CHAPTER 5. Mineral Growth

Size: px

Start display at page:

Download "CHAPTER 5. Mineral Growth"

Neil Jennings
5 years ago
Views:

1 CHAPTER 5 Mineral Growth

2 Figure 5.1 The rock cycle, a mineralogist s view. The rock cycle fundamentally involves mineralogic changes in response to different temperature pressure (T P) conditions.

3 Mineral Stability A bulk composition of a rock must contain components of a mineral for the mineral to form. The same bulk composition may conceivably lead to the formation of hundreds of minerals Which particular mineral forms depends on the stability and energy of formation of the mineral

4 Stability State with lower energy is the stable state Stable: The book on the floor has lower energy, in this case lower potential energy Unstable : will spontaneously move to a lower energy position Metastable: The book on the shelf has higher energy but it will not attain a lower energy state unless it is nudged from it s position. The energy required to nudge the book is the Activation Energy The stability of a mineral is judged with Gibb s Free Energy (G). It has units that of energy (calories or joules = cal) per mole Figure 5.2 Stability.

5 Gibb s Free Energy Free energy of formation from the elements ΔG f = energy difference between the free energy of the element in standard state (298 K and 1A) and the free energy of the element when it is bonded in a mineral structure at the P,T condition of interest. For two minerals with the same chemical composition (e.g. α-quartz and β- Quartz or calcite and aragonite the one with lower free energy under the specified P,T condition is the stable form. ΔG f of all minerals vary with P and T so, for example, under certain P,t condition calcite has lower free energy and under some other condition aragonite has lower energy (hence more stable)

6 New minerals form by chemical reactions. Let us consider the reaction: Muscovite + Quartz = K-feldspar+ Sillimanite + water KAl 2 (AlSi 3 O 10 )(OH)2 + SiO 2 = KAlSi 3 O 8 + Al 2 SiO 5 +H 2 O (Reactants) (Products) ΔG f (reaction) = ΔG f (products) - ΔG f (reactants) If ΔG f (reaction) is <0 i.e., -ve, the reaction will proceed towards right, If ΔG f (reaction) is +ve, i.e., >0 the reactants are more stable. At equilibrium ΔG f (reaction) = 0, For a given P,T,X condition, the assemblage with the lowest ΔG f is the stable assemblage In reality ΔG f of all the minerals in a rock is difficult to calculate Minerals commonly persist metastabily even though they are not in the lowest free energy in a given P,T,X condition.

7 Phase Diagram A phase can be a mineral, melt or gas. A phase diagram represents the stable phases for a given composition under a given P,T condition.

8 The metamorphic minerals: Kyanite, andalusite and sillimanite are the polymorphs of Al 2 Si 2 O 5 (or Al 2 O 3.SiO 2 ) ΔG f of formation of these three polymorphs vary with P,T as shown in the diagram. The lower figure shows the stability fields of different polymorphs under changing P,T condition. If Andalusite is heated Sillimanite forms. If pressure is increased Kyanite will form at the expense of Andalusite If metamorphic rock contains Andalusite, we can infer that the rock was metamorphosed under low P and low to moderate temp typical of contact metamorphism. If a metamorphic rock contains all three isomorphs, what is the P,T condition? Figure 5.3 Aluminum silicate stability relations.

9 Binary (two component) Eutectic Phase Diagram Liquidus: composition of liquids (or melt) in equilibrium with solids (crystals) at a particular temperature Solidus: composition of solids (or crystals) in equilibrium with melts at a particular temperature Eutectic: Where both components crystallize simultaneously. Eutectic temperature is always lower than the melting temp of components A or B The proportion of solid:liquid at any temp can be found by Lever Rule Figure 5.4 Crystallization in the system diopside (Di) anorthite (An). After Osborn (1942). See text for discussion

10 Figure 5.5 Lever rule.

11 Binary (two component) with continuous solid solution Phase Diagram Figure 5.6 Olivine crystallization at 1 atmosphere pressure. After Bowen and Schairer (1935). See text for discussion.

12 Binary (two component) with solvus Phase Diagram Solvus: curve that defines two co-existing phases that unmixes from a solid solution Figure 5.7 Alkali feldspar crystallization.

13 Mineral Nucleation Homogeneous Nucleation: Embryos have the chemical composition and mineral structure of a mineral and forms by chance aggregation of component ions number of the embryos decrease exponentially with size: most consist of a few atoms Embryos can only grow if the new mineral has a lower free energy than the melt. Crystals also contain surface energy due to disrupted chemical bonds at the surface of embryos. Magnitude of the surface energy is proportional to the surface area of the crystal The free energy change in forming a crystal of volume v from a melt is: ΔG v = (ΔG f(xl) ΔG f(melt) )*v + ΔG s Where ΔG s is the surface energy of the crystal ΔG s = ɣa (where ɣ = surface energy per unit area and a is the surface area of the crystal Figure 5.8 Free energy of formation of crystal nuclei from a melt as a function of size.

14 The free energy change in forming a crystal of volume v from a melt is: ΔG v = (ΔG f(xl) ΔG f(melt) )*v + ΔG s Where ΔG s is the surface energy of the crystal ΔG s = ɣa (where ɣ = surface energy per unit area and a is the surface area of the crystal) For a cubic crystal with edges of length c ΔG v = (ΔG f(xl) ΔG f(melt) )*c 3 + ɣ6c 2 For T 0 (equilibrium temp) : ΔG v = 0 but ΔG s is positive for all embryo size ensuring ΔGv is positive hence no crystal growth For T 1 (slight undercooling) = for embryos smaller than critical growth radius r c,δg v >0 but for larger embryo radius, ΔG v <0 : a few large crystals T 2 = r c smaller, more stable crystals T 3 =strong undercooling, ΔG v <0, r c even smaller, many nuclei can be stable

15 Crystal growth requires super cooling to provide the activation energy to overcome surface energy The required activation energy is low for slow cooling, large for fast cooling Plutonic rocks cool slowly few, large crystals Volcanic rocks cool rapidly high undercooling, many small crystals Metamorphic Rocks: rate of change of pressure/temp is low hence early formed crystals are few and large (porphyroblasts) Crystals grow as 1. temperature rises: increases the mobility of ions 2. smaller grains recrystallize to form larger grains as temp rises 3. Larger grains can become deformed and can recrystallize to form smaller grains due to strong deformation.

16 Heterogeneous Nucleation Epitaxial nucleation: new crystals grow on existing crystal face requiring less surface energy. example hematite growing on pre-existing magnetite Crystals can also grow on imperfections in preexisting crystals Figure 5.9 Epitaxial growth. Hematite crystal (shaded) may nucleate and grow on the (111) face of magnetite.

17 Figure 5.10 Growth on a crystal face.

The 111 faces on NaCl is all Na+ (attracts Cl-) or Cl- (attracts Na+) so this face grows fast The 100 face is made of equal number of Na+ and Clso no net charge no attraction.

19 The 111 faces on NaCl is all Na+ (attracts Cl-) or Cl- (attracts Na+) so this face grows fast The 100 face is made of equal number of Na+ and Clso no net charge no attraction. This face grows only by chance encounter with bumbling ions So each new 111 layer will be thicker than 100 layer which will make 111 progressively smaller The slowest growing face is the most prominent in a crystal. Face full of charged Na+ or Cl- has maximum surface energy so adding oppositely charged layers on that face will lower the surface energy the most hence that faces grows fastest Figure 5.11 Slow-growing faces become larger.

21 Law of Bravais: Most prominent face are those that cuts the greatest density of lattice nodes i.e., lattice nodes are most closely spaced.. Spacing d(100)>d(001)>d(102) planes So growth will be fastest normal to (102) face and slowest normal to (100) face Lattice node spacing on (102)>(001)>(100) So, 102 will grow fastest and will be the smallest The growth rate of crystal face is, in general, inversely proportional to the interplanar spacing of that face Figure 5.12 Growth rates of crystal faces are inversely proportional to interplanar (d) spacing.

22 Zoned Crystals Figure 5.13 Photomicrograph of a thin section (see Chapter 7) showing zoned crystals of pyroxene (P) (crossed polarizers).

23 Zoned plagioclase where crystals are not allowed to react with the melt Figure 5.14 Plagioclase phase diagram at 5 kbar water pressure. Adapted from Yoder and others (1957). (a) Equilibrium crystallization. (b) Fractional crystallization.

Structural Defects: Point Defects Line Defects Edge Defects Figure 5.15 Point defects. Point Defects a. Schottkey Defect: Vacanct cation balanced by vacant anion: no change in formula b.

24 Structural Defects: Point Defects Line Defects Edge Defects Figure 5.15 Point defects. Point Defects a. Schottkey Defect: Vacanct cation balanced by vacant anion: no change in formula b. Frenkel Defect: Cation out of place: cations are smaller and move more easily c. Interstitial Defect: Foreign ion push it s way in. Charge is balanced by elsewhere by substituting lower charge cation for higher charge cation d. Substitution Defect: substitutes a normal ion: should we call it a defect? More defects at higher temperature and also more diffusion

Line Defects: Ductile deformation of rocks require deformation of constituent minerals Deformation of minerals takes place by slip along favored crystallographic planes Slip System = crystallographic

25 Line Defects: Ductile deformation of rocks require deformation of constituent minerals Deformation of minerals takes place by slip along favored crystallographic planes Slip System = crystallographic plane (along which slip is taking) and slip direction e.g., {001}[010] in the figure Dislocation line: edges of propagating slip surface where bonds are being broken Boundary between slipped and not yet slipped domains Can be edge dislocation or screw dislocation Figure 5.16 Slip system in a crystal lattice. Slip occurs on a crystal plane parallel to (001) and in a direction parallel to [010] (the b a xis), so the slip system is { 001}[010].

26 Line Defects Buergers vector: Same as dislocation direction Start at any point on lattice nodes and trace a circuit around the dislocation making sure to move equal number of lattice nodes in opposite direction. The vector between the starting and finishing node is the Buergers vector Perpendicular to the dislocation line in Edge dislocation Parallel to dislocation line in screw dislocation Figure 5.17 Dislocations.

27 Planar Defects: Mismatch of crystal structure along a surface Grain Boundaries Stacking Faults: e.g., ABABCABAB in a hexagonal close packing structure Antiphase Boundaries: separates segments of crystal known as Antiphase domains that are related to each other by simple translation Figure 5.18 Unless terminated at the edge of a crystal, a dislocation line (DL) forms a continuous loop outlining a surface, equivalent to a fault, with movement parallel to the Buergers vector.

Twinning: Symmetrical intergrowth of two or more crystal segments of the same mineral Twin Operation: symmetry operation that relates the two segment Reflection, Rotation, Inversion Twin Law: Twin

28 Twinning: Symmetrical intergrowth of two or more crystal segments of the same mineral Twin Operation: symmetry operation that relates the two segment Reflection, Rotation, Inversion Twin Law: Twin operation + crystallographic plane or operation associated with twinning. E.g., reflection on {hkl} Composition plane: surface along which the two twin segments are joined Contact Twins: not intergrown joined along a plane Penetration Twins: Twin segments intergrown Simple Twins: Only two twin segments Multiple Twins Polysynthetic twins: successive parallel composition planes Cyclic twins: composition planes are not parallel Figure 5.20 Symmetry operations in twinning. (a) Twinning by reflection on {011} in rutile. (b) Twinning by rotation on [001] in K-feldspar to produce a Carlsbad twin.

29 Figure 5.21 Contact twins. (a) Octahedron of spinel twinned by reflection on { 11T} (spinel law). (b) Gypsum twinned by reflection o n {100}. ( c) Calcite twin with {001} composition plane.

30 Figure 5.22 Penetration twins. (a) Pyrite Iron Cross twin by 90 o rotation on [001]. (b) Staurolite twin by reflection on { 231}.

31 Figure 5.23 Multiple twins. (a) Polysynthetic twinning in plagioclase by repeated reflection on {010}. These twins are known as albite twins. (b) Cyclic twinning in rutile by repeated reflection on {011}.

32 Figure 5.24 Transformation twinning in leucite.

33 Figure 5.25 Deformation twinning in calcite can be produced by glide on {102} crystallographic planes.

34 Post Crystallization Processes: Ordering: in K-Feldspar polymorphs Twinning: often during polymorphic transition Recrystallization: Minerals tend to reduce their surface area to reduce the surface energy Done by smoothening irregular outlines Increasing grain size Higher temperature facilitates movement and diffusion of ions making recrystallization effective At high enough temperature, defects are healed. Exsolution Perthite (albite in K-feldspar) and antiperthite (K-Feldspar in Albite) Pseudomorphism: replacing mineral maintains the form of the original mineral Figure 5.26 Recrystallization.

35 Figure 5.27 Exsolution in alkali feldspar.

36 Figure 5.28 Photomicrograph of a thin section of mica schist showing dark pleochroic halos around radioactive zircon (Z) inclusions in biotite (B).

GY 302: Crystallography & Mineralogy

UNIVERSITY OF SOUTH ALABAMA GY 302: Crystallography & Mineralogy Lecture 5: Space Groups, Crystal Growth and Twinning Instructor: Dr. Douglas Haywick Last Time 1. Stereo Projections and the Wulff Net 2.