Full Idea
Unlike some of his contemporaries, Aquinas does not think that there is a 'spiritual matter' that angels or disembodied souls have as one of their components, but rather that they are separated forms that configure no matter at all.
Gist of Idea
Aquinas saw angels as separated forms, rather than as made of 'spiritual matter'
Source
report of Thomas Aquinas (Summa Theologicae [1265]) by Kretzmann/Stump - Aquinas, Thomas 10
Book Reference
'Shorter Routledge Encyclopaedia of Philosophy', ed/tr. Craig,Edward [Routledge 2005], p.39
A Reaction
'Separated forms' sounds like the modern concept of abstract entities, meaning that souls and angels exist in the way that platonists believe numbers exist. How else might Aquinas have understood them?