Indo-Iranian and Balto-Slavic languages are called satem languages, because in them the Proto-Indo-European palatovelars *ḱ, *ǵ, and *ǵʰ developed into sibilants or affricats, usually into [s]/[z]- or [ʃ]/[ʒ]-type sounds. In the centum languages (Hellenic, Celtic, Italic, and Germanic), the palatovelars merged with the plain velars *k, *g, *gʰ series.
Still, the Balto-Slavic languages share a word that does not follow that rule, in this word the PIE *ḱ changed into *k, and not to *s or *š as it would have been expected. Such a change is characteristic of the centum languages, but not the satem Balto-Slavic.
The word means 'stone, rock' and it developed so:
PIE *h₂éḱmō > Proto-Balto-Slavic *akmō, which gave the modern words:
- Latvian: akmens
- Lithuanian: akmuõ
- Proto-Slavic: *akmy > *kamy (with metathesis) which further gave Russian ка́мень (kámenʹ), Serbo-Croatian kȃm, kami, kȁmēn, Czech kámen, etc. Other IE languages treat the word according to their position within the centum-satem division: Proto-Germanic *hamaraz (English hammer), Sanskrit áçmā 'stone, rock', Old Persian asman- 'stone'.
Is there any plausible explanation for such an inconsistency of an otherwise consistent sound law? All the sources I found just say "unexplained centumization". But are there at least some theories why that happened, any attempts to explain that? Maybe the PIE source of the PBS *akmō had *k in the root and not *ḱ?