Introduction to MineralogyDr. Tark Hamilton
Chapter 10: Lecture 30-32Crystal Growth, Twinning,
Defects, Colour & Magnetism
Camosun College GEOS 250
Lectures: 9:30-10:20 M T Th F300
Lab: 9:30-12:20 W F300
6 foot Stalactites & HelictitesHuw Cordey,
Lechuguilla Cave, NMWhite Sands NM
Larry FellowsSatin Spar, Barry Marsh
AlabasterCarvings, Tom Joe
Gypsum
?
Gypsum: acicular & tabular S. AustraliaDesert Rose Bahia Argentina, M. Olsina
Martins da PedraEyre Peninsula, S. Aus.
Rob Lavinsky
Rob Lavinsky
Crystals = Selenite
Gypsum 2/m: Santa Eulalia, MXTabular & Cl on (010), Pinacoid & Cl on (001), Prism bevel (120), other faces rare
Gypsum - CaSO4•2H2O Variety.(Selenite) 2/mLeft: Hourglass Tw [010](010)(110) Right: (010)(001) Fishtail TwSouth Australia Rob Lavinsky (Twin Composition Plane = (100)
fig_10_01
Nucleus of 62 formula units of NaClCrystals React Through Their Exterior Surfaces
Interior ionsOctahedral &
SatisfiedIsodesmic
Bond energyChemical Activity = 1
Exterior ionsOctahedral &Unsatisfied
AnisodesmicLarge Free
Surface EnergyChemical Activity
Large &Ready to PPTOr Dissolve
fig_10_02
Surface Growth & Reactivity of CrystalsDepends Upon Unsatisfied Charges
Free Corner & Edges
Interior Satisfied Ions
Likeliest face to grow:Corner > Step > Terrace
Surface Clusters likely to redissolveFree Surface Energy ≈ Surface Area/Volume
fig_10_03
Common Forms are Lattice Planeswith high Site Density e.g.: AB, AC, AD
Filled Sites/Length= Site Density
Points alongHypotenuse /
Length = √
0.7071
0.2774
0.4472
0.3153
1.0000
fig_10_04
Crystal Forms,Variable Growth Rates, - Vectoral Properties
(100) EqualNa+ & Cl-
(111) AlternateNa+ & Cl- planes
Vectoral PropertiesDepend on Direction:
Hardness,Conductivity,Speed of light,Xray-diffraction
Discontinuous Vectoral Properties:
• Pertain only to certain planes or directions
• No intermediate values
• Cleavage
• Fracture
• Rate of Growth
• Rate of Solution
• Chemical or Ionic Diffusion
fig_10_05
Crystal Growth, Colour Zonation & Transformation of Forms
Octahedral nucleus
Cube Overgrowth
Mn & Fe Oxy-hydroxide Dendrites:Solnhofen Limestone Bavaria, P. Andresen
Dessication & Bedding PermeabilityMn more easily oxidized &
More easily precipitated than FeDip direction
fig_10_07
Point, Line & Mosaic Defectsin a Hexagonal closest Packed Layer of Spheres
Good toolBad 737
Tail
Point Defects• Point Defects Represent disorder, vacancy,
Higher Temperature locations in structure• Shottky Defects: Cations (or Anions) absent
from structure• Frenkel Defects: Dislocation of a Cation or
Anion into an adjacent (normally vacant) site• Shottky & Frenkel Defects don’t affect
Stoichiometry• Impurity Defects: Interstitial or Substitutional
can affect colour even at ppb or ppm levels e.g. Ti in Quartz to form Amythest or Rutillated form
fig_10_08
Crystal Defects: Point, Line & Plane
Shottky
FrenkelEdge Dislocation Lineage Structure
Screw DislocationImpurity Defect
Other Defects in Crystals• Stacking Faults: AB-AB-A –AB in Hexagonal,
ABC-ABC- BC-ABC in Cubic, TOT-TOT-T-TOT in clays (a missing layer)
• Omission solid solution: A more highly charged cation substitutes for 2 cations leaving 1 void as in 2K+AlSi3O8 Pb+2AlSi3O8 + □AlSi3O8
□Ca2Mg5Si8O22(OH)2 Na+Ca2Mg5Al3+Si7O22(OH)2
Also in Beryl, Zeolites & in defect structures like Pyrrhotite (Fe2+
1-3xFe3+2x)□ x S Fe6S7 – Fe11S12
• Colour Centers: electron for anion as in Fluorite• Chain width errors in amphiboles, clays: curls
fig_10_10
Chain Width Errors in InosilicatesHRTEM photo: a٭=asinβ
Cl = 56°& 124°
fig_10_11
Epitaxial Overgrowths (Energy) (oriented mineral-mineral contacts)
Also granoblastic textures: Quartzite, Marble & foliations in schist, gneiss also
Catalytic surfaces, templates, adsorbtion
Controls onExsolution &
Twinning
fig_10_12
Parallel Crystal Growth (C-axes)(Really all one crystal)
Scepter QuartzBarite Tablets
fig_10_13
Twinning: Symmetrical intergrowth of 2 or more crystals of the same mineral
Atoms in theComposition Plane
Fit both crystal lattices
Twin Laws have aComposition Plane
& or a rotational axisOr mirror,
as a single extrasymmetry element
3 Causes for Twinning• Growth twins are the interruption or change in the
lattice during formation due to deformation from a larger substituting ion
• Annealing or Transformation twins result of a change in crystal system during cooling as one form becomes unstable & the crystal structure re-organizes into a more stable form
• Deformation or gliding twins result from stress on the crystal after the crystal has formed, as during regional metamorphism
• HCP structure is the most likely to twin of the three common crystal structures: BCC, FCC, and HCP
• Epitaxis and Parallel growth simply reduces free surface energy and is not twinning
fig_10_14
Contact (CP) & Penetration TwinsSpinel LawCP=(111)
CarlsbadLaw
TA=[001]CP=(010)
FluoriteTA=[111]
Pyrite iron cross
TA=[001]
Japan Law CP=(1122)
fig_10_16
Polysynthetic: multiple parallel twins
(010)
(1012)
Carlsbad-albite Albite
TA={001}
Good
CP=(031)
Perfect(001)
Labradoresence
Plagioclase
CP=(011)
Rhomb-Diagonal
fig_10_17
Striations from Polysynthetic Twinning
Plagioclase Perfect(001)
CP=(010)
Albite twinning(Triclinic) also:
Ala-a, Ala-b,Acline, Pericline
MagnetiteOct-Dodec(111) twins
Pyrite 2/m 3Cube
(011) twins
MagnetiteStriated on
(110) by (111)
fig_10_19
Common Twins of Monoclinic Minerals
fig_10_20
Orthorhombic Twin Laws
CaCO3PbCO3
Fairy CrossesFe4Al16Si8O48H2
(monoclinic)
fig_10_21
(011) Tetragonal Twins (diagonal)
SnO2
TiO2
fig_10_22
Hexagonals twin most commonly
Calcite (0112)Negative rhombohedron
CalciteC=TA
Butterfly twin
Quartz twins:Brazil (1120)
Dauphine (0001)Japan (1122)
fig_10_24
Spectrum & Causes of Mineral Colour
fig_10_25
Near Infrared(Molecular Bands) Lattice Energy
Transition metal ions(unfilled d orbitals)
Visible Absorbtion
Visible & IR Spectrum ofBe3Al2Si6O18 ± H2O,CO2
fig_10_26
Crystal Field Splitting of d-electrons(promoted or demoted by octahedral anions)
RandomAnions
Planar demotionBetween anions
Axial promotionToward anions
dz2
fig_10_27
Absorbtion Spectra of 3 Gems
BeAl2O4 – Fe 3+ 6fold
(Mg,Fe)2SiO4 – Fe2+ 6fold
Fe3Al2Si3O12 – Fe2+ 8fold
fig_10_28
Differential promotion of Cr-d electronsin Ruby versus Emerald
Cr Absorbion Peaks In Ruby
Tra
nsm
issi
on
Transmission
table_10_01
fig_10_29
Molecular Orbital TheoryExplains the Blue Colour in Sapphire
Electron transfer ~ Vis+nearIRFe2+
A + Fe3+B Fe3+
A + Fe3+B
table_10_02
fig_10_30
Colour or f-centres in Purple Fluorite
f = farbe“colour” in German
fig_10_31
Hole Colour Centres in Smoky Quartz
Normal Quartz
Al+H SubstitutedQuartz with
Radiation damage(electron holes)
e- has excited states
table_10_03
Physical Processes for Colour• Admixture or inclusions of other minerals/matter Green Quartz (Adventurine) – chlorite inclusions Black Calcite – graphite, MnOxides, petroleum Pink K-spar – Hematite inclusions Red Jasper – Hematite inclusions Feldspar (Sunstone) – Native Cu inclusions
• Refraction of light for iridescence Feldspar – Labradorescence
• Irradiation or heat treating Blue Topaz from Yellow Yellow Citrine from Smoky Quartz or Amythest Sapphire or Ruby from Corundum
table_10_04Fe2+ Fe3+
2 O4 – Magnetite
Types of Magnetic Mineral Behaviour(in the presence of an external field)
• Diamagnetic: paired electrons, no moment, repelled by field. e.g. Calcite, Quartz, Feldspar
• Paramagnetic: few unpaired electrons, weakly attracted, thermal randomization dominates. e.g. Olivine, Augite, Hypersthene, Hornblende
• Ferromagnetic: dominantly aligned unpaired electron domains. Strongly attracted & capable of remnance. e.g. Taenite & Kamacite in FeNi
• Ferrimagnetic: aligned unpaired electons outweigh anti-aligned ones. e.g. Magnetite, Chromite, Pyrrhotite, Greigite, Smythite
fig_10_32
Unmagnetized & Magnetizedgranular multidomain magnetite
Alternate directionsFor grains > 10μ
Randomno magnetic moment
fig_10_33
Taenite
Native Fe0
Curie Point 770°C
Magnetite
Fe3+IV (Fe2+ Fe3+)VI O4
Curie Point 580°C
fig_10_34
Amorphous Alloy(disordered)
Crystalline Alloy(ordered)
