CHAPTER 5: CRYSTAL DEFECTS AND TWINNING. Sarah Lambart

CHAPTER 5: CRYSTAL DEFECTS AND TWINNING Sarah Lambart

RECAP CHAP. 4 Hermann-Mauguin symbols 32 crystal classes Miller indices Crystal forms

RECAP CHAP. 4 Crystal System Crystal Class Symmetry Name of Class 1 none Pedial Triclinic i Pinacoidal 2 1A 2 Sphenoidal Monoclinic m 1m Domatic 2/m i, 1A 2, 1m Prismatic Hermann-Mauguin symbols 32 crystal classes Orthorhombic Tetragonal 222 3A 2 Rhombic-disphenoidal mm2 (2mm) 1A 2, 2m Rhombic-pyramidal 2/m2/m2/m i, 3A 2, 3m Rhombic-dipyramidal 4 1A 4 Tetragonal- Pyramidal 4 Tetragonal-disphenoidal 4/m i, 1A 4, 1m Tetragonal-dipyramidal 422 1A 4, 4A 2 Tetragonal-trapezohedral 4mm 1A 4, 4m Ditetragonal-pyramidal 2m 1 4, 2A 2, 2m Tetragonal-scalenohedral 4/m2/m2/m i, 1A 4, 4A 2, 5m Ditetragonal-dipyramidal 3 1A 3 Trigonal-pyramidal 1 3 Rhombohedral 32 1A 3, 3A 2 Trigonal-trapezohedral Miller indices Hexagonal 3m 1A 3, 3m Ditrigonal-pyramidal 2/m 1 3, 3A 2, 3m Hexagonal-scalenohedral 6 1A 6 Hexagonal-pyramidal 1 6 Trigonal-dipyramidal 6/m i, 1A 6, 1m Hexagonal-dipyramidal 622 1A 6, 6A 2 Hexagonal-trapezohedral Crystal forms Isometric 6mm 1A 6, 6m Dihexagonal-pyramidal m2 1 6, 3A 2, 3m Ditrigonal-dipyramidal 6/m2/m2/m i, 1A 6, 6A 2, 7m Dihexagonal-dipyramidal 23 3A 2, 4A 3 Tetaroidal 2/m 3A 2, 3m, 4 3 Diploidal 432 3A 4, 4A 3, 6A 2 Gyroidal 3m 3 4, 4A 3, 6m Hextetrahedral 4/m 2/m 3A 4, 4 3, 6A 2, 9m Hexoctahedral

RECAP CHAP. 4 Hermann-Mauguin symbols 32 crystal classes Miller indices Crystal forms

RECAP CHAP. 4 Hermann-Mauguin 32 crystal classes Miller indices Crystal forms symbols

CONTENT CHAP. 5 (2 LECTURES) Crystal defects Twinning Polymorphism and Isomorphism

TWINNING Twinning: the symmetrical intergrowth of 2 or more crystals of the same substance; usually caused by stress or change in growing conditions (P, T) pyrite quartz Staurolite

TWINNING Twinning: the symmetrical intergrowth of 2 or more crystals of the same substance; usually caused by stress or change in growing conditions (P, T) Crystal are sharing lattice points

TWINNING Symmetry operations: Reflection across a mirror plan (twin plan) Rotation about a common axis (twin axis) Inversion about a common point (twin point) Twin laws: that add elements of symmetry above that provide by normal symmetry operations of the crystal (i.e., point group).

TYPES OF TWINNING Contact twin: where a twin plane acts as a compositional surface separating 2 individual crystals Ex. orthoclase Repetition of the contact twins Ex. chrysoberyl Ex. plagioclase

Ex. Plagioclase http://minerva.union.edu/ hollochk/c_petrology/ ig_minerals.htm

TYPES OF TWINNING Penetration twin: where an irregular surface generated by a twin axis or a twin center separates two individual crystals Ex. orthoclase

CAUSES OF TWINNING Transformation twins: Change of the crystal system Ex.: Leucite: Cubic at HT to orthorhombic at LT Growth twins: Presence of a crystal defect Ex.: plagioclase and feldspar Michael M. Raith, Steinmann Institut, University of Bonn

CRYSTAL DEFECTS Point defects Dislocations (Linear defects) Planar defects (2D) Inclusions (3D)

POINT DEFECT Point defects are where an atom is missing or is in an irregular place in the lattice structure.

POINT DEFECT Point defects are where an atom is missing or is in an irregular place in the lattice structure. Interstitial atom: an extra atom into an interstitial void in the crystal structure

POINT DEFECT Point defects are where an atom is missing or is in an irregular place in the lattice structure. Interstitial atom: an extra atom into an interstitial void in the crystal structure Vacancy: empty spaces where an atom should be, but is missing major process for diffusion

POINT DEFECT Point defects are where an atom is missing or is in an irregular place in the lattice structure. Frankel pair: vacancy + interstitial atom due to atom migration

POINT DEFECT Point defects are where an atom is missing or is in an irregular place in the lattice structure. Substitution: an atom of a different type than the bulk atoms

LINEAR DEFECT Linear defects are called dislocations. Edge dislocations: an extra half-plane

LINEAR DEFECT Linear defects are called dislocations. Edge dislocations: an extra half-plane

LINEAR DEFECT Linear defects are called dislocations. Screw dislocation:

LINEAR DEFECT Linear defects are called dislocations. Edge dislocation: Screw dislocation: Zhang group, UNC Charlotte

PLANAR DEFECT Planar defects: interfaces between homogeneous regions of the material. Stacking Faults and Twin Boundaries Few atomic spacings Ex.: ABABABCABAB (i.e., switching for hcp to fcc) Many atomic spacings

INCLUSIONS Solid, liquid or gaz Fluid inclusion Solid inclusion (garnet in quartz) Melt inclusion

ISOMORPHISM AND POLYMORPHISM Polymorphism = many forms : that a single chemical composition can exist with two or more different crystal structures. Change of structure = polymorphic transformations: Reconstructive transformation Displacive transformation Order-disorder transformation

POLYMORPHISM Reconstructive transformations: extensive rearrangement of the crystal structure (breaking of chemical bonds and reassembling the atoms into a different crystal structure) a large change in energy of the structure very slow rate presence of metastable polymorphs Example: carbon - Diamond to graphite

POLYMORPHISM Example: quartzα-quartz at T> 580 C β-quartz at T<580 C Displacive transformations: Small rearrangement of the crystal structure (no broken bonds) no change of energy Instantaneous and reversible no metastable polymorph

POLYMORPHISM Order disorder transformations: Perfect order: Only at the absolute zero (0K or -273.15 C) High temperature forms more disordered Example: KAlSi 3 O 8 - HT: sanidine (2/m) MT: orthoclase (2/m) BT: microcline (1) Continuous transition (no specific transformation temperature) Potential presence of metastable polymorphs (if the change of temperature is rapid)

IMPORTANT POLYMORPHS Carbon 2 polymorphs: - HP/HT: diamond (isometric) - LP/LT: graphite (hexagonal) Diamonds can form only along here Reconstructive transformation Require a lot of energy (from the hardest mineral to one of the softest one) presence of diamond at the Earth s surface Pressure Diamond Graphite 5 GPa 1,500 C Temperature

IMPORTANT POLYMORPHS Al 2 SiO 5 3 polymorphs: - HP: Kyanite (triclinic) - HT: sillimanite (orthorhombic) - LP/LT: Andalusite (orthorhombic) Reconstructive transformations Use to define metamorphic zones: Andalusite: contact metamorphism Sillimanite: Regional metamorphism

IMPORTANT POLYMORPHS CaCO 3 3 polymorphs: - Aragonite (orthorhombic)and Vaterite (hexagonal): metastable at the Earth s surface conditions - Calcite (hexagonal): Stable at the Earth s surface conditions Reconstructive transformations

SiO 2 T IMPORTANT POLYMORPHS 6 polymorphs Low pressure: Cristobalite (isometric) Tridymite (hexagonal) β-quartz (hexagonal) R D α-quartz (trigonal) Coesite (monoclinic) Stishovite (tetra) R Temperature ( C) P 1600 1200 800 400 0 Liq. Cristobalite High Quartz Tridymite Low Quartz R P Coesite Stishovite 0 10 20 30 40 50 10 70 80 90 100 Pressure (kb)

IMPORTANT POLYMORPHS KAl 2 SiO 8 3 polymorphs Fast cooling Order-disorder transformations slow cooling HT polymorph = sanidine (monoclinic): found in volcanic rocks that have cooled rapidly Slower cooling: sanidine is transformed into orthoclase, then microcline Sanidine & orthoclase: Carlsbad twinning:

PSEUDOMORPHISM Pseudomorphism = false form : mineral that has the appearance of another mineral: internal structure and chemical composition are changed but its external form is preserved. 3 mechanisms: Substitution Encrustation Alteration

PSEUDOMORPHISM MECHANISMS Substitution: Chemical constituents replaced by other chemical constituents Examples: Petrified forest: wood fibers replaced by quartz Fluorite alteration: fluorite replaced by quartz (trigonal) but looked isometric

PSEUDOMORPHISM MECHANISMS Encrustation: thin crust of a new mineral forms on the surface of a preexisting mineral Alteration: only partial removal of the original mineral and only partial replacement by the new mineral has taken place Examples: serpentine pseudomorphed after olivine or pyroxene anhydrite (CaSO 4 ) pseudomorphed after gypsum (CaSO 4.2H 2 O) limonite [FeO.(OH).nH 2 O] after pyrite (FeS 2 )

PSEUDOMORPHISM MECHANISMS Paramorphism: Different external and internal structures Example: Graphite after diamond in the Beni Bousera pyroxenites (Morocco) Diamonds can form only along here P > 5 Gpa (asthenospheric depths) Pressure Diamond Graphite 5 GPa 1,500 C Temperature

ISOMORPHISM Isomorphism = solid solutions: mineral with the same crystal structure in which specific sites can be occupied by two or more elements, ions, or radicals. Example: olivine Forsterite Mg 2 SiO 4 Fayalite Fe 2 SiO 4 Mg 2+ can be substituted by Fe 2+ : olivine = (Mg,Fe) 2 SiO 4 compositional variations in the minerals

ISOMORPHISM Rules for substitution 1) Extent of substitution more enhanced at higher T 2) Electrical neutrality has to be maintained (in most cases)

ISOMORPHISM 3) If difference in charge (Z) between substituting ions > 1: difficult substitution 4) The substitution should involve similar ion sizes: Size difference < 15%: extensive substitution Size difference: 15-30%: limited or incomplete Size difference > 30%: almost impossible 5) If two ions are competing for the same site, the one with the highest charge (Z) and smaller radius (r) is favored (i.e. higher Z/r or ionic potential is favored). Rules 3-5: Goldschmidt s rules for ionic substitution

ISOMORPHISM Type: of substitutions Simple substitution: substitution of cations with the same charges Ex.: Olivine (Mg,Fe)2SiO4 Tetrahedral sites are occupied by Si4+. Octahedral sites are occupied by either Mg2+ or Fe2+: 10 are occupied by Mg2+ (red) and 3 are occupied by Fe2+ (yellow): the olivine formula is (Mg70Fe30)2SiO8 (called Forsterite seventy ) Modified from Nesse, 2000, Fig. 4.15a

ISOMORPHISM Coupled substitution: maintain charge balance by coupling one substitution that increases the charge with one that reduces the charge Ex.: Plagioclase: albite: (NaAlSi 3 O 8 ) anorthite (CaAl 2 Si 2 O 8 ) Substitution of N a+ for Ca 2+ is balanced by the substitution of Si 4+ for Al 3+ Nesse, 2000, Fig. 4.15b

ISOMORPHISM Omission substitution: maintain charge balance by leaving structural sites vacant: (n+1)m n+ nm n+1 + Ex.: Pyrrhotite 3Fe 2+ 2Fe 3+ + The amount of Fe 3+ that can replace Fe 2+ is limited to less than 20%: Fe (x-1) S with x = 0 to 0.2 Nesse, 2000, Fig. 4.15c

ISOMORPHISM Interstitial substitution: maintain charge balance by placing ions in sites that normally are vacant: Ex.: Beryl Al 2 Be 3 SiO 6 O 18 + Si 4+ Al 3+ + (K +, Rb +, Cs + ) Insertion of large cations in the open channel is balanced by the substitution of Si 4+ by Al 3+ Nesse, 2000, Fig. 4.15d