Gray white potassium feldspar and plagioclase are often difficult to distinguish on hand specimens. At this point, we need to carefully observe the twin crystal characteristics of these two types of feldspar, because plagioclase has clustered twin crystals. When rotating the hand specimen, we can see regular bright and dark clusters on the plagioclase crystal, while potassium feldspar is a card type twin crystal, manifested as two halves of crystals with different brightness levels.