The image below is the definition of 一員 in the 明鏡国語辞典
what is this ─ヰン?
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These characters show the spelling of the word according to the old spelling rules (歴史的仮名遣い). In this specific case, it means that the word was originally spelled as イチヰン/いちゐん in kana.
I do not know about hiragana and katakana use in 明鏡国語辞典, but in 大辞泉 katakana is used in this place when showing the old spelling of 漢語 (words derived from Chinese) and hiragana when spelling 和語 (native Japanese words).
You can typically find this information about the format of dictionary entries in a section labelled 凡例.