In addition to well-known soft /s/ and hard /k/ pronunciations, 'c' is sometimes pronounced /ʃ/ e.g. special, liquorice? Is there any rule?
Most examples of "c" sounding like /ʃ/ come from palatalization, a process which turns a /sj/ sound into an /ʃ/ sound and a /zj/ sound into a /ʒ/ sounds. This process is still going on in English, and there are some words which are pronounced both ways. For example, nausea can be pronounced either /nɔːzɪə/ (predominantly U.K.) or /nɔːʒə/ (predominantly U.S.).
In the spelling of these words, you can generally see the original /sj/ sound. For example,
have an "iV" or an "eV" after the "c".
Then there are words like licorice that just have exceptional spellings. There's no rule for these.
I do not know of a specific rule (yet), but I can think of four examples of this in modern English which begin to form a pattern:
So far I have noticed a few trends among these words:
- In all of these examples, the word originates from an old form of French (according to etymonline)
- Google disputes this for 'luscious", but instead suggests it comes directly from 'delicious'
- In all four words above, the 'c' is followed by an 'i' or 'e', both vowels which cause 'c' to produce a fricative sound (as illustrated in this answer to a related question)
- in three of the four words, the 'c' is preceded by another vowel, and in 'luscious' the 'sc'is pronounced as a single consonant, and is also preceded by a vowel
- In all four examples, the French word from which it originates uses a 'c' in the same place, whereas words which do not follow the pattern (such as 'device') use an 's' instead in their originating French.
Clearly there is some sort of trend here, though I think more exploration is needed to determine an actual rule. One example which does not follow the pattern described above is 'decide'. I'm not sure yet how it differs from the others but I suspect syllable stress may be relevant.