PostPost-crystallization process

Changes in structure and/or composition following crystallization

Examples

Ordering

e.g. in the K-feldspars KChanges result from cooling

Exsolution another example of phase diagram Recrystallization Radioactive decay Structural defects Twinning

Exsolution

Common in alkali feldspars, also occurs in the plagioclase feldspars

High T: complete solid solution between K and Na Low T: limited solid solution Distribution of solid solution shown on phase diagram

Alkali FeldsparPH2O = 1.96 kb Only limited temperature range with complete solid solution Works exactly like the plagioclase feldspar except binary minimum

phase diagram

Solid homogeneous alkali feldsparsAlbite matrix K-spar matrix

Homogeneous compositions not allow Split into two separate phases

Fig. 5-27

Exsolution occurs in solid state

Time and temperature depending Most have sufficient time for diffusion to move ions

Perthite term for albite exsolution lamellae in K-spar matrix KAntiperthite K-spar exsolution lamellae in albite matrix

Alkali FeldsparPH2O = 5 kb Solvus line intersects the Liquidus and Solidus curves Crystallization continues as usual until point d eutectic Albite and K-spar crystallize as individual crystals with limited solid solution

phase diagram

Examples of post-crystallization post

Ordering

e.g. in the K-feldspars KChanges result from cooling

Exsolution another example of phase diagram Recrystallization Radioactive decay Structural defects Twinning

Recrystallization

Surfaces are high energy environment because of terminated bonds Minerals will change to minimize the surface area Grains become larger Edges become smoother

Smoother boundaries from recrystallization

Larger grain size from recystallization

Fig. 5-22 5-

Pseudomorphism

Replacement of one mineral by another Preserves the external form of original mineral Example:

Goethite (orthorhombic) replacing pyrite (isometric)

Radioactivity

Generate new elements cause substitution defects

Decay of 40K to 40Ca and 40Ar Below closing T, Ar trapped, used for dating

Alpha decay

Alpha particle dislodges atoms

Causes defect in crystal structure

Metamict minerals form if long enough time and high enough radioactivity Change physical properties because loss of long range order

Less dense Darker Optical properties change

Also may change physical properties of surrounding minerals

Structural Defects

Disruptions in ordered arrangement of crystals

Common in natural minerals

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

Systematic throughout crystal lattice

I will only talk about types of point defects

Point Defects

Schottky Defect - Vacant Sites Frenkel defect - Atoms out of correct position Impurity defects: defects:

Extraneous atoms or ions Substituted atoms or ionsSimilar to solid solution series or substitutions Difference is magnitude of substitution

Schottky defects

Vacancy i.e. both cation and anion missing 1:1 ratio vacancy if similar charge e.g. Halite Can be more complex with higher charge

Frenkel Defects

Dislocation defects Generally cations because they are smaller No change in the charge balance

Frenkel and Schottky

Mechanism for changes in solid state

Diffusion through minerals Allows metamorphism

Impurity Defects

Interstitial defects

Ions or atoms in sites not normally occupied Requires charge balance of mineral Substitution of one ion for another ion in the structure Identical to substitution , but depends on expectation of pure composition

Substitution defects

Interstitial defect foreign cation located in structure

Substitution defect foreign cation substitutes for normal cation

Fig. 5-11 5-

Twinning

Intergrowth of two or more crystals Related by symmetry element not present in original single mineral Several twin operations: operations:

Reflection Rotation Inversion (rare)

Twin Law describes twin operation and axis or plane of symmetry

Reflection

Two or more segments of crystal Related by mirror that is along a common crystallographic plane Can not be a mirror in the original mineral

Rutile TiO2Crystallographic axes

Twin law: Reflection on (011)

Reflection on {011}

Fig. 5-20

Rotation

Two or more segments of crystal Related by rotation of crystallographic axis common to all Usually 2-fold 2Can not duplicate rotation in original mineral

Twin Law: Rotation on [001] Very common in Kspars called Carlsbad twins

Fig. 5-16 5-

Twin terminology

Composition surface plane joining twins, may be irregular or planar Composition plane if composition surface is planar; referred to by miller index Contact twin no intergrowth across composition plane

Contact TwinsSpinel reflected on {111}

Gypsum

reflected on {100}

Calcite

reflected on {001}

Fig. 5-17 5-

Penetration twin inter-grown twins, intertypically irregular composition surfaces

Pyrite 90 rotation on [001]

Staurolite reflection on {231}

Fig. 5-18 5-

Simple twins two twin segments Multiple twins three or more segments repeated by same twin law Polysynthetic twins succession of parallel composition planes (plagioclase) Cyclic twins succession of composition planes that are not parallel

Polysynthetic Twins

Cyclic Twins

Plagioclase repeated reflection on {010}

Rutile repeated reflection on {011}Fig. 5-19 5-

Mechanism forming twins

Growth occur during growth of minerals Transformation displacive polymorphs

Occurs during cooling of minerals E.g. leucite, transforms from cubic to leucite, tetragonal system - @ 665 C Space change accommodated by twins

Isometric above 665 C

Tetragonal below 665 C

Can be elongate along any three directions

Leucite KAlSi2O6 A feldspathoidTwinned crystals can fill all available space

Fig. 5-20 5-

Deformation twinning

Result from application of shear stress Lattice obtains new orientation by displacement along successive planes

Fig. 5-20 5-