Slate is the first evolved state of a metamorphosed shale or mudstone when subjected to low-grade regional metamorphism following the Barrovian sequence. This sequence outlines the stages of increasing degrees of metamorphism, beginning with the evolution of shale. Shale will initially be metamorphosed into slate and with increasing heat and pressure and further into phyllite, schist, and gneiss. Direct heat and pressure cause clay minerals to recrystallize into very fine-grained micaceous minerals, such as biotite and chlorite. These minerals adopt a well-developed parallel and fissile foliation called slaty cleavage. Slate can often be mistaken as shale as they are both fissile, fine-grained, and typically are similar in colour with a dull lustre. They differ by their formational processes and composition, as slate is crystalline, and its foliation is due to the alignment of micaceous minerals from compressional deformation. This makes it slightly more durable, whereas shale's lamination is due to the clastic compaction of organic materials, giving it a clay-like texture. Additionally, slate and phyllite also share similar characteristics, such as slaty cleavage. Phyllite has undergone a greater degree of metamorphism which typically creates coarser micas, a more metallic sheen, and a less perfectly flat cleavage surface, differentiating it from slate (see Phyllite samples).