Metamorphic Shale Formation: Which Mineral Would Form?

What mineral would form if shale is buried to a depth of 20 km and the temperature is raised to 450 °C?

Garnet would form in metamorphosed shales (pelites) buried at a depth of 20 km and a temperature of 450 °C. This is because garnet is indicative of medium-grade metamorphism and typically forms under such conditions. Therefore, the correct option is 4) Garnet.

Formation of Garnet in Metamorphosed Shales

Garnet Formation: Garnet is a common mineral found in metamorphic rocks and its presence can provide clues about the conditions under which the rock was formed. In the case of shale being buried to a depth of 20 km and exposed to a temperature of 450 °C, the metamorphic conditions are suitable for the formation of garnet.

Metamorphic Grade and Index Minerals

Metamorphic Grade: The metamorphic grade of a rock refers to the intensity of the metamorphic conditions it has been subjected to. Different minerals form at different grades of metamorphism, with garnet typically forming in medium-grade metamorphic rocks.

Index Minerals: Minerals like chlorite, biotite, garnet, kyanite, and sillimanite are known as index minerals and can be used as geothermobarometers to interpret the metamorphic conditions of a rock.

Formation of New Minerals in Metamorphic Rocks

Appearance of New Minerals: In metamorphic rocks, the appearance of new minerals, often as porphyroblasts, indicates the change in mineral composition due to metamorphism.

Temperature Conditions: The temperature of 450 °C falls into the range of medium-grade metamorphism where garnet would start to form as a new mineral, aligning with the conditions described in the question.

Overall, the metamorphic conditions of shale buried to a depth of 20 km and exposed to a temperature of 450 °C are ideal for the formation of garnet as a new mineral, indicating medium-grade metamorphism.