I am citing the index from Niketas Choniates’ book O City of Byzantium translated by Harry J. Magoulias. Obviously, Magoulias did not translate the index, he is the original author of that section of the book, so it would not be accurate to cite Choniates if I am referencing the index. As of now, I wrote this:
Harry J. Magoulias, “Index,” in O City of Byzantium, pp.425, 433.
Note that I had already written a long note for that book previously.
In your case, the publication could be considered two things, either the whole book or the index, if you consider the index something akin to a chapter.
No author is given for your index. Often the index is compiled by some helper of the editor (e.g. a student assistant) or some employee in the publishing house. We cannot assume that it was created by the translator (unless you have a source that verifies this). So if you want to cite the index in the way you would cite a chapter, you would have to cite it as an anonymous work, e.g. something like this (in the bibliography):
"Index." In O City of Byzantium, Annals of Niketas Choniatēs, translated by Harry J. Magoulias, 422-441. Detroit: Wayne State University Press, 1984.
and, in a footnote:
- "Index", pp. 425, 433.
But usually an index is not considered a chapter but a part of the back matter of a book. So it would probably make more sense to cite the book, e.g. in the bibliography:
O City of Byzantium, Annals of Niketas Choniatēs. Translated by Harry J. Magoulias. Detroit: Wayne State University Press, 1984.
and in a footnote:
- O City of Byzantium, pp. 425, 433.
