Question

Kyanite, andalusite and sillimanite

1) are index minerals formed from clay-rich parent rocks.
2) form from the same parent rocks under different combinations of pressure and temperature.
3) can transform from one to the other depending on changes in temperature and pressure.
4) have the same chemical composition.
5) all of the above

Answer 1

Kyanite, andalusite, and sillimanite are index minerals with the same chemical composition, originating from clay-rich parent rocks and can transform from one to the other, indicative of various metamorphic conditions.

Step-by-step explanation:

Kyanite, andalusite, and sillimanite are all minerals that are known to be polymorphs, which means they have the same chemical composition but different internal structures.

These minerals indeed are index minerals that originate from clay-rich parent rocks and are indicative of the metamorphic conditions under which they formed. Each of these minerals is stable under different pressure and temperature conditions during metamorphism. Rocks with kyanite and sillimanite generally indicate medium-pressure terranes, often associated with continent-continent collision zones.

Conversely, rocks with andalusite and sillimanite are characteristic of low-pressure terranes, possibly reflecting elevated geothermal gradients due to divergent zones or specific ocean-continent collision zones.

Finally, kyanite, andalusite, and sillimanite can indeed transform from one to the other, given changes in pressure and temperature conditions. This transformation is important for geologists to understand the environmental history of metamorphic rocks.