Crystal ChemistryCrystal Chemistry
Mineral – “…defined, but generally not Mineral – “…defined, but generally not fixed, chemical composition…”fixed, chemical composition…”
Modern geology – Geochemistry or Modern geology – Geochemistry or GeophysicsGeophysics GeophysicsGeophysics – application of physical – application of physical
principles to study of earthprinciples to study of earth GeochemistryGeochemistry – application of chemical – application of chemical
principles to study of earthprinciples to study of earth high T or low Thigh T or low T
Coming up… a review of basic Coming up… a review of basic chemistrychemistry ElementsElements
protons, neutrons, electronsprotons, neutrons, electrons Bond types and controls on bondsBond types and controls on bonds
Nuclear ChemistryNuclear Chemistry
Atomic number (Z) – number of Atomic number (Z) – number of protonsprotons Specific for particular elements (periodic Specific for particular elements (periodic
table)table) Neutrons – about same mass as Neutrons – about same mass as
protons, different number of neutrons protons, different number of neutrons make isotopesmake isotopes
Atomic weight – sum of weight of Atomic weight – sum of weight of neutrons and protonsneutrons and protons Isotopes - Superscript in front of Isotopes - Superscript in front of
element symbol, atomic weight exact element symbol, atomic weight exact Elements – atomic weight is the average Elements – atomic weight is the average
of abundance of isotopesof abundance of isotopes
Stable IsotopesStable Isotopes Oxygen – Z = 8; three isotopesOxygen – Z = 8; three isotopes Average bulk earth abundance:Average bulk earth abundance:
1616O – 99.757%O – 99.757% 1717O – 0.038%O – 0.038% 1818O – 0.205%O – 0.205%
Materials (minerals, water, air, shells, etc) Materials (minerals, water, air, shells, etc) have variable ratios of these isotopeshave variable ratios of these isotopes
1818O = ratio of O = ratio of 1818O/O/1616OOsamplesample to to 1818O/O/1616OOstandardstandard
Radioactive isotopeRadioactive isotope Potassium (z=19)Potassium (z=19)
4040K has 21 neutronsK has 21 neutrons Natural abundance = 0.0117%Natural abundance = 0.0117% RadioactiveRadioactive Decays to Decays to 4040Ar, basis of one type of age datingAr, basis of one type of age dating Half life = 1.248 x 10Half life = 1.248 x 1099 a a
3939K has 20 neutronsK has 20 neutrons Natural abundance = 93.3%Natural abundance = 93.3% Stable (not radioactive)Stable (not radioactive)
4141K has 22 neutronsK has 22 neutrons Natural abundance = 6.7%Natural abundance = 6.7%
Chemical ReactionsChemical Reactions
Based on electron transfers, charge Based on electron transfers, charge balancebalance
If number of electrons = number of If number of electrons = number of protons, no electrical chargeprotons, no electrical charge
Orbit nucleus in systematic wayOrbit nucleus in systematic way Organized according to energy levelsOrganized according to energy levels Shells filled according to energyShells filled according to energy
Electron Quantum numberElectron Quantum number
Quantum number - reflects energy of Quantum number - reflects energy of electronelectron
Unique for each electronUnique for each electron No two electrons in atom can have same No two electrons in atom can have same
quantum numberquantum number Controls how electrons fill shellsControls how electrons fill shells Controls their chemical reactivityControls their chemical reactivity
Formation of ionsFormation of ions
Ions – excess or deficit of electrons Ions – excess or deficit of electrons relative to protonsrelative to protons Anions – net negative chargeAnions – net negative charge Cations – net positive chargeCations – net positive charge
Valence or Oxidation state is the Valence or Oxidation state is the value of the charge on the ionvalue of the charge on the ion
Configuration of valence electrons Configuration of valence electrons controls whether gain or lose controls whether gain or lose electronelectron Metals – typically lose one or two Metals – typically lose one or two
valence electron: form valence electron: form cationscations Non-metals – typically require a few Non-metals – typically require a few
electrons to fill valence shells: form electrons to fill valence shells: form anionsanions
Valence shells fill systematically – see Valence shells fill systematically – see table 3-3 for how shells filledtable 3-3 for how shells filled Atomic number 1-20 and 31-38 – fill s & p Atomic number 1-20 and 31-38 – fill s & p
subshellssubshells Between atomic number 20 and 31 – Between atomic number 20 and 31 –
shells fill from internal subshells – fill 3d shells fill from internal subshells – fill 3d shell (4s shell filled)shell (4s shell filled)
Transition metalsTransition metals Elements may have differing numbers of Elements may have differing numbers of
shells filledshells filled E.g. Ferrous and Ferric ironE.g. Ferrous and Ferric iron
Fig. 3-Fig. 3-33
Noble Gases, He, Ne, Ar, KrNoble Gases, He, Ne, Ar, Kr
Lose electrons (cations) Lose electrons (cations) to become noble gas coreto become noble gas core
Note the various Note the various oxidation states for the oxidation states for the transition metalstransition metals
Ferric Fe (+3)
Ferrous Fe (+2)
Metallic FeGain
electrons (anions)
Clearly – gain or loss of electrons Clearly – gain or loss of electrons importantimportant
““Quantified” as property called Quantified” as property called ElectronegativityElectronegativity
ElectronegativityElectronegativity Defined by Linus PaulingDefined by Linus Pauling Propensity of element to gain or lose Propensity of element to gain or lose
electronelectron Based on arbitrary scale: Li = 1, C = 2.5, Based on arbitrary scale: Li = 1, C = 2.5,
F = 4F = 4 Low electronegativity - the more likely Low electronegativity - the more likely
to lose electron form cationsto lose electron form cations High electronegativity – likely to gain High electronegativity – likely to gain
electron to form anionselectron to form anions See Table 3-4 for valuesSee Table 3-4 for values
Coming upComing up
1)1) Abundance of elements on earthAbundance of elements on earth
2)2) Types of electron sharing bonds – Types of electron sharing bonds – ionic, covalent, metallicionic, covalent, metallic
3)3) How to estimate bond types from How to estimate bond types from electronegativityelectronegativity
Earth abundances of Earth abundances of elementselements
What elements are most abundant?What elements are most abundant? These elements will make up common These elements will make up common
mineralsminerals What part of earth do they occur?What part of earth do they occur?
Crust?Crust? Bulk earth?Bulk earth?
Crust - 8 common elementsCrust - 8 common elements OO2-2-, Si, Si4+4+, Al, Al3+3+, Fe, Fe2+,3+2+,3+, Ca, Ca2+2+, Na, Na++, K, K++ and Mg and Mg2+2+
Most minerals are made of these elementsMost minerals are made of these elements Determination of crustal abundance – simply collect Determination of crustal abundance – simply collect
large number of samples and measurelarge number of samples and measure Bulk earth compositionBulk earth composition The same 8 elements are common in bulk, but different ratiosThe same 8 elements are common in bulk, but different ratios
Bulk Earth composition:Bulk Earth composition: Difficult to assess – impossible to directly Difficult to assess – impossible to directly
sample mantle or coresample mantle or core Estimated byEstimated by
Mass and density based on geophysical Mass and density based on geophysical measurementsmeasurements
Composition of mantle magmas and Composition of mantle magmas and xenolithsxenoliths
Composition of meteoritesComposition of meteorites
Chemical bondingChemical bonding Eight common elements (plus all others) Eight common elements (plus all others) bondbond to to
form mineralsform minerals Bonding controls spatial arrangement of atomsBonding controls spatial arrangement of atoms
Two categoriesTwo categories Sharing of valence electrons: Sharing of valence electrons: ionic, covalent and ionic, covalent and
metallicmetallic No sharing: No sharing: van der Waals and hydrogenvan der Waals and hydrogen
These 5 types of bonds are “end members”These 5 types of bonds are “end members” Rarely just one type or the otherRarely just one type or the other
However: We’ll consider most minerals to be However: We’ll consider most minerals to be ionically bondedionically bonded
Ionic BondingIonic Bonding Transfer of electron(s) from one Transfer of electron(s) from one
element to anotherelement to another Results in filled valence shells of bothResults in filled valence shells of both The electrostatic attraction keep atoms The electrostatic attraction keep atoms
togethertogether The distance between ions depends The distance between ions depends
on attractive forces (Coulomb law) on attractive forces (Coulomb law) and repulsive forces (Born repulsion)and repulsive forces (Born repulsion)
Fig. 3-4Fig. 3-4
Attractive forces
Repulsive forces
Face centered Face centered cubic lattice cubic lattice arrangement of arrangement of halitehalite
Bonding in HaliteBonding in Halite
Equilibrium distance = 2.8 Å
Fig. 2-10Fig. 2-10
Ionic bondingIonic bonding Ions bond so that positive = negative Ions bond so that positive = negative
chargescharges Minerals Minerals mustmust be electrically neutral be electrically neutral NaCl (Halite), Na(Mg,Fe,Li,Al)NaCl (Halite), Na(Mg,Fe,Li,Al)33AlAl66[Si[Si66OO1818]]
(BO(BO33))33(O,OH,F)(O,OH,F)44 (tourmaline – not all ionic (tourmaline – not all ionic bonds here) bonds here)
Characteristics:Characteristics: Ions act like spheresIons act like spheres Alternating cations and anionsAlternating cations and anions One of the strongest bondsOne of the strongest bonds Brittle because like ions repelBrittle because like ions repel Cleavage is commonCleavage is common
Covalent BondsCovalent Bonds Electrons shared when orbitals of two Electrons shared when orbitals of two
different elements overlapdifferent elements overlap Shared by only two atomsShared by only two atoms Differs from metallic (later – all atoms Differs from metallic (later – all atoms
share electrons)share electrons) Electrons move around nucleus of both Electrons move around nucleus of both
atomsatoms
Examples – Diamond and GraphiteExamples – Diamond and Graphite Diamond Diamond
Stable Ne configuration by either gain or loss Stable Ne configuration by either gain or loss of 4 electronsof 4 electrons
Ionic bonding not possible because all Ionic bonding not possible because all electrons exactly the same electronegativityelectrons exactly the same electronegativity
One carbon won’t “steal” electron from One carbon won’t “steal” electron from anotheranother
Instead share electrons – very strong bondingInstead share electrons – very strong bonding
Fig. 3-5Fig. 3-5
Covalent Covalent bonding in bonding in diamonddiamond
• 4 orbitals shown 4 orbitals shown as bonds, call as bonds, call bondsbonds• bonds distortedbonds distorted
Each bold line Each bold line represents represents another similar another similar bondbond
GraphiteGraphite
Fig. 3-6
Similar Similar bonds, but bonds, but only in layersonly in layers
Additional Additional Sharing Sharing electrons, electrons, bonds. bonds.
Metallic bondsMetallic bonds
A type of covalent bondA type of covalent bond Electrons shared without systematic Electrons shared without systematic
change in orbitalschange in orbitals Free to move throughout crystal Free to move throughout crystal
structurestructure Formed with low electronegativity – Formed with low electronegativity –
weakly held valence electronsweakly held valence electrons
Relation between valence-Relation between valence-dependent bondsdependent bonds
Most bonds not purely ionic, covalent Most bonds not purely ionic, covalent or metallicor metallic
Amount of bond type depends on Amount of bond type depends on electronegativity (tendency to give electronegativity (tendency to give up electrons)up electrons)
Greater difference in Greater difference in electronegativity between ions electronegativity between ions means more ionic characteristicmeans more ionic characteristic
Only 1 anion (of 8 common Only 1 anion (of 8 common elements)elements) OxygenOxygen Electronegativity of O = 3.5Electronegativity of O = 3.5 Electronegativity of other common Electronegativity of other common
elements range from 0.8 (K) to 1.8 (Si)elements range from 0.8 (K) to 1.8 (Si)
Qualitative difference in electronegativity:Qualitative difference in electronegativity: O-K = 3.5 – 0.8 = 2.7, more ionic O-K = 3.5 – 0.8 = 2.7, more ionic
characteristicscharacteristics O-Si = 3.5 – 1.8 = 1.7, less ionic O-Si = 3.5 – 1.8 = 1.7, less ionic
characteristicscharacteristics Possible to quantify % ionic bonding:Possible to quantify % ionic bonding:
O-element bonding of 8 common elements O-element bonding of 8 common elements ranges from 50% ionic (Si-O) to 80% ionic (K-ranges from 50% ionic (Si-O) to 80% ionic (K-O)O)
Eq. 3.4Eq. 3.4Fig. 3-10Fig. 3-10
% ionic character = 1 – e-0.25(Xa – Xc)2
Note negative sign, typo in 1st edition
O-Si ~50 % ionic
O-K ~80 % ionic
X = electronegativity of a, anion and c, cation
Native elementsNative elements
Examples: S, Fe, Au…Examples: S, Fe, Au… No differences in electronegativityNo differences in electronegativity Bonding intermediate between Bonding intermediate between
covalent and metalliccovalent and metallic Low electronegativity values (Cu, Ag, Low electronegativity values (Cu, Ag,
Au) favor metallic bondingAu) favor metallic bonding High electronegativity values (non-High electronegativity values (non-
metals, C, S) favor covalent bondingmetals, C, S) favor covalent bonding
Fig. 3-9Fig. 3-9
Con
tinuo
us v
aria
tions
Continuous variations
Limited variations
100% covalent, metallic or ionic
50 % covalent & 50% metallic
Part covalent, part metallic, and part ionic
Range of possible mixtures of electron-Range of possible mixtures of electron-sharing valence bond typessharing valence bond types
PercentagesNot
Allowed
Physical Properties caused by Physical Properties caused by Valence bondsValence bonds
Electrical conductanceElectrical conductance Ionic and covalentIonic and covalent have little have little
conductanceconductance MetallicMetallic highly conductive highly conductive
SolubilitySolubility IonicIonic highly soluble (think halite) highly soluble (think halite)
BrittlenessBrittleness IonicIonic highly brittle – cleavage common highly brittle – cleavage common
Halite – perfect {001} cubic cleavageHalite – perfect {001} cubic cleavage
HardnessHardness Covalent Covalent – strongest bonding, so – strongest bonding, so
hardest. Think diamondhardest. Think diamond MalleableMalleable
MetallicMetallic easily worked easily worked
Non-valence bondsNon-valence bonds
Result of asymmetric charge Result of asymmetric charge distributiondistribution Create electrostatic forcesCreate electrostatic forces Two typesTwo types
Van der Waals and HydrogenVan der Waals and Hydrogen
Hydrogen BondingHydrogen Bonding Ice exampleIce example
HH22O is polar moleculeO is polar molecule 2 H atoms at angle to O atom (not straight 2 H atoms at angle to O atom (not straight
line)line) O is more electronegative than HO is more electronegative than H O = 3.5, H = 2.1O = 3.5, H = 2.1 O “claims” more of the electronO “claims” more of the electron Net negative charge on O side of moleculeNet negative charge on O side of molecule
The asymmetric charges allow solidifying The asymmetric charges allow solidifying liquid when T < 0º C @ 1 atm Pliquid when T < 0º C @ 1 atm P
Fig. 3-11 & 18-2Fig. 3-11 & 18-2
Asymmetrical charge - polar
Hydrogen bond
Hexagonal symmetry
Ice – viewed down c axis
Van der WallsVan der Walls Carbon exampleCarbon example
Graphite – carbon bonded in sheetsGraphite – carbon bonded in sheets Bonding within sheets is covalent – Bonding within sheets is covalent – bonds bonds Over time electrons evenly distributedOver time electrons evenly distributed At given time, excess electrons on one side At given time, excess electrons on one side
of sheetof sheet Creates weak electrostatic attractionCreates weak electrostatic attraction
Physical propertiesPhysical properties Typically softTypically soft Graphite good lubricantGraphite good lubricant
Fig. 3-12Fig. 3-12
Covalent bonds within the sheets
Van der Waal forces between the sheets,Caused by bonds on top of sheets
Other examples:talcserpentine/smectite
Atoms and ion sizeAtoms and ion size
Assume that atoms are spheresAssume that atoms are spheres Clear simplification – electron Clear simplification – electron
distributions are not sphericaldistributions are not spherical Assumption works well for arrangement Assumption works well for arrangement
in solidsin solids Atoms pack together in regular Atoms pack together in regular
arrangementarrangement
If we assume the ions are spheresIf we assume the ions are spheres Can assume an Can assume an effective radiuseffective radius Measure distance between adjacent atoms in the Measure distance between adjacent atoms in the
solidsolid Measured with X-ray diffraction, d spacingMeasured with X-ray diffraction, d spacing
Effective radius a measure of size of the atomsEffective radius a measure of size of the atoms
Very important – one control of how atoms Very important – one control of how atoms pack togetherpack together
Bond length – sum of effective radius of Bond length – sum of effective radius of two adjacent atomstwo adjacent atoms Metallic bonds: all same effective radiusMetallic bonds: all same effective radius
½ distance between nuclei½ distance between nuclei Ionic bonds: effective radius different between Ionic bonds: effective radius different between
two atomstwo atoms Not ½ distance between nucleiNot ½ distance between nuclei
Fig. 3-13Fig. 3-13
Metallic bondingBond length = d spacingIonic radius = ½*d spacing
Covalent and ionic bondingBond length = d spacingd spacing = Ra + Rc
Clearly – what types of ions present Clearly – what types of ions present control ionic radiuscontrol ionic radius
Primary variables controlling ionic Primary variables controlling ionic radius:radius: Oxidation state Oxidation state – i.e. charge on ion– i.e. charge on ion Coordination number Coordination number – i.e. number of – i.e. number of
ions surrounding central ionsions surrounding central ions
Oxidation stateOxidation state
Inversely relatedInversely related More oxidized (less negative, more More oxidized (less negative, more
positive) means smaller effective radiuspositive) means smaller effective radius e.g., Fee.g., Fe3+3+ or Fe or Fe2+2+
Cations smaller than anions, OCations smaller than anions, O2-2- very very largelarge
Positive charge holds electron closer to Positive charge holds electron closer to nucleusnucleus
Fig. 3-15Fig. 3-15ChargeCharge
(higher oxidation state)(higher oxidation state)
Ion
ic r
ad
ius
(Å)
Coordination numbers
CoordinationCoordination
Positive correlation – high Positive correlation – high coordination number, smaller ionscoordination number, smaller ions Think of solids as large anions Think of solids as large anions
surrounding small spaces filled by cationssurrounding small spaces filled by cations Size of space determined by effective Size of space determined by effective
radius of anionsradius of anions Cation effective radius changes to fill Cation effective radius changes to fill
spacespace