Introduction

The speaker welcomes the audience to the third part of the lecture series on mineralogy and introduces the topic of crystal chemistry.

Crystal Chemistry

• The lecture focuses on crystal chemistry and the minerals present in the earth's mantle and crust.

• Oxygen, silicon, and aluminum are the three most abundant elements in the crust, while oxygen, magnesium, and silicon are the most abundant in the mantle.

• Minerals formed from these elements have different mineralogies due to their abundance in different minerals.

• Silicon tetrahydron is a common structure found in many minerals. It consists of a silicon atom connected to four oxygen atoms at its center.

• Bridging oxygen connects tetrahedrons at their vertices while non-bridging oxygen is not connected to other oxygens in a crystal structure.

Physical Properties

• Bond lengths between silica-oxygen show slightly lesser values than calculated due to covalency present in silicate structures.

• Bond angle between silica-oxygen-silica is approximately 140 degrees.

Mineral Formula

• The lecture covers how to write down mineral formulas for different types of silicate structures such as disilicate ionosilicates.

Feldspar and Mineral Composition of Earth's Crust

This section discusses the composition of feldspar and minerals in the Earth's crust.

Feldspar Composition

• Plagioclase makes up almost 39% of feldspar present in the Earth's crust.

• Alkali feldspar makes up around 12% of feldspar present in the Earth's crust.

• Combining plagioclase and alkali feldspar, more than 50% of feldspar is present in the Earth's crust.

Mineral Composition

• The most abundant elements in the Earth's crust are oxygen, silicon, aluminum, calcium, and iron.

• Plagioclase and alkali feldspar are the two most abundant minerals in the Earth's crust due to their high aluminum content.

• Quartz is the third most abundant mineral with a presence of almost 12 weight percent.

• Orthopyroxene and clinopyroxene combine to make up almost 11% of pyroxene present in the Earth's crust.

• Minerals found in the Earth's crust are rich in different elements such as aluminum, silicon, calcium, and iron.

Layers of Mantle

This section discusses how mantle is divided into three layers: upper mantle, transition zone, and lower mantle.

Upper Mantle

• The upper mantle starts from where the earth's crust ends (30 km beneath continents or 10 to 11 km beneath oceans).

• The upper mantle extends up to 410 km.

Transition Zone

• The transition zone starts from 410 km to 660 km depth.

Lower Mantle

• The lower mantle starts from 660 km to 2900 km.

• A diagram shows the abundance of different minerals in the mantle, which is only applicable when considering ultramafic rocks.

• Ultramafic rock is made up of mafic minerals that have a high magnesium and iron content.

• Peridotite is an ultramafic rock that has more than 40% volume percent of olivine.

• Xenolith studies show that most xenoliths are ultramafic or harzburgitic in composition.

Mineral Composition of the Upper Mantle

This section discusses the mineral composition of the upper mantle, which is mostly made up of ultramafic composition.

Mineral Composition of the Upper Mantle

• The upper mantle is composed of four minerals: olivine, garnet, orthopyroxene, and clinopyroxene.

• Olivine makes up 60% of the upper mantle's volume fraction.

• Garnet makes up 12-15% of the upper part of the upper mantle.

• Orthopyroxene is not stable below a depth of 300 km.

• Clinopyroxene becomes more abundant in deeper parts of the upper mantle.

Mineral Structures

• Olivine has an orthorhombic structure and is a nesosilicate.

• Garnet has a cubic structure and is also a nesosilicate.

• Clino pyroxenes are chain silicates.

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