The double C is usually pronounced either /k/ or /ks/. It would be so much more useful if this diagraph (two letters that make one sound) could represent one sound, but English is a messy language and C is an especially messy letter. The usual guideline is that double C often has a /ks/ before the vowels I, E or Y. Otherwise the pronunciation is usually /k/.
Words with the CC before an I are usually from Latin. For example1: accident (accidentem), Occident (occidentem) and succinct (succinctus). Sometimes Latin, being an ancient dead language, does not have words for modern things. So New Latin words are created, for example: staphylococci is thanks to the Scottish surgeon and bacteriologist Alexander Ogston (1844-1929).2 Newly created English-Latin words do not always pronounce CC before an I as /ks/.
Words with CC before an E are also usually from Latin. For example3: accede (accedere), accelerate (acceleratus), accent (accentus), accept (acceptare), access (accessus), eccentric (eccentricus) and success (successus). A commonly mentioned exception to this pattern is the word soccer. This exception is important to note, because it is not from Latin. It is late 19th century university slang for their version of the game.4 Another exception is the word cappuccino (with a ch-sound) which is from Italian. So the guideline is that words with the CC before an E make a /ks/ if they are from Latin.
Words with the pattern CCO come from a variety of sources. Some are Latin, but many are not and they usually have a /k/ pronunciation. Example: accolade and accord are from French; accommodate is from Latin; accost, broccoli and piccolo are from Italian; raccoon and tobacco are from native American languages.
Fewer words have the pattern CCU. Generally these words are from Latin and have a /k/ pronunciation. For example5: accurate (accuratus), accuse (accusare), occult (occultus meaning: hidden or concealed), occupy (occupare), occur (occurrere)... An exception is the word hiccup, which is from the Danish word hikke.6
Few common words have the combination CCA. Most of these words are from French and/or Latin have a /k/ pronunciation. For example7: buccaneer (French: boucanier), impeccable (Middle French: impeccable), occasion (Old French: ochaison / Latin: occasionem), peccadillo (Latin: peccatum). An (non-Latin/French) exception is moccasin, a type of native American leather shoe made from deerskin or soft leather.
The only common word with a CC before Y is coccyx (coccyx), the bone at the bottom of the spine. So we can ignore Y in this rule.
Not counting acronyms, CC usually comes before a vowel. Of course there are exceptions! These include Bacchus, the Greek god of wine; zucchini, from the Italian plural of zucchino; acclaim from the Latin acclamare; accredit from the French ; accrue from the Old French acreue. So the guideline is, words with the CC before an I or E make a /ks/, if they are from the original Latin, otherwise the pronunciation is usually /k/. The variation CK will be covered next week.