The sequence of increasing grades of metamorphism of a mudstone (shale, claystone, or siltstone) parent rock (protolith) is: slate (lowest grade), phyllite, schist, gneiss (highest grade). 1. Describe the change in grain size from slate to schist
2. How does the texture of phyllite differ from that of schist?
3. Why do you think that the micas (lat minerals) in these rocks are all parallel, r nearly so, to one another?