Final answer:
Mica is the mineral responsible for the strong foliation in a schist, as it forms visible crystals that align and contribute to the rock's flaky structure and sheen.
Step-by-step explanation:
The mineral responsible for the strong foliation in a schist is mica. Mica is the key component that contributes to the pronounced layering or foliation seen in schist. Schist is characterized by having medium- to high-grade metamorphism which promotes the growth of visible mica crystals, often muscovite or biotite, which align due to pressure and give schist its typical sheen and flaky structure. Other minerals such as quartz and feldspar can be present in schist but do not contribute significantly to the rock's foliation as mica does.