A minor, tierce de picardie - when a piece in a minor key ends a section with a major chord, in bar 23(c major) The tonality is sometime unclear because the ground bass uses chromatic notes
key changes to related keys happen frequently
the key changes happen in the following bars,
bar 14 e minor, bar 16 g major, bar 21 c major, bar 23 a minor, bar 27 e minor
the music returns to A minor from bar 28.