Vote for your favorite mineral in #MinCup26! - Sphalerite vs. Anorthite Both zinc ore Sphalerite and calcium plagioclase feldspar Anorthite have perfect cleavage that will split cleanly if you hit them with a hammer, but only one can claim a larger split of your votes!
Cooper & Stanley (1990): "Minute, poorly crystalline brownish aggregates on quartz have been identified (BM(NH): XRD) as leadhillite-susannite (B. Yates Collection)."