Scarlatti Sonata in d minor, K 213

This Scarlatti Sonata is another piece off Bela Fleck's Perpetual Motion Album. The sonata is played by banjo and mandolin. The piece is in a slow simple meter, possibly four-four. The piece definitely seems more like the first movement to a sonata because it is not long enough and does not have all three movements. Halfway though the piece, the key changes from the minor to the major key. The key change sounds like it goes to D major and not to the relative major key of F major. The piece begins with the banjo playing quarter notes, sol-do-me-sol. When the mandolin comes in on the second measure it takes the quarter notes over and the banjo plays on the and of the beat. The mandolin plays do-fi-sol-me while the banjo plays fa-me-re-do-ti-do. The first phrase is five measures long and ends in a perfect authentic cadence. The next phrase is also five measures long and ends with another authentic cadence. This noticeable melody in this phrase is the banjo playing sixteenth notes, starting on do and going down the d minor triad for two octaves, do-sol-me-do-sol-me-do-sol. This pattern is repeated a half step lower and then a third time another half step lower. Next, the banjo merely plays the opposite of these, beginning low and going high. The rest of the minor section, before the piece modulates, sounds a lot like a huge cadential extension. The reason why I say this is because it keeps sounds like it will have a cadence but it just keeps going on. The melody in the major key is still played by the banjo and does a cute little do-re-do-sol pattern that keeps repeating the neighboring non-chord thing on different starting pitches. The section slyly creeps back into the minor. During this section of major and minor harmony is done by the mandolin where it plays the same rhythm as the banjo a third away for a nice major harmony so that when it switches to minor the key change is apparent. The piece ends with a perfect authentic cadence.
The thing I enjoy most about this piece is that the banjo has the melody throughout the entire sonata while the mandolin plays the accompanying part. I enjoy this because far too often mandolins are featured and other instruments like the banjo don’t get so much melodic glory. I like the key change and the first two phrases on the first half of the song, but I am confused as to the actual form of the piece.