There's absolutely nothing in any of Tolkien's works to suggest such a mineral.
The opening paragraphs of the Tale of Years for the Third Age (Return of the King Appendix B) so include the following statement:
Moria for long remained secure, but its numbers dwindled until many of its vast mansions became dark and empty.
It's possible that whatever source you recall was based on that, but the actual reason for this dwindling is never expanded on by Tolkien.
Looking at the Mithril element, even in the earliest conceptions, and at the point at which the name and idea of Mithril enters the stories (in History of Middle-earth 7), we read:
The dwarves tell no tale, but even as mithril was the foundation of their wealth so also it was their destruction: they delved too greedily and too deep, and disturbed that from which they fled.
This is almost identical to the account in Fellowship of the Ring, with the exception of the words: "Durin's Bane", but despite that it establishes that the connection between Mithril and the abandoning of Moria was that the dwarves had disturbed and fled from something (the early plot outlines in History of Middle-earth 6 indicate even then that a Balrog was in Moria).
This one must be attributed to fan-fiction or other non-canon works.