Limonite after Pyrite is formed when Pyrite begins to decompose through chemical weathering and the iron present in the mineral starts to rust. Limonite is any impure hydrated iron oxide. Limonite is mostly clay but also may contain phosphates and silica. Once the rust has started, Limonite slowly starts to form. Limonite does not have one specific shape or crystal structure. Limonite pseudomorphs (takes the external shape) of other minerals such as Pyrite. Most Limonite has a brownish red color and leaves varying streak of color as the composition of Limonite could be made up of a different amount of phosphates or silica.