Structural defects and twinning

No crystal has 100% flawless structure

Crystal defects Defects can affect Strength Conductivity Deformation style Color

Crystal Defects Steel spheres: a) Regular packed array with 3 point defects b) Point and line defects c) Mosaic (or domains) separated by defect boundaries Fig 3.50 of Klein and Hurlbut, Manual of Mineralogy, © John Wiley and Sons

Point defects Higher density of defects at higher T Defects represent disorder - disorder favored at higher T Point defects Vacant sites Atoms out of correct position Extraneous atoms Substituted atoms

Crystal Defects 1. Point Defects a. Schottky defect 1. Point Defects Schottky (vacancy) - seen with steel balls in last frame Need to maintain charge balance! b) Impurity Foreign ion replaces normal one (solid solution) Not considered a defect Foreign ion is added (interstitial) Both combined b. Interstitial (impurity) defect

Crystal Defects 1. Point Defects c) Frenkel (cation hops from lattice site to interstitial) = a + b combination b. Frenkel defect

Line defects Crystal deformation controlled by crystal structure Planes/locations are favored for deformation based on bond strength… Bond breakage doesn’t happen throughout entire structure simultaneously Lump in carpet

Crystal Defects 2. Line Defects d) Edge dislocation Migration aids ductile deformation Fig 10-4 of Bloss, Crystallography and Crystal Chemistry.© MSA Dislocation direction is perpendicular to the dislocation line

Crystal Defects 2. Line Defects e) Screw dislocation (aids mineral growth) Fig 10-5 of Bloss, Crystallography and Crystal Chemistry. © MSA Dislocation direction is parallel to the dislocation line, vertical dislocation line

Planar defects Mismatch of the crystal structure across a surface Officially grain boundaries count as planar defect

Crystal Defects 3. Plane Defects f) Domain structure (antiphase domains) Has short-range but not long-range order Fig 10-2 of Bloss, Crystallography and Crystal Chemistry. © MSA

Crystal Defects 3. Plane Defects g) Stacking faults Common in clays and low-T disequilibrium A - B - C layers may be various clay types (illite, smectite, etc.) ABCABCABCABABCABC AAAAAABAAAAAAA ABABABABABCABABAB

Twinning Rational symmetrically-related intergrowth Lattices of each orientation have definite crystallographic relation to each other A variety of planar structural defect

Twinning Aragonite twin Note zone at twin plane which is common to each part Although aragonite is orthorhombic, the twin looks hexagonal due to the 120o O-C-O angle in the CO3 group Redrawn from Fig 2-69 of Berry, Mason and Dietrich, Mineralogy, Freeman & Co.

Twinning 1) Reflection (twin plane) Examples: gypsum “fish-tail” Twin Operation is the symmetry operation which relates the two (or more) parts (twin mirror, rot. axis) 1) Reflection (twin plane) Examples: gypsum “fish-tail” 2) Rotation (usually 180o) about an axis common to both (twin axis): normal and parallel twins. Examples: carlsbad twin 3) Inversion (twin center)

Contact & Penetration twins Both are simple twins only two parts Pass around staurolite, muscovite, selenite, plag Can’t ID them 2006 since we haven’t seen them all yet

Polysynthetic twins Albite Law in plagioclase Multiple twins (> 2 segments repeated by same law) Cyclic twins - successive planes not parallel Polysynthetic twins Albite Law in plagioclase

Twinning Mechanisms: 1) Growth Growth increment cluster adds w/ twin orientation Epitaxial more stable than random Not all epitaxis  twins Usually simple & penetration synneusis a special case

Twinning Mechanisms: 1) Growth Feldspars: Plagioclase: Triclinic Albite-law-striations a-c a-c b b

Twinning Mechanisms: 1) Growth Feldspars: Plagioclase: Triclinic Albite-law-striations

cyclic twinning in inverted low quartz Mechanisms: 2) Transformation (secondary) SiO2: High T is higher symmetry High Quartz P6222 Low Quartz P3221

Twinning Mechanisms: 2) Transformation (secondary twins) Feldspars: Orthoclase (monoclinic)  microcline (triclinic) a-c a-c Monoclinic (high-T) Triclinic (low-T) b b

Twinning Mechanisms: 2) Transformation (secondary) Feldspars: K-feldspar: large K  lower T of transformation “tartan twins” Interpretation wrt petrology!

Twinning Mechanisms: 3) Deformation (secondary) Results from shear stress greater stress  gliding, and finally rupture Also in feldspars. Looks like transformation, but the difference in interpretation is tremendous

Mechanisms: 3) Deformation (secondary) Results from shear stress. Plagioclase

Mechanisms: 3) Deformation (secondary) Results from shear stress. Calcite

Isostructural minerals 2 minerals with identical structure NaCl, PbS Different chemical and physical properties, identical symmetry, cleavage, habit

Isostructural group Group of isostructural minerals realted by common anion or anionic group Calcite group: calcite, magnesite, rhodochrosite, siderite

polymorphism Ability of a chemical compound to crystallize with more than 1 structure SiO2, Al2SiO5, KAlSi3O8

polymorphism Ability of a chemical compound to crystallize with more than 1 structure SiO2, Al2SiO5, KAlSi3O8

Polymorphism 1. Displacive polymorphism    quartz at 573oC at atmospheric pressure 1000 2 4 High-Quartz Low-Quartz 500 Temperature Coesite Pressure (GPa)

Polymorphism 1. Displacive polymorphism High 1. Displacive polymorphism Note: higher T  higher symmetry due to more thermal energy (may twin as lower T) Transition involves small adjustments and no breaking of bonds Easily reversed and non- quenchable (low E barrier) P6222 Low P3221

Polymorphism 2. Reconstructive polymorphs More common: other quartz polymorphs, graphite- diamond, calcite-aragonite, sillimanite-kyanite- andalusite Transition involves extensive adjustments, including breaking and reformation of bonds High E barrier, so quenchable and not easily reversed (still find Precambrian tridymite) Stable Unstable Metastable

Pseudorphism May be confused with polymorphs A completely different thing Complete replacement of one mineral by one or more other minerals such that the new minerals retain the external shape of the original one Limonite after pyrite Chlorite after garnet Brucite after periclase Forsterite after tremolite Can use the shape to infer the original mineral Very useful in petrogenetic interpretations