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I am writing an essay on the origin of life and I was reading an article on hydrothermal vents. (Peptides by Activation of Amino Acids with CO on (Ni,Fe)S Surfaces: Implications for the Origin of Life).

It appears that (Ni,Fe)S has played an important role in protein synthesis, but I wanted to know more about the substance its self. It is a compound, a group of chemicals? Or is it simply nickel, iron, and sulfur individually? I'm not very clued up when it comes to chemistry and I haven't come across that sort of notation before.

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When two materials precipitate at the same time, you'll often get an extremely intimate mix of the two. I'm assuming that the notation you've got here means a precipitate consisting of a mixture of both nickel (II) ions, iron (II) ions, and sulfide ions, or a mix of nickel and iron sulfides (NiS & FeS).

Iron and nickel (II) have similar ionic radii and charge densities, so it should be possible to create crystal domains containing both metal ions interchangeably, in any ratio.

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Aesin's answer is almost there. In geosciences and mineralogy, the notation (A,B)X means that both elements A and B exist in the same crystal structure in one mineral. This is not a mechanical mixture of iron and nickel sulfides, nor it is an amorphous mixture of ions.

The elements are sorted according to their abundance, and whatever is in parentheses should stoichiometrically sum up to whatever subscript there is. Thus (Ni,Fe)S has more Ni than Fe, and together they sum up to 1 atom(s) per formula unit. An example of what the formula can be is Ni0.8Fe0.2S or Ni0.55Fe0.45S

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