A rare mineral compound; a technical term used in mineralogy for a specific crystalline substance.
This appears to be a mineralogical term, likely named after a person (possibly 'Harst' or similar) or a place, following the standard naming convention for minerals which often use '-ite' as a suffix (like 'magnetite' or 'hematite').
Minerals get named in really interesting ways—sometimes after scientists who discovered them, sometimes after where they're found, and the '-ite' ending signals 'this is a mineral'—there are thousands of them with equally specialized names!
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