Lecture 2 PPT

Lecture 2: Minerals

Overview of Minerals

• Minerals are defined as naturally occurring inorganic solids that possess several key characteristics:

  • An orderly (or internal) crystalline structure

  • A specific chemical composition

Example Minerals

• Olivine (common in basalt, a volcanic rock):

  • Chemical composition: $(Mg, Fe)<em>{2}SiO</em>{4}$

• Quartz:

  • Also known as Herkimer Diamond

  • Chemical composition: $SiO_{4}$

Defining Characteristics of Minerals

• Inorganic: Means that minerals are not derived from living organisms.

• Solid: Minerals maintain a solid state under typical Earth surface conditions.

• Orderly Crystalline Structure:

  • Atoms are arranged in a systematic and repeating pattern.

• Specific Chemical Composition:

  • Each mineral has a defined formula.

Notable Mineral Examples

• Realgar & Orpiment

• Azurite & Malachite

• Cuprite

• Garnet

Chemical Composition of Minerals

• To determine the chemical composition of a mineral:

  • Utilize a chemical formula which serves as a shorthand representation of the elements in a molecule and/or compound.

  • Symbols for elements can be found on the periodic table of elements.

Example Chemical Formulas

• Water:

  • $H_{2}O$

  • Hydrogen (H): 2 atoms

  • Oxygen (O): 1 atom

• Ilmenite:

  • $FeTiO_{3}$

• Amphibole:

  • $(Na,K)<em>{0-1}Ca</em>{2}(Mg,Fe,Al)<em>{5}(Si,Al)</em>{8}O<em>{22}(OH)</em>{2}$

Questions

• What does Ti stand for in the ilmenite formula?

• How many Fe, Ti, and O atoms are needed to make the mineral ilmenite?

Silicate Minerals

• Silica Tetrahedra:

  • Also called the silicon-oxygen tetrahedron.

  • Composed of 1 silicon atom surrounded by 4 oxygen atoms.

  • Essential building block for ALL silicate minerals.

  • Bonds are strong between atoms in the tetrahedron.

• Stability Requirement:

  • Silica tetrahedra must bond with positively charged ions or with other tetrahedra, resulting in a net zero charge for stability.

Examples of Silicate Minerals

• Plagioclase Feldspar:

  • Chemical composition: $CaAl<em>{2}Si</em>{2}O_{8}$

• Pyroxene Group:

  • Chemical composition: $CaMgSi<em>{2}O</em>{6}$

Tetrahedral Sharing

• When silica tetrahedra share with other tetrahedra, it is primarily oxygen that is being shared.

Types of Silicate Minerals

• Olivine:

  • Commonly found in the mantle.

  • Simplest silicate structure; categorized as isolated with no oxygen sharing with other silica tetrahedra.

  • Chemical composition: $(Mg,Fe)<em>{2}SiO</em>{4}$

• Pyroxene Group:

  • Characterized as a single chain silicate where one oxygen is shared with other tetrahedra.

  • Example: Diopside: $CaMgSi<em>{2}O</em>{6}$

• Amphibole Group:

  • Known as a double chain silicate where two oxygens are shared with other tetrahedra.

  • Example: Hornblende:

  • Chemical composition: $(Na,K)<em>{0-1}Ca</em>{2}(Mg,Fe,Al)<em>{5}(Si,Al)</em>{8}O<em>{22}(OH)</em>{2}$

• Mica Group:

  • Classed as sheet silicates where three oxygens are shared with other tetrahedra.

  • Example: Biotite:

  • Chemical composition: $K(Fe,Mg)<em>{3}AlSi</em>{3}O<em>{10}(OH)</em>{2}$

• Quartz:

  • Described as framework silicates which are the most complex silicate structures.

  • All oxygens are shared with other tetrahedra, forming a three-dimensional network.

• Potassium Feldspar:

  • Chemical composition: $KAlSi<em>{3}O</em>{8}$

  • Also a framework silicate with a complex structure.

• Plagioclase Feldspar:

  • Variants include:

  • Albite: $NaAlSi<em>{3}O</em>{8}$

  • Anorthite: $CaAl<em>{2}Si</em>{2}O_{8}$

Complexity of Silicate Structures

• Silicate mineral structures can range from:

  • Isolated -> Single chain -> Double chain -> Framework -> Sheet.

• As the structure becomes more complex from isolated to framework, the number of linking silica tetrahedra increases.

Importance of Water in Silicate Structures

• Chemical classification of minerals differentiates between silicates and non-silicates.

• Some silicates are classified as hydrous minerals which mean:

  • They utilize water to form, hence water is represented in their chemical formula.

  • Water can be identified in the formula as either OH or H2O depending on the bonding.

Importance of Water

• Water influences magma:

  • Occurs as a gas phase dissolved in magma.

  • When minerals crystallize from magma, elements are extracted, increasing the abundance of remaining elements in the magma.

  • If water remains in the magma, as it moves closer to the surface, dissolved gases (including water) can form bubbles.

  • Trapped bubbles within magma can lead to explosive eruptions.

Notable Example of Explosive Eruptions

• Mount Etna and Stromboli

Role of Minerals in Volcanic Activity

• Images of mineral presence during volcanic events:

  • 2020 Eruption of Kilauea:

  • Observations of plagioclase vesicles (bubbles) and minerals like clinopyroxene.

  • Bubbles trapped in magma illustrated via different scales (200 μm, 0.1 mm, etc.).

  • 2022 Eruption of Mauna Loa:

  • Similar mineralogical traits observed.

Visual Representation in Eruptions

• Various microscopic images help in understanding the mineralogical components and processes occurring during eruptions.