But how do you account for the A and D in the seventh? I disagree with the analysis that this chord is a type of dominant chord or augmented sixth, because it never serves the functions of those chords. Also, since Scriabin was into quartal harmony, and since the chord is spelled using diminished, perfect, and augmented fourths, I'm inclined to view and analyze it as a chord built of fourths (a quartal chord).
Yes, it is a quartal hexachord, but it is really more than just that. The so-called “mystic chord” is but one of many Scriabin used in which a whole-tone dominant chord is suspended over a tonic root. Scriabin uses this chord not only in the vertical sense, but in the horizontal sense as well. Although neither whole tone nor octatonic, the mystic chord contains elements of both. When arranged horizontally, the mystic chord has, as stated before, elements of both.
Jim Samson, in his book “Music in Transition: A Study of Tonal Expansion and Atonality” points out that the mystic chord fits in well with Scriabin’s predominately dominant quality sonorities and harmony as it may take on a dominant quality on C or F# (another tritone).
In modern Jazz notation, this can be understood as a fully extended dominant-thirteenth chord. The tritone root motion also resembles the 'tritone substitution' common in Jazz harmonic procedures. In Classical terms, what is tonic for common practice becomes dominant in Scriabin; and what is dominant becomes 'departure dominant'.
Nicolas Slonimsky compares the synthetic chord to a "typical terminal" chord of jazz, rag-time, and rock, the major tonic chord with an added sixth and ninth (if the root is C: C, G, E, A, D), and to Debussy's post-Wagnerian "enhanced" dominant seventh chords. If one moves the F# up to G and the A up to Bb, one is left with a familiar dominant seventh (added ninth).
So it really does take on the function and form of an ALTERED dominant seventh in a similar way that (in classical period terms) a French sixth chord is altered (flat ^5).
I won’t argue that it is not a quartal chord (because in fact, it is), but I’m just pointing out that the scope is a bit broader.