Chapter 2: Problem 11

Why is biotite called a ferromagnesian mineral, while muscovite is not?

Short Answer

Expert verified

Biotite is called a ferromagnesian mineral because it contains significant amounts of iron (Fe) and magnesium (Mg) in its chemical composition, with the formula \(K(Mg,Fe)_3AlSi_3O_10(OH,F)_2\). In contrast, muscovite, with a formula of \(KAl_2(AlSi_3O_10)(OH)_2\), lacks significant amounts of iron and magnesium, making it a non-ferromagnesian mineral.

Step by step solution

Introduce Biotite and Muscovite

Biotite and muscovite are both types of mica, which are silicate minerals characterized by their sheet-like structure. Biotite is also known as black mica, while muscovite is commonly referred to as white mica. To understand why biotite is classified as a ferromagnesian mineral and muscovite is not, let's first explore the meaning of "ferromagnesian."

Define Ferromagnesian

Ferromagnesian is a term used to describe minerals that contain significant amounts of iron (Fe) and magnesium (Mg). These elements are typically present as cations within the mineral's structure, which contributes to their distinct properties, such as higher density and dark color. Now that we understand the meaning of ferromagnesian, let's examine the chemical compositions of biotite and muscovite.

Chemical Composition of Biotite

Biotite is a complex silicate mineral with the general chemical formula: \(K(Mg,Fe)_3AlSi_3O_10(OH,F)_2\). As you can see, it contains both magnesium (Mg) and iron (Fe) in the form of cations, contributing to its dark color and higher density compared to other mica minerals.

Chemical Composition of Muscovite

Muscovite has a chemical formula of \(KAl_2(AlSi_3O_10)(OH)_2\). Unlike biotite, muscovite does not contain significant amounts of iron (Fe) or magnesium (Mg) in its structure, making it a non-ferromagnesian mineral. Instead, its primary metal cation is aluminum (Al), giving it a lighter color and lower density compared to biotite.

Compare the Compositions and Classify the Minerals

Comparing the compositions of biotite and muscovite, it becomes clear that biotite contains both iron (Fe) and magnesium (Mg), making it a ferromagnesian mineral. On the other hand, muscovite's composition lacks significant amounts of iron and magnesium, which is why it is not classified as a ferromagnesian mineral.

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Why is biotite called a ferromagnesian mineral, while muscovite is not?

Short Answer

Step by step solution

Introduce Biotite and Muscovite

Define Ferromagnesian

Chemical Composition of Biotite

Chemical Composition of Muscovite

Compare the Compositions and Classify the Minerals

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