Post-crystallization process Changes in structure and/or composition following crystallization Changes in structure and/or composition following crystallization.

Post-crystallization processPost-crystallization process

Changes in structure and/or Changes in structure and/or composition following crystallizationcomposition following crystallization

ExamplesExamples OrderingOrdering

e.g. in the K-feldsparse.g. in the K-feldspars Changes result from coolingChanges result from cooling

Exsolution – another example of phase Exsolution – another example of phase diagramdiagram

RecrystallizationRecrystallization Radioactive decayRadioactive decay Structural defectsStructural defects TwinningTwinning

Idealized feldspar structure

Fig. 12-6

Si or Al

K (or Na, Ca)

Si or Al

Fig. 4-13Fig. 4-13

Al migrates through structure with cooling:Sanidine to Orthoclase to Microcline as Al restricted

ExsolutionExsolution

Common in alkali feldspars, also Common in alkali feldspars, also occurs in the plagioclase feldsparsoccurs in the plagioclase feldspars High T: complete solid solution between High T: complete solid solution between

K and NaK and Na Low T: limited solid solutionLow T: limited solid solution Distribution of solid solution shown on Distribution of solid solution shown on

phase diagramphase diagram

Alkali Feldspar – complete phase Alkali Feldspar – complete phase diagramdiagram

PPH2OH2O = 1.96 kb = 1.96 kb

Only limited Only limited temperature range temperature range with complete solid with complete solid solution (770 to 680)solution (770 to 680)

Works exactly like the Works exactly like the plagioclase feldspar plagioclase feldspar except binary except binary minimumminimum

Fig. 5-7a

Fig. 5-27Fig. 5-27

Solid homogeneous alkali feldspars

Homogeneous compositions not allowHomogeneous compositions not allowSplit into two separate phasesSplit into two separate phases

Albite matrix

K-spar matrix Start

Exsolution occurs in solid stateExsolution occurs in solid state Time and temperature dependentTime and temperature dependent Most have sufficient time for diffusion to Most have sufficient time for diffusion to

move ions, separate two phasesmove ions, separate two phases PerthitePerthite – term for albite exsolution – term for albite exsolution

lamellae in K-spar matrixlamellae in K-spar matrix AntiperthiteAntiperthite – K-spar exsolution – K-spar exsolution

lamellae in albite matrixlamellae in albite matrix

PPH2OH2O = 5 kb = 5 kb

Solvus line intersects the Solvus line intersects the Liquidus and Solidus Liquidus and Solidus curvescurves

Crystallization continues Crystallization continues as usual until point d – as usual until point d – eutectic, Kseutectic, Ks5353 and Ks and Ks1919 crystallize until solidcrystallize until solid

With more cooling, Albite With more cooling, Albite and K-spar “unmix” and and K-spar “unmix” and become more “pure” become more “pure” phases. Still limited solid phases. Still limited solid solution.solution.

Alkali Feldspar – phase Alkali Feldspar – phase diagramdiagram

RecrystallizationRecrystallization

Surfaces are high energy Surfaces are high energy environment because of terminated environment because of terminated bondsbonds

Minerals change to minimize the Minerals change to minimize the surface areasurface area Edges become smootherEdges become smoother Grains become largerGrains become larger

Fig. 5-26Fig. 5-26

Smoother boundaries from recrystallizationMinimize surface area

Contact metamorphismContact metamorphism

Larger grain size from recrystallization

PseudomorphismPseudomorphism Replacement of one mineral by anotherReplacement of one mineral by another

Low – T phenomenon usually, weatheringLow – T phenomenon usually, weathering Preserves the external form of original Preserves the external form of original

mineralmineral Example:Example:

quartz (hexagonal) replacing fluorite quartz (hexagonal) replacing fluorite (isometric)(isometric)

Cubic Quartz??

Radioactivity – Beta decayRadioactivity – Beta decay

Generate new elements cause Generate new elements cause substitution defectssubstitution defects Decay of Decay of 4040K to K to 4040Ca and Ca and 4040ArAr Beta decay (electron or positron emitted)Beta decay (electron or positron emitted) The newly created elements are not same The newly created elements are not same

size or charge as the original elementsize or charge as the original element Not typically substituted in mineralNot typically substituted in mineral

Below closing T, Ar trapped, used for Below closing T, Ar trapped, used for datingdating

Radioactivity - Alpha decayRadioactivity - Alpha decay

Alpha particle dislodges atomsAlpha particle dislodges atoms Causes defect in crystal structureCauses defect in crystal structure

MetamictMetamict minerals form if long enough minerals form if long enough time and high enough radioactivitytime and high enough radioactivity

Change physical properties because loss Change physical properties because loss of long range orderof long range order Less denseLess dense DarkerDarker Optical properties changeOptical properties change

Also may change physical properties of Also may change physical properties of surrounding mineralssurrounding minerals

Structural DefectsStructural Defects

Disruptions in ordered arrangement of Disruptions in ordered arrangement of atoms within crystalsatoms within crystals Common in natural mineralsCommon in natural minerals

Occur as point, line, or plane defectOccur as point, line, or plane defect Different from compositional variationDifferent from compositional variation

Systematic throughout crystal latticeSystematic throughout crystal lattice I will only talk about types of point I will only talk about types of point

defectsdefects

Point DefectsPoint Defects

Schottky Defect Schottky Defect - Vacant Sites- Vacant Sites Frenkel defect Frenkel defect - Atoms out of correct - Atoms out of correct

position – position – Impurity defectsImpurity defects::

Extraneous atoms or ionsExtraneous atoms or ions Substituted atoms or ionsSubstituted atoms or ions

Similar to solid solution series or Similar to solid solution series or substitutionssubstitutions

Difference is magnitude of substitutionDifference is magnitude of substitution

Schottky defectsSchottky defects Vacancy – i.e. both cation and anion Vacancy – i.e. both cation and anion

missingmissing 1:1 ratio vacancy if similar charge – 1:1 ratio vacancy if similar charge –

e.g. Halite missing equal amount of Cle.g. Halite missing equal amount of Cl-- and Naand Na++

Can be more complex with higher Can be more complex with higher chargecharge

Fig. 5-15a

Frenkel DefectsFrenkel Defects

Dislocation defectsDislocation defects Generally cations because they are Generally cations because they are

smallersmaller No change in the charge balanceNo change in the charge balance

Fig. 5-15b

Frenkel and SchottkyFrenkel and Schottky

Mechanisms for changes in solid Mechanisms for changes in solid statestate Diffusion through mineralsDiffusion through minerals Allows metamorphismAllows metamorphism

Impurity DefectsImpurity Defects

Interstitial defectsInterstitial defects Ions or atoms in sites not normally occupiedIons or atoms in sites not normally occupied Requires charge balance of mineralRequires charge balance of mineral

Substitution defectsSubstitution defects Substitution of one ion for another ion in the Substitution of one ion for another ion in the

structurestructure Identical to “substitution”, but depends on Identical to “substitution”, but depends on

expectation of pure compositionexpectation of pure composition Example – radioactive decay, Example – radioactive decay, 4040K to K to 4040ArAr

Fig. 5-11Fig. 5-11

Interstitial defect – Interstitial defect – foreign cation foreign cation located in located in structurestructure

Substitution defect Substitution defect – (1) foreign cation – (1) foreign cation substitutes for substitutes for normal cationnormal cation(2) Radioactive (2) Radioactive decaydecay

TwinningTwinning Intergrowth of two or more crystalsIntergrowth of two or more crystals Related by symmetry element not Related by symmetry element not

present in original single mineralpresent in original single mineral Several Several twin operationstwin operations (i.e. symmetry (i.e. symmetry

element):element): ReflectionReflection RotationRotation Inversion (rare)Inversion (rare)

““Twin Law” Twin Law” – describes twin operation – describes twin operation and axis or plane of symmetryand axis or plane of symmetry

ReflectionReflection

Two or more segments of crystalTwo or more segments of crystal Related by mirror that is along a Related by mirror that is along a

common crystallographic planecommon crystallographic plane Can not be a mirror in the original Can not be a mirror in the original

mineralmineral

Fig. 5-20Fig. 5-20

Crystallographic axes

Reflection on {011}

Twin law: Reflection on (011)

Rutile TiO2 - Tetrahedral

RotationRotation

Two or more segments of crystalTwo or more segments of crystal Related by rotation of Related by rotation of

crystallographic axis common to allcrystallographic axis common to all Usually 2-foldUsually 2-fold Can not duplicate rotation in original Can not duplicate rotation in original

mineralmineral

Fig. 5-16Fig. 5-16

Twin Law: Rotation Twin Law: Rotation on [001]on [001]Very common in K-Very common in K-spars – called spars – called “Carlsbad twins”“Carlsbad twins”

Twin terminologyTwin terminology

Composition surface Composition surface – plane joining – plane joining twins, may be irregular or planartwins, may be irregular or planar

Composition plane Composition plane – if composition – if composition surface is planar; referred to by surface is planar; referred to by miller indexmiller index

Contact twin Contact twin – no intergrowth across – no intergrowth across composition planecomposition plane

Fig. 5-21Fig. 5-21

Contact Twins

Spinel isometricSpinel isometric – – reflected on {111}reflected on {111}

Gypsum MonoclinicGypsum Monoclinic – – reflected on {100}reflected on {100}

Calcite hexagonalCalcite hexagonal – – reflected on {001}reflected on {001}

Fig. 5-22Fig. 5-22

Penetration twin Penetration twin – inter-grown twins, – inter-grown twins, typically irregular composition surfacestypically irregular composition surfaces

Pyrite Isometric – Pyrite Isometric – 180º rotation on 180º rotation on [001][001]

Staurolite Staurolite Monoclinic – Monoclinic – reflection on reflection on {231}{231}

Simple twins Simple twins – two twin segments– two twin segments Multiple twins Multiple twins – three or more segments – three or more segments

repeated by same twin lawrepeated by same twin law Polysynthetic twins Polysynthetic twins – succession of – succession of

parallel composition planes (plagioclase)parallel composition planes (plagioclase) Cyclic twins Cyclic twins – succession of composition – succession of composition

planes that are not parallelplanes that are not parallel

Fig. 5-23Fig. 5-23

PolysynthetiPolysynthetic Twins c Twins

Plagioclase:Plagioclase:Albite twinning: repeated Albite twinning: repeated reflection on {010}reflection on {010}Allows Michel – Levy techniqueAllows Michel – Levy technique

Cyclic Twins

Rutile – repeated Rutile – repeated reflection on reflection on {011}{011}

Mechanism forming twinsMechanism forming twins

GrowthGrowth – occur during growth of – occur during growth of mineralsminerals

TransformationTransformation – displacive – displacive polymorphspolymorphs Occurs during cooling of mineralsOccurs during cooling of minerals E.g. leucite, transforms from cubic to E.g. leucite, transforms from cubic to

tetragonal system - @ 665º Ctetragonal system - @ 665º C Space change accommodated by twinsSpace change accommodated by twins

Fig. 5-20Fig. 5-20

Isometric above 665º C Tetragonal below 665º C

Can be elongate along any three directions

Twinned crystals can fill all available space

LeuciteKAlSi2O6

A feldspathoid

Fig. 5-20Fig. 5-20

Deformation twinningDeformation twinning Result from application of shear stressResult from application of shear stress Lattice obtains new orientation by Lattice obtains new orientation by

displacement along successive planesdisplacement along successive planes