Briefly explain why the rock you mentioned have crystal so small that they can only be seen with a microscope?

The rock may have undergone rapid cooling, preventing the crystals from growing to a larger size. This could be due to the rock being formed in a volcanic eruption or being subjected to high pressure and temperature conditions deep within the Earth's crust. The rapid cooling process does not allow enough time for the crystals to grow to a visible size, resulting in microscopic crystals.