2.5 structural defects and twins
TRANSCRIPT
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-