Why is "cementerio" spelled with "n"?

Question

I am aware that cementerio has nothing to do with cemento. It comes from Latin coemeterium which comes from Greek κοιμητήριον, koimeterion = sleeping place, bedroom, dormitory.

I have not found any plausible explanation for the occurrence of the n in the Spanish (and Catalan) spelling. Where does it come from? Is there a more general tendency in Spanish to insert n while adopting Latin words, or is cementerio the only instance?

http://etimologias.dechile.net/?cementerio (the entries authored by the mysterious "Helena" are usually trustable :) ) says (emphasis by me) that

... jamás en la Edad Media las tumbas se cementaban. La forma además alterna con cimenterio desde antiguo. La gente era enterrada directamente en la tierra y los nobles y reyes en pétreos sarcófagos. Nadie ha estudiado realmente a qué se debe esa forma con la nasal.

So the influence of cemento to the spelling and pronunciation of cementerio is not plausible. However, I know that sometimes just a sound similarity of words might influence an alteration of one of them so I am still open to this option.

Answer 1

Spanish etymologist Corominas states that the origin of that middle -n- is due mostly to a prolongation of the nasalization produced by the previous -m-. He also states that the word could have been influenced by Latin caementum (Spanish 'cimiento', 'cemento') just by the similarity of how both words sound, but that cannot have been the only reason.

Note that you can find cimenterio, cimeterio and cemeterio in Spanish texts from the 15th to 17th centuries, maybe emphasizing the influence of cimiento. Quevedo used cimenterio in his works, while cimeterio and cemeterio were quite less used, but still used in the Diálogos familiares de la agricultura cristiana (1589) by Juan de Pineda as Corominas states.

As I have researched, that prolongation of the nasalization may not be something exclusive of the word cementerio. It also seems to appear in other words:

• Remanso comes from Latin remansum, and this from remanēre. In this case, the added -n- already happened in Latin, but still it can be considered a prolongation of the nasalization of the previous -m-.
• Ninguno comes from Latin nec unus (Spanish 'ni uno'). Again, the added -n- can be a prolongation of the nasalization of the initial n-. In fact, there was an intermediate form neguno (and therefore nenguno), according to Corominas. Note also that in this case, the added -n- could have also been influenced by the nin version of Spanish ni. But even so, nin itself can be considered as having a prolongation of the nasalization of the initial n- (or influenced by Latin non, but again...).

There are other cases of an extra -n- added to a word, but those are usually explained by contamination from other words, such as alondra, from Latin alaudŭla, probably influenced by golondrina (from Latin hirundo, -ĭnis). Both words refer to a kind of bird.