We measured a microstructure of hydrothermal Au by EBSD and EDS which contains locally a small amount of Ag. The pure gold areas are not separated as grains but obviously linked to large angle grain boundaries only, see the attached image. Has anybody seen a similar microstructure or can explain the occurence of (secondary) Au?
What impurities other than silver might be present?
In several places it looks as if the pure gold areas might be in cellular colonies (discontinuous precipitation of lamellae of Au and another unknown minority phase containing silver) growing away from high angle grain boundaries with the orientation determined by the grain away from which the colony grows.
Are you sure it is single phase? The simplest explanation is that the grains rich in Ag are a second phase.
This is a natural gold particle collected by a gold washer in Finland. Therefore it is hard to say anything about the history. We assume a typical hydrothermal formation (big grains, twins). Beside trace elements which were under the detection limit for EDS but certainly existent no other element could be detected by EDS.
The Ag-containing part is a solid solution, but is this then another phase? Well, I defined it as another phase in the phase map (red Au, green AuAG) but surprisingly we do not see any phase boundary.
In my opinion it will be hard to see any phase boundary at all. Both Au and Ag are fcc and form continuous range of solid solutions across the phase diagram. AuAg phase can be considered a phase by itself as it differs from pure Ag and Au. As history of the particle is not definable it is hard to say more. Hope it helps.
In fact you are right, but in the EDS map you can see a well defined step from one phase to the other. It is not only an artifact. The questions are, does it make sense to discuss it as seperate phase (chemistry says so), and if yes, how this can happen or can be interpreted? No doubt, it takes some time to distinguish both isostructural phases by EBSD (and EDS).
and... These are the raw data, i.e. no result finishing has been applied.
You are right as well and so is the chemistry. As I have no further info on the golden particle the gold washers have found it is hard to say how it came about. Maybe you should try to modify the idea of the hydrothermal formation of the golden nugget or particle in question. Maybe some friction between golden particle and silver rich particle occured during the hydrothermal formation at some stage?
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