How does slate differ from a phyllite? How does phyllite differ from a schist? How does schist differ from a gneiss? (E)

Slate is a fine-grained metamorphic rock that is formed from the low-grade metamorphism of shale or mudstone. It is characterized by its strong cleavage, which allows it to be split into thin, even layers known as "slaty cleavage". This makes slate an ideal material for roofing, chalkboards, and flagstones. The mineral composition of slate mainly includes quartz, clay minerals, and small amounts of feldspar, calcite, and other minerals. Phyllite, on the other hand, is formed through the continued metamorphism of slate, resulting in a slightly coarser-grained rock. Its distinctive feature is a sheen caused by the presence of tiny mica flakes, which are not found in slate. Phyllite's physical properties lie between slate and schist, displaying more pronounced foliation than slate but not as distinct as schist.

As mentioned earlier, phyllite is an intermediate metamorphic rock between slate and schist. The main difference between phyllite and schist lies in their mineralogy and foliation. Schist is formed at higher temperatures and pressures than phyllite, which results in the growth of larger, visible mica crystals and other sheet silicate minerals like biotite and chlorite. This gives schist a shiny appearance and a more pronounced foliation that is easily visible to the naked eye. The individual minerals in schist can often be identified without using a microscope, unlike slate or phyllite, where the minerals are typically very small or submicroscopic.

Gneiss is a high-grade metamorphic rock formed from the further metamorphism of schist or other rocks under higher temperature and pressure conditions. The main difference between schist and gneiss is the degree of metamorphism and the appearance of their foliation. Gneiss has a banded or "gnarled" appearance consisting of alternating light and dark bands, known as "gneissic banding". This banding is a result of the segregation of mineral grains into bands based on their composition, mainly due to the higher temperatures and pressures experienced during its formation. Gneissic banding consists of bands of felsic minerals (like quartz and feldspar) and bands of mafic minerals (like biotite and amphibole). In contrast, schist has a more homogenous appearance and lacks the distinct banding of gneiss. Instead, its foliation is characterized by the alignment of sheet silicate minerals like mica.