muscovite mica with cassiterite from Xuebaoding, Pingwu County, Sichuan Province — one of the most celebrated mineral localities in China
the feathery golden crystals are muscovite, a potassium aluminum phyllosilicate (KAl₂(AlSi₃O₁₀)(OH)₂) with that characteristic platy cleavage. the dark metallic crystals at the center are cassiterite — SnO₂, tin dioxide, the primary ore of tin. cassiterite has been mined since the Bronze Age and the name comes from the Greek "kassiteros" meaning tin
what makes Xuebaoding specimens like this remarkable is the co-crystallization story: both minerals formed together in a granite pegmatite environment under very specific hydrothermal conditions. the mica grew outward in these radiating "feather" plates while cassiterite developed its adamantine high-relief crystals in the same fluid system. they're essentially a record of the same geological event frozen in two completely different mineral forms
the combination of silky mica and mirror-bright cassiterite in one hand specimen is genuinely hard to beat
