Illustration of four major types of of reserve forms of nutrients

Close but not quite exactly "stored in the mineral" .... That particular arrow that you mention is indicating the pathway of mineral (or ionic) form of an element becoming plant unavailable through a process of chemical precipitation (ie formation of a solid crust.
Without going into deeper explanation here, what would be important to remember from that particular slide is that for each nutrient there are processes that make it available to plants, and those that make them unavailable to plants.

Thank you!