In "World of Ice" (ep. 9), there is music that starts off with something that sounds like a harpsichord, after Himari enters a hidden door in the library (around 6:11). Strings (mostly playing pizzicato, I think) and a clarinet come in later. Normally I'd assume that this might be something original created from the anime — for instance, I've heard stuff that could pass as classical music in the Monogatari Series.
However, I've noticed pieces that were clearly not original being used in this series, such as the beginning of Mozart's Piano Sonata K. 331 (when Himari is still in the normal section of the library), or the famous theme from Dvořák's 9th symphony (in "Because I love him"). Thus, it seems plausible that this is not original music.
What is the music playing here? I tried to look up the soundtrack or the episode on the fan-made wiki, but neither gave a full listing. Is this original music, and if not, where is this music from?
Update: If I'm not mistaken, the same track appears in the first half of ep. 21 ("The Door of Fate We Choose"). This would suggest to me that it's more likely to be original music, but I'm not even completely sure right now if it's the same music.
Update 2: I tried to search Musipedia, which largely is a classical music identification site, for the beginning of this section of the soundtrack and came up with no results that seemed correct. I also listened to this section of "World of Ice" a second time and it struck me as less "Baroque" and "classical" than I initially thought. Original music seems more likely, but I'm still having trouble obtaining the title of this music. (It's definitely nothing listed on this Tumblr.)