Help identify this mite?
This appears to be a bird mite (or possibly rat mite) in the genus Ornithonyssus of the parasitic family Macronyssidae.

 Credit: user Aewills on bugguide.com 
Distinguishing between species is difficult.
In fact, according to idtools.org, identification of Mesostigmata to family or lower can usually only be accomplished only if the specimen is an adult female.
Blaine Mathison mentions the following features useful for identifying these mites to lower taxonomic levels under magnification [source: bugguide.com]:

chelicerae shape
dorsal plate breadth and degree of dorsal coverage
spiracle present between legs
genitoventral plate width
sternal plate shape and presence of setae

You can find existing answers to posts about bird mites on Bio.Stackexchange below:

Bug Identification: Tiny Bugs
Tiny white very hard cell bug! Please help to identify!