The Publication Manual of the American Psychological Association (APA) refers to archival materials as "archival documents and collections." The manual includes several examples, but also states that the general format for citing archival materials can be modified to be more or less specific as appropriate for different types of collections, keeping in mind the purpose of the citation is to direct the reader to the cited source.
See the Publication Manual of the APA, especially the preamble to section 6.11 and section 7.10.
