According to the Grimoire and the Destiny app after killing 2500 enemies in a locations(i.e ocean of storms or ishtar sink) you will receive extra materials(i.e. spin metal, or spirit bloom) from a material node in that area. I find that I still only receive one and occaisionally receive two. Is this a bug? or should I only occaisionally be receiving an extra material? if the latter is so then the text should be changed to state that
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Occasionally I would open a chest a receive engrams and around 8 to 12 material resources unique to the planet I'm on. I never thought it was attributed to kills though. I believed it was random chance. Aside from those I would come across chests with more items than usual such as uncommon items like ship schematics.