I think you have hit on an important point most flake and other graphite deposits are inhomogenous and therefore it is very important to assay the daylights out of them for each sample particularly if they come from various parts of the deposit. The beauty of the Albany deposit is that it was formed by a single cataclysmic event which was likely of very limited duration and so the deposit is likely to be very homogenous and thus confirmative purity testing will likely be addative and does not need to be repeated exhaustively.