Flashcards Mineralogy Test 1

What is a mineral?

solid, crystalline, chemically consistent, naturally occurring, inorganic

Minerals vs non-minerals

Minerals: quartz, ice, biotite, diamond, olivine

Non-minerals: opal, volcanic glass, water, quartz sand, amber

hexagonal closest packing (HCP)

crystalline structure in which close-packed layers of atoms or ions are stacked as a series of two alternating layers of different relative orientations (AB). CN of 12

cubic closest packing (CCP)

crystalline structure in which planes of closely packed atoms or ions are stacked as a series of three alternating layers of different relative orientations (ABC). CN of 12

Pauling's Rule #1

-maximum number of anions surround a cation as allowed by size =coordination number (count anions)

-determined by radius ratio (rcat / ran ), their relative size

-coordination polyhedron shape made by connection centers of ions

-polyhedra are building blocks of minerals

Pauling's Rule #2:

electrostatic valency principle

Electrostatic valency: in a stable crystal structure, the total the valency bonds that reach an anion from all the neighboring cations is equal to the charge of the anion

bond strength e.v. = z/CN

Pauling's Rule #3

Sharing of edges or faces of coordination polyhedra generally destabilizes astructure -> because cations prefer to stay at maximum distance

Pauling's Rule #4

Cations with high valence charges and small coordination number tend not to share polyhedral elements. Their large positive charges tend to repel

Pauling's Rule #5

principle of parsimony or simplicity rule: the number of different kinds of coordination polyhedra or constituent "sites" in a crystal structure tends to be small

Fluorite structure

e.g CaF2 and UO2

Halite structure

KCl, MgO, PbS

CN:6

ferropericlase

Perovskite Structure

ABO3

CN of A: 10-12

CN of B: octahedrons

MgSiO3

bridgmanite

Spinel Structure

AB2O4

CN of A: octahedrons

CN of B: tetrahedrons

ringwoodite

Polymorphs

Minerals with the same composition but different crystalline structures

How are coordination polyhedra held together?

bonds

How do we visualize mineral structures?

spheres

Why are Pauling's rules important to mineral structures?

Guidelines for looking at minerals

Tell configurations of different elements

When you can build a stable mineral structure

Describe the structure of MgO

Halite

CN of Mg: 6

CN of O: 6

What is a unit cell?

the smallest unit of a structure (or pattern) that can be infinitely repeated to generate the entire structure

Three characteristics of a unit cell?

1. as many 90 degree angles as possible. 2. wants to be as small as possible but needs integer number of structural units. 3. must repeat in its entirety throughout the crystal lattice

Triclinic Crystal System

a≠b≠c

α≠β≠γ≠90

feldspars, e.g. anorthite

Monoclinic Crystal System

Particles in each unit cell form rectangular bases, two rectangular faces, and two faces that are parallelograms

a ≠ b ≠ c; α, γ = 90 o, β > 90o

mica, biotite

Orthorhombic crystal system

a ≠ b ≠ c; α = β = γ = 90°

olivine

Tetragonal crystal system

a = b ≠ c

α = β = γ = 90°

zircon

Hexagonal crystal system

a=b≠c α=β=90 γ=120

isometric crystal system

a = b = c

α = β = γ = 90°

what is a crystal lattice?

the regular pattern in which a crystal is arranged

In real minerals there are....

local violations of the perfect arrangement

Examples of local violations in crystals:

one atom being replaced by impurity atoms

disorder among the cation sites (such as Si, Al disorder)

stacking mistakes in silicates

point defects

vacancy atoms, interstitial atoms, substitutional atoms

Line Defects

Edge dislocation: linear defect where an extra half-plane of atoms is inserted partway into the crystal structure, distorting the surrounding atomic spacing

Screw dislocation: linear defect where a shear stress displaces parts of the crystal lattice in a helical, or screw-like, spiral ramp pattern around the dislocation line.

Burgers vector

A vector that denotes the magnitude and direction of lattice distortion associated with a dislocation.

Delta G equation

delta G = delta H - T delta S

What is important to remember for enthalpy?

-the creation of point defects requires energy → inc enthalpy

-increase disorder → increase entropy

products-reactants

-change in enthalpy is relted to change in heat capacity

What does it mean if delta g is negative?

the reaction is spontaneous, or energetically favorable. energy is lost and the final state is more stable

ex. diamond to graphite is

Why are metallically bonded minerals opaque?

light is immediately absorbed as electron moves into excited state

continuum of unoccupied e- states results in continuum of absorbed photon energy• => metal is opaque

Insulator

large band gap

Semiconductor

energy difference between filled orbital and lowest empty band is only slightly below, not overlapping (small band gap)

Conductor

overlapping bands

How to calculate miller index

1. take the reciprocal of the lattice intersects

2. clear fractions

3. delete axis labels

What are directions in?

[square brackets]

What do bars over miller indexes mean?

negative

Structure of the Earth

lithosphere, upper mantle, lower mantle, inner core, outer core

Where is olivine found?

upper mantle

Where is ringwoodite found?

upper mantle 520-610km

Where is bridgmanite and ferropericlase found?

lower mantle

mirror

imaginary plane that divides a crystal into two halves, each of which in a perfectly developed crystal is the mirror image of the other

center of symmetry (point of symmetry)

is present in a crystal if an imaginary line can be passed from any point on its surface through its center and the same point is found on the line that is at an equal distance beyond the center on the opposite side

rotoinversion axis

rotation with inversion

What is the coordination number of a tetrahedral shape?

4

What is the coordination number of a octahedral shape?

6

What is the coordination number of a hexahedral shape?

8

bond strength=

charge/CN

What is hardness related to?

Strength of bonds

What are the (8) major bind forming elements?

Oxygen (O), Silicon(Si), Aluminum(Al), Iron(Fe), Calcium (Ca), Sodium(Na), Potassium(K), Magnesium(Mg)

Oxidation state of Oxygen?

typically -2

Oxidation state of Silicon?

+4

Oxidation state of Aluminum?

+3

Oxidation state of Iron?

0

Oxidation state of Calcium?

+2

Oxidation state of Sodium?

+1

Oxidation state of Potassium?

+1

Oxidation state of Magnesium?

+2

What are the properties of an Ionic bond?

-strong bond strength, moderate-high hardness, brittle

-poor conductors in solid state, liquids and solutions conduct by ion transport

-med to high melting point

-soluble in polar solvents

ex. Halite-NaCl, Fluorite-CaF2

What are the properties of metallic bonds?

-Variable bond strength, generally moderate hardness, high plasticity

-sectile, ductile, malleable

-good conductors of heat and e- by e- transport

-variable melting point, commonly low

-insoluble except in acids or alkalis by chemical reaction

-non-directed structures of high coordination and symmetry

ex. Copper-Cu, Silver-Ag, Gold-Au, most metals

What are the properties of covalent bonds?

-very strong bond strength, high hardness, brittle

-insulators in solid & melted state

-high melting point and very low solubility

-highly directional structure, structures of lower coordination and symmetry

ex. diamond-C, sphalerite-ZnS: organic molecules, graphite-C

How to find the coordination number?

The number of atoms surrounding a center atom is the CN

what is a minerals classification based on?

anionic group (the negatively charged ion)

What are the properties of Van der Walls bonds?

-soft and somewhat plastic mechanical structure

-mostly insulators in solid and liquid state

-low melting point

-soluble in organic solvents

-Low symmetry

-ex. Sulfur, graphite, organic compounds

What are the properties of a hydrogen bond?

-stronger bonds than van der walls but weaker than ionic and covalent

-high boiling and melting points

-soluble in water

What is electronegativity?

A measure of the tendency of an element to gain or lose
electrons

-Elements with low electronegativity lose their outer valence
electrons readily to form cations, whereas those with high
electronegativity have a strong afinity for extra electrons and
tend to form anions

Gibbs free energy of a mineral

-The free energy of a mineral is the amount of chemical energy available in the mineral to do work or undergo a reaction at a constant temperature and pressure

-GBE determines stability! whether the system will be a solution or melt

-Higher GFE means decreased stability

-lower GFE means increased stability

Why do minerals grow in the first place?

to achieve Gibbs Free Energy minimum-changes in physical and chemical conditions may occur in response to maintaining the minimum value

-cations and anions in the structure may become more or less disordered.

How does a mineral change if it undergoes transformations? (cation substitution, solid solutions, cation ordering)

-Cation substitution leads to changes in the crystal lattice which in turn adjusts the weight, density, optics (light passing through the crystal-color changes), and distorts the overall crystal lattice

-transformations in solid solutions include changes to melting and freezing points and changes to mineral stability

-Cation ordering lowers the structural symmetry of a mineral

entropy

measure of disorder, randomness, or unusable energy in a physical or thermodynamic system

enthalpy

a thermodynamic property that measures the total heat content of a system at constant pressure