Combining words in Elvish

[Galin]

"shine (with white/silver light)" - sil/thil (I'm not sure how the language division works here)

As far as sound goes anyway, a version of unvoiced th existed in early Quenya but it merged with -s- in that language. Feanor did not agree that this should be allowed and would have called his mother Therinde not Serinde.

Back in Etymologies Tolkien imagined SIL- and THIL- as variant roots, but even if this held true for the later scenario, generally speaking words that include -th- are Sindarin, although since we have silivren in Sindarin too, for example, along with Ithil...

... rather let's say words with -th- are not (later) Quenya