The statement is partially correct. While the transition zone does lie within the mantle, it separates the upper mantle from the lower mantle.

Here's why:

* Upper Mantle: This layer extends from the Earth's crust down to about 410 km (255 mi). It contains minerals like olivine and pyroxene.

* Transition Zone: This zone lies between 410 km and 660 km (255 mi and 410 mi). It's characterized by significant changes in mineral structure due to the increasing pressure. Minerals like olivine and pyroxene transform into denser forms.

* Lower Mantle: This layer extends from 660 km to the Earth's outer core (about 2,900 km). It's mostly composed of silicate minerals.

So, the transition zone is the boundary layer between the upper and lower mantle, marking a significant change in mineral composition and structure.