i initially planned to draw out a diagram for this, then realized that i'd need to draw out several diagrams to explain everything i wanted to, including how magic is perceived by the society this setting is focused on and a more accurate representation of how it actually functions, and... well, let's get into it.
still no diagrams, sorry. but i AM going to elaborate on what exactly is up with how the elements are organized... and talk about a second set of elements, bringing the total to... either an uncountable amount, 2, or a secret third thing. it depends on how you look at it, really.
firstly, it should be noted that the classical non-Shadow elements are technically all the same type of magical particle vibrating at differing wavelengths. Light is the "basic" (read: most plentifully produced in nature) form, and the other 6 elements in question correspond to their own wavelengths. notably, this behaves somewhat similarly to... well, light and colors. as a consequence, the range of magic functions more as a spectrum than a small and neatly-organized set of elements... sort of.
in the case of spirits specifically, most (though not all) are constrained to absorbing and manipulating magic within a specific frequency band. while the exact range of this band will vary slightly between two spirits of the same "element" (using the classical system), most (though not all) will have a similar breadth and be clustered around the same center frequency. "Light-elemental" spirits are not as restricted by wavelength, but still have finite bounds within the "standard" spectrum of Light magic. there is a reason for these things that belongs in a separate post about spirit biology, because i still have to talk about Shadow.
Shadow magic, meanwhile, is a state of a different particle entirely. it, too, has a set of wavelengths that could correspond to a set of elements, ones that could similarly be grouped into 6 broad "named" elements. they are not, however, named, due both to lack of knowledge and lack of necessity.
Shadow magic is, comparatively, extraordinarily rare. it is generated, in varying frequencies, by all living things, but when it interacts with Light magic, the two bind together to create.... something else i'll talk about in part 3 because this post is ALSO getting long. regardless, this interaction ends up removing most of it from the world, due to the large amounts of Light magic generated by the sun. even places where light (visual kind) does not reach have low concentrations, as Light (magic kind) travels through most solid matter easily.
Shadow spirits form very rarely, as a result, and any that are not "true" Shadow-elemental cannot gather enough of the proper frequency of their band of Shadow magic to survive, and quickly dissipate if they coalesce at all. thus, the standards for determining the names of elements (how the magic of spirits of that "element" behaves) do not properly exist for Shadow magic.
incidentally, it also means Shadow spirits must live in highly-populated areas, though not necessarily populated by sapient beings.