I am doing some research on the algebra of relations (Tarski/Givant axiomatization), and I notice that in a proper relation algebra, an element (relation) is an equivalence element if and only if it is transitive and symmetric. I am wondering as to why "reflexive" is not included. Is there even a possible sentence or set of sentences in the language of relation algebras such that in a proper relation algebra, an element is reflexive if and only if it satisfies it (the sentence or set of sentences)? I am trying to figure out if there is a way to formalize reflexivity in the theory of relation algebras. It would be preferable if there were an equational axiom that could capture reflexivity.