux Mines on my old Arsenic specimen they have on display. Both of these are in epitaxi on tetrahedrite. I first noticed this at the GSC in Ottawa where they have isolated needles from Henderson II, Quebec. I hope you can follow this up and will give you any help I can.

Hi All,

I have also published some observations on silver-rich cubanite from the Silvana Mine in an article on my website. [www.davidkjoyceminerals.com] look in the "Articles" section for the articla on cubanite. From that same locality, we also found acicular cubanite crystals coated with tiny acanthite crystals. There are some interesting SEM pictures in that article.

I also have acicular crystalsof cubanite from Grey Rocks, BC and the Advocate Mine, Newfoundland. I may have some acicular "cubanite" crystals on matrix from these localities available.

Regards,
David K. Joyce

Thanks Dave, that's a great article!!! So maybe Mark's argetopyrite observation was an acanthite contamination of the cubanite??? If that's the case then my belief was wrongly founded. Still one doesn't know until one looks.

Rob,

Not necessarily. All we observed were acanthite crystals encrusting acicular crystals of something. Could be cubanite, silver-rich cubanite or argentopyrite! Don't know what the substrate crystals are. I still have that specimen if anyone is interested in working on it.

David K. Joyce