sorok

Anybody has an idea why turkic kIrk "fourty" was loaned as sorok into
russian - I mean, the svabhakti vowel is normal, but what about the
palatalisation of the initial an why an s- ? I supposed it is due to
the second palatalisation, but in this case the turkic form should have
been xIrk , and not kIrk. Does anybody know a turkic language where
this k- is palatalised (it is surprising in front of a back vowel) ?
Any other explanation for the russian form (are there other slavonic
languages that dropped the old word for fourty ?).

Guillaume