💎Mineralogy Unit 2 – Mineral Chemistry and Atomic Structure

Key Concepts and Definitions

• Minerals defined as naturally occurring, inorganic, crystalline solids with a definite chemical composition and an ordered atomic arrangement
• Chemical elements are the building blocks of minerals and consist of atoms with a specific number of protons in their nuclei
• Atoms composed of protons, neutrons, and electrons; protons and neutrons form the nucleus while electrons orbit in shells
• Isotopes are variations of an element with different numbers of neutrons in the nucleus
• Chemical bonding involves the interaction between atoms through the transfer or sharing of electrons
• Crystal systems describe the geometric arrangement of atoms in a mineral based on the symmetry and length of the crystallographic axes (cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic)
  • Cubic system has three equal axes at right angles to each other
  • Hexagonal system has four axes, with three equal axes in a plane and a unique fourth axis perpendicular to the plane

Atomic Structure Basics

• Atoms are the fundamental units of matter and consist of a dense nucleus surrounded by a cloud of electrons
• The nucleus contains positively charged protons and neutral neutrons, while negatively charged electrons orbit the nucleus in shells
• Atomic number is the number of protons in an atom's nucleus and determines the element's identity
• Mass number is the sum of protons and neutrons in an atom's nucleus
• Electron configuration describes the arrangement of electrons in an atom's shells (K, L, M, N) and subshells (s, p, d, f)
  • Electrons fill shells and subshells in a specific order based on energy levels
  • Valence electrons in the outermost shell participate in chemical bonding
• Periodic table organizes elements based on their atomic number and electron configuration, with elements in the same group having similar chemical properties

Chemical Bonding in Minerals

• Chemical bonding is the attraction between atoms that allows the formation of molecules and crystals
• Ionic bonding involves the transfer of electrons from one atom to another, creating positively and negatively charged ions that attract each other (halite, NaCl)
• Covalent bonding involves the sharing of electrons between atoms, forming strong directional bonds (diamond, C)
• Metallic bonding occurs in metals where valence electrons are delocalized and shared among many atoms (native copper, Cu)
• Van der Waals bonding is a weak attraction between molecules resulting from temporary dipoles (graphite, C)
• Hydrogen bonding is a special type of dipole-dipole interaction involving hydrogen atoms bonded to highly electronegative elements like oxygen or nitrogen (ice, H2O)
  • Hydrogen bonding plays a crucial role in the properties of water and the structure of many minerals

Crystal Systems and Structures

• Crystals are solid materials with a regular, repeating arrangement of atoms in three dimensions
• The seven crystal systems describe the geometric arrangement of atoms based on the symmetry and length of the crystallographic axes
• Unit cell is the smallest repeating unit that defines the crystal structure and contains the complete symmetry of the crystal
• Bravais lattices are the 14 unique three-dimensional arrangements of points in space that describe all possible crystal structures
• Miller indices (hkl) are used to describe the orientation of planes and faces in a crystal
• Polymorphism occurs when a chemical compound can crystallize in more than one crystal structure (diamond and graphite, both C)
  • Polymorphs have the same chemical composition but different physical properties due to their distinct crystal structures

Mineral Properties and Identification

• Minerals are identified based on a combination of physical, chemical, and optical properties
• Hardness is a mineral's resistance to scratching and is measured using the Mohs scale (1-10)
• Cleavage is the tendency of a mineral to break along smooth, flat surfaces parallel to weak bonding planes in the crystal structure (mica, muscovite)
• Fracture describes the appearance of a mineral's broken surface when it does not exhibit cleavage (conchoidal, even, uneven)
• Luster is the appearance of a mineral's surface in reflected light (metallic, vitreous, resinous, pearly)
• Streak is the color of a mineral's powder when it is scraped across a streak plate (hematite, red)
• Specific gravity is the ratio of a mineral's density to the density of water and can be used to distinguish minerals with similar appearances
• Optical properties, such as color, transparency, and pleochroism, can be observed using a microscope and are useful for identification

Common Minerals and Their Compositions

• Silicates are the most abundant mineral group and contain silicon and oxygen tetrahedra (SiO4) as their basic building blocks (quartz, feldspars, micas)
  • Silicates can form various structures, such as isolated tetrahedra, chains, sheets, and frameworks
• Carbonates contain the carbonate ion (CO3) and typically form in sedimentary environments (calcite, dolomite)
• Oxides are compounds of oxygen and one or more metallic elements (hematite, magnetite)
• Sulfides are compounds of sulfur and one or more metallic elements and are important ore minerals (pyrite, galena)
• Native elements occur in their pure form without combining with other elements (gold, silver, copper)
• Halides are compounds of the halogen elements (fluorine, chlorine, bromine, iodine) and a metallic element (halite, fluorite)
  • Halides typically have a cubic crystal structure and exhibit perfect cleavage

Analytical Techniques in Mineralogy

• X-ray diffraction (XRD) is used to determine the crystal structure and identify minerals based on their unique diffraction patterns
• Scanning electron microscopy (SEM) provides high-resolution images of mineral surfaces and can be used to study crystal morphology and chemical composition
• Transmission electron microscopy (TEM) allows for the direct imaging of crystal structures and defects at the atomic scale
• Electron microprobe analysis (EMPA) is used to determine the chemical composition of small mineral grains or specific points within a sample
• Raman spectroscopy uses inelastic scattering of monochromatic light to identify minerals based on their unique vibrational modes
• Fourier-transform infrared spectroscopy (FTIR) is used to study the molecular structure and bonding in minerals by measuring the absorption of infrared light
  • FTIR is particularly useful for identifying hydrous minerals and studying the incorporation of water in mineral structures

Applications in Geology and Beyond

• Mineralogy is fundamental to understanding the composition, structure, and evolution of the Earth's crust and mantle
• Mineral assemblages in rocks provide information about the pressure, temperature, and chemical conditions during their formation (metamorphic facies)
• Ore deposits are concentrations of economically valuable minerals that are often studied using mineralogical techniques to optimize extraction and processing
• Environmental mineralogy investigates the role of minerals in controlling the distribution and mobility of contaminants in soil and water
• Biomineralization is the process by which living organisms produce minerals, such as calcium carbonate in shells and calcium phosphate in bones and teeth
• Materials science and engineering rely on the principles of mineralogy to develop new synthetic materials with desirable properties (zeolites, perovskites)
• Planetary science uses mineralogical data from meteorites, asteroids, and other planetary bodies to understand their formation and evolution
  • The presence of specific minerals, such as clays and carbonates, on Mars suggests the past existence of liquid water and potentially habitable environments