Thomasclarkite-(Y) is a rare mineral which was known as UK-93 until 1997, when it was renamed in honour of Thomas H. Clark (1893–1996), McGill University professor.
The mineral is one of many rare-earth element minerals from Mont Saint-Hilaire.
The only reported occurrence is in an alkalic pegmatite dike in an intrusive gabbro-nepheline syenite.
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