Mineralogy Midterm!!!

Mineral: a naturally occurring, crystalline solid, with a definite chemical composition. >6000 minerals to date

**Glass is not a mineral because it lacks long-range atomic order (non-crystalline)

Biomineral: minerals that constitute an integral part of biologic structures. (ex. calcite and aragonite CaCO3, tooth enamel made by hydroxylapatite!)

Mineraloid: minerals that lack long-range crystalline structure, including amorphous solids and glasses.

Crystal: mineral bounded by regular crystal faces produced as the crystal grew.

Mineral Formulas: Cations first and in order of decreasing coordination number!!!

Unit Meshes:

square, rectangle, diamond

Crystal Systems

  • CHEAT SHEET

  • Triclinic —> no symmetry!! (ex. K-feldspar)

  • Monoclinic —> 1 axis of symmetry, all axes are different (ex. orthoclase, muscovite)

  • Orthorhombic —> all axes different, but all angles are equal

  • Tetragonal —> one axes is different length, but all angles are equal

  • Hexagonal (Trigonal, Rhombohedral) —> self explanatory

  • Isometric (cubic) —> self explanatory

Crystal Chemistry

Ionic Bond: Electrostatic attraction between positive and negative ions

Covalent: Sharing of electrons when atomic orbitals overlap

Metallic Bonding: lattice of cations in a sea of delocalized electrons

OXIDATION IS LOSS. REDUCTION IS GAIN. (OF ELECTRONS)

PROTONS (ATOMIC NUMBER Z) DEFINES ELEMENT!!!

Crystal Structure + Pauling’s Rules

  1. Coordination Principle: coordination number (number of nearest cation neighbors is determined by radius ratio (radius of cation over radius of anion)

  2. Electrostatic Valency Principle: strength of bond = cation charge / coordination number (ei. the larger the charge and the closer the bond, the stronger)

  3. Sharing of Polyhedral Elements 1: corners < edges < faces shared destabilize the structure the most

  4. Sharing of Polyhedral Elements 2: if there are differently charged cations, the highly charge cations minimize the number of anions to keep a max distance apart

  5. Principle of Parsimony: crystal constituents are simple.

  • Isostructural: two or more minerals whose atoms are arranged in same type of crystal structure (ex. NaCl and PbS have same symmetry, cleavage, and crystal habit (isometric))

  • Polymorphism: the ability of a chemical compound to crystallize into more than one structure (ex. SiO2 => quartz, tridymite, coesite)

    • high pressure => high density structures, low pressure => low density structure

    • Reconstructive: REQUIRES ENERGY TO BREAK BONDS then rearranges atoms/ions into new structure

    • Displacive: NO BREAKING BONDS just bending of crystal structure (ex. alpha quartz becomes beta quartz at high temperatures)

    • Order-Disorder: RAPID COOLING —> cation distribution disorder, SLOW COOLING —> order (ex. K-feldspar sanidine is rapid cooling and disordered, K-feldspar microcline is slow cooling and ordered (Si and Al tend to be the same sites))

      • rapid cooling —> volcanic igneous, slow cooling —> plutonic igneous

Mineral Growth

NO QUARTZ IN MAFIC ROCKS (basalt), QUARTZ AND OTHER SILICA POLYMORPHS IN FELSIC VOLCANIC ROCKS (rhyolite)

  • Simple Substitution: cations with same charge and size

  • Coupled Substitution: basically simple substitutions

  • Omission: ions with different charge leave sites vacant (ex. swapping 3 Fe2+ for 2 Fe3+)

  • Interstitial: ions with different charge, extra ions placed in normally vacant sites (ex. swapping Si4+ with Al3+ and K+)

Homogeneous Nucleation: atoms/ions find each other spontaneously and randomly form the nucleus of a crystal

Heterogeneous Nucleation (dominant mechanism): new minerals nucleate by taking advantage of an existing crystal structure

Contact Twinning: twinning from one point

Penetrative Twinning: crystals directly overlap and are intergrowth (related by rotation) (ex. staurolite, pyrite)

X-Ray Crystallography

  • Powder X-ray Diffraction: finely ground powder

  • X-rays generated when stream of high energy electrons strike material, diffraction occurs when electrons hit atoms of object

  • d = distance between atoms in a crystal structure

  • Diffraction intensity differs among minerals because it depends on number and kinds of atoms present (the heavier the element, the more electrons, thus the more diffraction)

Physical Properties

  • Zircon: ZrSiO4, tetragonal, prismatic crystals

  • Transition elements are mainly responsible for colour in minerals

  • Colours:

    • Idiochromatic: “self-coloured” due to compositon

    • Allochromatic: coloured due to impurities

    • Pseudoachromatic: “false coloured” due to light diffraction tricks

  • Fluorescence: when a higher energy electron drops into a lower energy level, releasing energy in the form of EMF causing photoluminescence

  • Magnetism: due to unpaired electrons in orbitals

    • Curie Temperature: temp at which magnetic material lose magnetism when heated

      • For magnetite: 585 C

    • Magnetite acquires magnetization parallel to earth’s magnetic field

  • Cleavage:

    • Perfect: breaks easily

    • Good: breaks easily but not on even/continuous surfaces

    • Distinct/Indistinct/Poor: decreasing quality of break surfaces

    • Types:

      • Cubic (galena)

      • Octahedral (fluorite)

      • Rhombohedral (calcite)

      • Prismatic (amphiboles)

      • Basal (micas)

Mineral Classification

Mineral Class: mostly based on anions or anionic properties

(Silicate) Mineral Subclass: bonding of tetrahedra is used to group structurally similar minerals (ex. tectosilicates) —> depends on how many O2- are shared between tetrahedra

Mineral Group: two or minerals with similar structure and chemically similar elements (feldspars within tectosilicates)