Datapoint Norf'k/The voiced sibilant [z]
[z] is encountered in some words in free variation with [s] (e.g. sip, zip ‘a zip’) but there is no phonemic contrast. All the words with initial z in Buffett’s ‘Encyclopedia’ are peripheral.
Values 
[z] exists only in loanwords
- Example 24-226:
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sip, zipa zip