EESC 200 Mineralogy Term Test

Lectures 1 - 8

Mineral

Naturally occurring

Inorganic

Solid

Regular crystal structure

Defined chemical formula

Made of single or combined elements

Rocks

Aggregates of minerals

Ionization

In a chemical bond an element either loses or gains electrons to achieve a stable electron configuration.

Valence Related Bonding

Ionic

Covalent

Metallic

Asymmetrical Charge Distribution Bonding

Hydrogen

Van der Waals

Ionic Bonding

Valence electrons are transferred from one element to another to achieve a stable valence configuration.

eg. Halite, fluorite, etc.

Mineral Properties with Ionic Bonds

High symmetry

Moderate to high hardness

Brittle

Moderate to high melting point

Low expansion coefficient

Soluble in polar solvents, solutions are conductors

Covalent Bonding

Electrons are shared between two atoms to create a stable electron configuration. Very strong bond.

eg. Diamonds, sphalerite, graphite, etc.

Mineral Properties with Covalent Bonds

Lower symmetry

High hardness

Brittle

High melting point

Low coefficient of expansion

Metallic Bonding

Free movement of electrons around a positively charged nuclei in close-packed structures.

eg. gold, silver, copper, etc.

Mineral Properties with Metallic Bonds

Low hardness

Malleable

Good electron conductors

Good heat conductors

High symmetry - mostly cubic

Hydrogen Bonding

Weak attraction between water dipoles. Important in ice and clay minerals

Van der Waals Bonding

Weak attraction between neutral molecules. Residual attraction attributed to dynamic charge fluctuations.

Coordination Principle

A smaller cation surrounds itself with as many larger anions as possible to form a symmetrical polyhedra

Coordination Number (CN)

The number of anions surrounding/ bonding the cation

Electron probe microanalysis

Common technique to non-destructively determine elemental composition in minerals. Compositions reported as oxides

Substitution Solid Solution

Interchange of one cation for another in a structural site. Ion sizes must be similar. Charge neutrality must be maintained.

Simple Substitution

2 cations of identical charge substituted

Couples Substitution

2 cation substitutions to maintain the charge

Omission Solid Solution

Maintains charge balance when ions of different charge substitute for each other by leaving structural sites vacant.

Interstitial Solid Solution

Variation of coupled substitution in which charge balance is maintained by placing ions in sites that are normally vacant.

Basic Rules for Ionic Solids

Cations are written first

Charges must balance

Cations in same structural site are grouped together

Cations in different structural site are listed in order of decreasing coordination number

Crystallography

Describes the shape, symmetry and crystal structure of minerals.

Motif

Fundamental part of a symmetrical design that, when repeated, creates the whole pattern. eg Crystal faces.

Operation

The act that reproduces the motif to create the pattern

Symmetry element

Anchor of an operation located at a particular point.

Enantiomorphs

Two motifs related by reflection. Non-superimposable.

Two Fold Rotation Point Symmetry

Oval symbol. 180 degree rotation.

Three Fold Rotation Point Symmetry

Triangle symbol. 120 degree rotation.

Centre of Symmetry

Two equivalent faces diagonal to each other and equidistant from the centre of the crystal.

Rotoinversion

Requires the rotation of the axis in combination with an inversion centre.

2 Fold Rotoinversion

Rotate 180 degrees. Invert through the centre of symmetry.

Law of Haüy

Crystal faces make simple rational intercepts on crystal axes.

Law of Bravais

Common crystal faces are parallel to lattice planes that have high lattice node density.

Miller Indices

Describes the orientation of crystal faces and crystallographic planes. Consist of 3 numbers that describe the relationship of a crystal face to a unit cell.

Zone

Collection of crystal faces parallel to a common line called a zone axis, which is parallel to the intersection of two faces,

Crystal Form

Collection of equivalent crystal faces that are related to each other by symmetry. Can be open or closed.

Isometric Forms

Cubic system. 15 total, all closed.

Nonisometric Forms

Pedions, pinacoids, dihedrons, prisms, pyramids, dipyramids, trapezohedrons, scalenohedrons, rhombohedrons, tetrahedrons. Can be open or closed.

Pedion (O)

A single face with no geometrically equivalent face on the crystal.

Pinacoid (O)

Two parallel faces on opposite sides of the crystal related by inversion or a mirror.

Dihedron (O)

Two nonparallel faces. AKA a dome if the two faces are related only by a mirror plane or by a sphenoid if the two faces are related by a 2-fold rotation axis.

Prism (O)

A collection of 2, 4, 6, 8, 12 faces that intersects in a set of mutually parallel edges forming a tube.

Pyramid (O)

A collection of 3, 4, 6, 8 or 12 nonparallel faces that can intersect at a point.

Dipyramid (O)

Two pyramids, one on each end of the crystal, with a total of 6, 8, 12, 16 or 24 faces. The two pyramids are related to each other by reflection across a mirror plane.

Trapezohedrons (C)

Forms consisting of 6, 8 or 12 trapezoid faces. The faces on one end of the crystal are offset relative to the faces on the other end.

Scalenohedrons (C)

Forms consisting of 8 or 12 faces, each of which is a scalene triangle.

Tetrahedrons (C)

A form consisting of four triangular faces. In the tetragonal system the four faces are identical isosceles. In the orthorhombic system the faces form two pairs of different isosceles triangles.

Enantiomorphous Forms

Lack a centre of symmetry and mirrors. Typically have a screw axis symmetry element.

Rules for Combining Crystal Forms

Simple crystals must exist in only a single closed form

Complex crystals can exist in multiple open and closed forms

At least 2+ open forms must be present for a crystal to remain open

There can’t be any gaps in the crystal structure

Forms must be consistent with symmetry of the crystal class it belongs in

Equant/ Isometric

Equal in all dimensions, eg. garnet

Granular

Small rounded grains, eg. olivine

Prismatic

Prisms with tetragonal, hexagonal, trigonal or rhombic cross sections, eg. quartz

Blocky

Rectangular brick like, eg. orthoclase

Tabular

Book-like. Thicker than platy but not as elongated as bladed, eg. barite

Columnar

Long and rounded, eg. beryl

Platy

Flat and thin flakes, eg. biotite

Bladed

Elongated and flattened like a knife blade, eg. kyanite

Stubby

Short columnar crystals, eg. ruby

Acicular

Needle like, eg. actinolite

Fibrous

Dense hairy masses, eg. serpentine

Massive

No discernable form, eg. Calcite

Globular

A colloform mass that is roughly spherical

Banded

Bands or layers of mineral with different colours or textures

Dendritic

Mineral growth in a branching form somewhat like fern

Lamellar

Composed of layers, somewhat like the pages of a book

Pauling’s Rules

The coordination principle

Electrostatic Valency principle

Sharing of polyhedral elements

Principle of parsimony

The Coordination Principle

Cation and anion distances are equal to the sum of their effective ionic radii. Cation coordination numbers are determined by the ratio of cation to anion radii.

Coordination Polyhedra

Shape defined by the anions bonded with the cations

Electrostatic Valency Principle

The strength of an ionic bond is equal to ionic charge divided by the coordination number. Only applies if all bond strengths are uniform

Anisodesmic Bonding

Some anion-cation bonds take more than half of the anion charge. eg Anhydrite

Mesodesmic Bonding

Some anion-cation bonds take exactly half the anion charge. Non-uniform bonding. eg Silicates

Sharing of Polyhedral Elements

Inherently unstable. Cations of higher valence and smaller coordination numbers tend to not share or share less anions with other cations.

Principle of Parsimony

The number of different components in a crystal tend to be small. Lowest energy arrangements are prefered.

Isodesmic

Uniform bonding, leads to symmetrical packing of anions. eg. halides, sulfides and oxides.

Isostructural groups

Groups of chemically related minerals with identical structures, common anion or anionic group

Polymorphism

High pressure = tight packing

High temperature = open packing

Reconstructive Polymorphs

Conversion of one polymorph to another requires that chemical bonds of one structure must be broken so that the atoms/ions can be rearranged to form the new structure.

Displacive Polymorphs

Inversions do not require breaking bonds; the difference between polymorphs is simply a distortion or bending of the crystal structure

Order-disorder Polymorphs

The mineral structure remains similar but the cation distribution within structural sites changes

Polytypism

Polymorphs only differ in the stacking sequence of identical sheets

Crystallization

Ordering of atoms or molecules to form a well-defined solid crystal lattice. Can grow as magma cools, crystals precipitating from water, crystals forming from chemical reactions.

Crystallization Requirements

All constituents must be present and available in the system. eg. to form SiO2 you need both Si and O

Stability

The relative term between two states, the lowest energy configuration is the most stable

Nucleation

Process by which the first few atoms of a mineral chemically bond together to form the nucleus of the mineral

Homogenous Nucleation

Occurs in melts and aqueous solutions where atoms move around and bump into each other to form chemical compounds.

Heterogenous Nucleation

New minerals nucleate by taking advantage of already existing mineral structures. No empryo needed

Growth rate

The slowest growing faces are the most prominent. Surfaces with the most unsatisfied bonds grow faster

Crystal defects

Irregularities in ordered arrangement of atoms in a crystalline solid

Point Defects

Vacancy: Missing ion

Frenkel: Misplaced ion

Interstitial: Foreign atom or ion occupies a normally unoccupied site

Substitution: Ion substitutes another in a pure crystal

Line Defects

Edge dislocation or screw dislocation

Planar defects

Grain boundaries: Surface or face of crystal

Stacking faults: Irregularities in stacking sequence

Antiphase boundaries: Separates segments of a crystal that are related to each other by simple translation

Twin Boundary

Segment between two intergrown crystals

Twinning

Intergrowth of two or more crystals of the same mineral. Lattices of each orientation are symmetry related. Can be a simple twin (2) or a multiple twin (3+). Can form by Growth twinning or Deformation twinning (stress)

Contact Twins

Share one crystal face or crystallographic plane

Penetration Twins

One individual penetrates the other at an angle

Cyclic Twins

Twin planes radiate from a central axis

Polysynthetic Twins

Repeating parallel twin planes

Igneous Minerals

Form from a magma. Formed through slowing kinetic energy. eg. biotite, olivine, etc.

Aqueous Minerals

Precipitate from water or an aqueous solution. Affected by temperature and saturation. eg. Calcite, etc.