Question

Rearrange the following minerals in order of increasing metamorphic grade: biotite, garnet, sillimanite, chlorite.

Short answer

The rearranged mineral order representing increasing metamorphic grade is: chlorite, biotite, garnet, sillimanite.

Step-by-step solution

Understanding Metamorphic Grade

Metamorphic grade refers to the level of heat and pressure experienced by a rock during metamorphism. As the grade increases, the minerals present in the rock change due to the increased temperature and pressure. Typically, mineral assemblages found in rocks of various metamorphic grades are used as indicators to determine the metamorphic grade of the rock.

Recognizing Index Minerals

Index minerals are minerals that are indicative of a specific metamorphic grade or metamorphic facies. The following minerals are commonly used as index minerals to identify different metamorphic grades (from lowest to highest grade): chlorite, biotite, garnet, staurolite, kyanite, and sillimanite.

Rearrange the Given Minerals

Now, we can easily rearrange the given minerals in the order of increasing metamorphic grade by referencing the list of index minerals. The order is as follows: 1. Chlorite (lowest grade) 2. Biotite 3. Garnet 4. Sillimanite (highest grade) So, the rearranged mineral order representing increasing metamorphic grade is: chlorite, biotite, garnet, sillimanite.

Key concepts

Index Minerals

Index minerals play a crucial role in understanding metamorphic rocks. These minerals form under specific ranges of temperature and pressure, making them reliable markers for different metamorphic grades.
You can think of index minerals like a thermometer, except instead of measuring temperature, they provide clues about the conditions under which the rock formed.
Some commonly referenced index minerals include:

• Chlorite - indicating low-grade metamorphism
• Biotite
• Garnet
• Staurolite
• Kyanite
• Sillimanite - indicating high-grade metamorphism

Understanding these minerals helps geologists determine the history and environmental conditions of metamorphic rocks.

Metamorphism

Metamorphism involves the alteration of existing rocks by exposure to high heat, pressure, and chemically active fluids. This process transforms the mineralogical and structural characteristics of the rocks, leading to a wide variety of metamorphic textures and minerals.
Metamorphism is a dynamic process that can lead to the formation of new minerals and changes in the rock's composition. It occurs mainly in two environments:

• Contact Metamorphism: When rocks are heated by nearby molten magma.
• Regional Metamorphism: Where large areas of rock are subjected to high temperature and pressure over a long period.

The degree of metamorphism can vary, leading to different grades that are marked by specific index minerals.

Mineral Assemblages

Mineral assemblages are groups of minerals that occur together in metamorphic rocks. These assemblages provide valuable information on the metamorphic conditions experienced by the rock.
As rocks transform, the original minerals may become unstable under new conditions, leading to the formation of new stable assemblages.
The specific minerals present in these assemblages can indicate the pressure and temperature conditions at the time of formation. For example, a garnet-staurolite assemblage suggests intermediate to high-grade metamorphic conditions.

• The occurrence of specific assemblages can also reveal the extent and duration of metamorphic conditions.
• Understanding these assemblages helps geologists to reconstruct past geological environments and events.

Heat and Pressure

The driving forces behind metamorphism are heat and pressure. They facilitate the transformation of rocks and minerals into new forms.
Heat increases with depth beneath the Earth's surface and plays a key role in promoting chemical reactions that lead to new mineral formations. It can come from deep burial of rocks or from nearby magma bodies.

• Heat results in recrystallization and the emergence of new mineral grains.
• This can lead to denser and more compact rock forms.

Pressure, also increasing with depth, influences the orientation and texture of minerals. It can cause minerals to align in a preferred direction, creating a feature known as foliation.

• Pressure can transform the rock's physical structure and enhance its strength.

Together, these factors determine the types of metamorphism and the resulting metamorphic grades.