@Andre The "k" form is from the Greek (in Greek it's spelled with kappa). The "c" form is a Latinized spelling (which seems a bizarre thing to do, since the Romans used k specifically for spelling Greek words containing kappa - they'd have been likely to use k). Both spellings are common these days; my preference is to spell it the Greek way (transliterate it more "directly"), I don't see a good reason to make it 'c'.

Hmm. On reading about it some just now, it looks like using "k" for Greek words was only used for some words in Roman times, so maybe they would have used 'c'. The Romans had 'c', 'q' and 'k' all giving 'k' sounds...

Okay, so I have to drop my earlier assertion of oddness. 'k' is from the original Greek, 'c' would presumably be the spelling if we take the word via Latin. I still lean toward 'k', personally (but as I said, both seem to be acceptable).

Thank you.