2.3 Polymorphism substitution

2.3 Polymorphism substitution - Mineral Structures From...

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Unformatted text preview: Mineral Structures From definition of a mineral: How do Pauling's rules manifest themselves as crystals? How can crystal structure make one mineral different from another? Can mineral structures be used to group minerals (e.g. classify them)? "...an ordered atomic arrangement..." Illustrations of mineral structures 2D representation of 3D materials Ions represented as spheres drawn to scale Stick and ball method Polyhedron method Hydrid: Stick and Ball, plus polyhedron Map view unit cell dimensions Olivine view down a crystallographic axis Fig. 49 Structures Isostructural minerals Polymorphism polymorphic minerals Same structure, different composition Same composition, different structures Isostructural Minerals Many minerals have identical structures, different compositions Example: halite (NaCl) and Galena (PbS) Differ in many physical properties composition Identical symmetry, cleavage, and habit elemental arrangement Isostructural group Several isostructural minerals Have common anion group Much substitution between cations Example: calcite group Polymorphism The ability for compounds with identical compositions to crystallize with more than one structure Polymorphs Polymorphic groups Caused by balance of conflicting requirements: Attraction and repulsion of cations and anions (charge) Fit of cations in coordination site (size) Geometry of covalent bonds Polymorphs controlled by P and T conditions: Composition of environment unimportant High P favors tightly packed lattice, high density High T favors open lattice, low density, wide substitution All same elements in polymorphs Presence or absence of polymorphs provide information on P and T conditions Four types of mechanisms to create polymorphs: 1. 2. 3. 4. Reconstructive break bonds Orderdisorder cation placement Displacive kink bonds Polytypism stacking arrangement 1. Reconstructive polymorphism Requires breaking bonds major reorganization Example: C Symmetry and/or structural elements may differ between polymorphs Symmetry and/or structural elements may be similar because identical composition C = Diamond and Graphite Diamond all 100% covalent bonds Graphite covalent bonds within sheets, van der Waal bonds between sheets What conditions cause one mineral or the other to form? Graphite stable at earth surface T and P Diamond stable only at high P and T but found on earth surface Won't spontaneously convert to graphite Minerals that exists outside of their stability fields are metastable What are temperatures at these depths? Found on a Phase Diagram e.g. for single component Increasing Depth ~200 km depth ~100 km depth Single component = C Where on (in) the earth would diamond form/be stable? Fig. 410 Diamond stability versus geothermal gradient Kimberlite Diamond window Lithosphere Asthenosphere Phase diagram Conceptual model of earth Red line is geothermal gradient Stability Boundary of Diamond and Graphite Metastable minerals occur because of energy required for conversion Quenching "frozen": e.g. Kfeldspars Bonds must be broken to switch between polymorphs Cooling removes energy required to break bonds Rate of cooling often important for lack of conversion e.g. fast cooling removes energy before reactions occur Example of Orderdisorder polymorphism 2. Orderdisorder polymorphism The mineral structure remains same between polymorphs Difference is in the location of cations in structure Good examples are the Kfeldspars Idealized feldspar structure Si or Al K (or Na, Ca) Kfeldspar has 4 tetrahedral sites called T1 and T2 (two each) Fig. 126 "Kspars" KAlSi3O8 one Al3+ substitutes for one Si4+ High Sanidine (high T) Al can substitute for any Si completely disordered Low Microcline (low T) Al restricted to one site completely ordered Orthoclase (Intermediate T) Intermediate number of sites with Al Orderdisorder in the Kfeldspars High Sanidine Al3+ equally likely to be in any one of the four T sites Microcline Al3+ is restricted to one T1 site. Si4+ fills other three sites Fig. 412 Degree of order depends on T Sanidine formed in magmas found in volcanic rocks quenched at disordered state: metastable Microcline found in plutonic rocks slow cooling allows for ordering to take place Over time, sanidine will convert to microcline High T favors disorder Low T favors order 3. Displacive Polymorphism No bonds broken and quartz are good examples quartz (AKA high quartz) quartz (AKA low quartz) 1 atm P and > 573 C, SiO2 has 6fold rotation axis. 1 atm P and < 573 C, SiO2 distorted to 3fold axis quartz View down c axis 6fold rotation axis quartz 3fold rotation axis Conversion can not be quenched, always happens Never find metastable quartz Fig. 411 External crystal shape may be retained from conversion to low form Causes strain on internal lattice Strain may cause twinning or undulatory extinction Must have sufficient space for mineral to form (4) Polytypism Stacking diffrences Common examples are micas and clays Orthorhombic, single stacking vector, 90 Orthorhombic, two stacking vectors, not 90 Monoclinic, single stacking vector, not 90 Fig. 413 Eventually will get to controls on compositional variations First some "housekeeping" necessary skills: Scheme for mineral classification Rules for chemical formulas A graphing technique ternary diagrams Mineral Classification Based on major anion or anionic group Consistent with chemical organization of inorganic compounds Families of minerals with common anions have similar structure and properties Cation contents commonly quite variable Follows from Pauling's rules 1, 3, and 4 (coordination polyhedron & sharing of polyhedral elements) anions define basic structure 2: (electrostatic valency principle) anionic group separate minerals Mineral group Native elements Oxides Hydroxides Halides Sulfides Sulfates Carbonates Phosphates Silicates Anion or anion gp N/A O2 OH Cl, Br, F S2 SO42 CO32 PO43 SiO44 Mineral Formulas Rules Cations first, then anions or anionic group Charges must balance Cations of same sites grouped Cations listed in decreasing coordination number Thus also decreasing ionic radius Also increasing valence state Examples Diopside a pyroxene: CaMgSi2O6 Charges balance Ca 8 fold coordination: +2 valence Mg 6 fold coordination: +2 valence Si 4 fold coordination: +4 valence Substitution within sites indicated by parentheses: Ca(Fe,Mg)Si2O6 (more on "solid solution" in a moment) Intermediate of Diopside (CaMgSi2O6) Hedenbergite (CaFeSi2O6) complete solid solution series Can explicitly describe substitution Alternatively: Can describe composition by relative amounts of end members: E.g. Olivine: (Mg2x,Fex)SiO4 0 x 2 Forsterite = Fo Fayalite = Fa All of the following are the same: (Mg0.78Fe0.22)2SiO4 Mg1.56Fe0.44SiO4 Fo78Fa22 (here numbers are percentages of amount of each mineral) Fo78 (here implied that the remainder is Fa22) Fa22 How to calculate chemical formulas Eg. Plagioclase feldspars: Albite NaAlSi3O8 What is composition of say Ab25An75? Anorthite CaAl2Si2O8 Graphic representation Common to have three "end members" Ca2+, Mg2+ and Fe2+ common substitutions between silicate minerals Ternary diagrams Used to describe distribution of each end member Total amount is 100% Ca2Si2O6 8% Fs Pyroxenes: (Mg,Fe,Ca)2Si2O6 50% Wo Composition is: En42Fs8Wo50 (Mg0.42Fe0.08Ca0.5)2Si2O6 Mg2Si2O6 42% En Fe2Si2O6 p.7273 Compositional Variation Think of minerals as framework of anions Form various sites where cations reside Not all sites need to be filled Some sites can accommodate more than one type of ion (e.g. polymorphism in feldspar, solid solution in olivine) Solid solution Anions can substitute for each other, but this is rare Occurs when different cations can occur in a particular site Three types: Substitution, omission, and interstitial Terms Substitution series or solid solution series: the complete range of composition of a mineral End members: the extremes in the range of compositions E.g. olivine: Forsterite and Fayelite Terms Continuous or complete solid solution series: all intermediate compositions are possible Incomplete or discontinuous solid solution series: a restricted range of compositions E.g. Olivine E.g Calcite magnesite Substitutional Solid Solution Two requirements for substitution Size substituting ions must be close in size Charge electrical neutrality must be maintained Size Comes from Pauling rule 1: coordination In general size of ions must be < 15% different for substituition Tetrahedral sites: Si4+ and Al3+ Octahedral sites: Mg2+, Fe2+, Fe3+, Al3+ Larger sites: Na+ and Ca2+ Temperature is important Example is K and Na substitution in alkali feldspar (Sanidine and Albite) Size difference is about 25% Complete solid solution at high T Limited solid solution at low T Results in exsolution Types of substitution Substitutional solid solution Omission substitution Interstitial substitution Different types have to do with where the substitution occurs in the crystal lattice Simple substitution Coupled substitution Simple Substitution Occurs with cations of about same size and same charge Example: Olivine Olivine (Fe.22Mg.78)2SiO4 View down a axis 22% Fe 78% Mg Fig. 414 Coupled Substitution Coupling two substitutions Example: Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8) One that raises charge Linked one that decreases charge Ca and Na occupy distorted 8fold site Al and Si occupy tetrahedral sites Coupled substitution: Na+ + Si4+ = Ca2+ + Al3+ Fig. 414 Coupled substitution The substitution doesn't always have to be different sites Corundum (Al203) Can couple cations and anions Fe2+ and Ti4+ substitute for 2Al3+ (makes sapphire). Cr3+ makes Ruby Both elements are in octahedral sites Hornblende: Fe2+ and OH substitutes for Fe3+ and O2 Omission substitution Charge balance maintained by leaving site vacant Pyrrhotite: variable amounts of Fe2+ and Fe3+ Formula: Fe(1x)S where 0<X<0.13 General substitution: (n+1)Mn+ = nM(n+1)+ + where is vacant 14Fe2+ = 8 Fe2+ + 4 Fe3+ + 2 28+ = 28+ 14 sites = 14 sites Fig. 414 Interstitial substitution Type of omission substitution Difference is that regular lattice framework site is not location of substitution Example: Beryl, a ring silicate Large openings can have K+, Rb+ and Cs+ Structure of Beryl Be3Al2Si6O8 Rings Substitution important: Cr substition makes emerald, other substitutions make Aquamarine blue green variety of emerald Al 6fold coordination Be 4fold coordination Fig. 156 Charge balance maintained by interstitial substitution Al, Be substition for Si Fig. 414 ...
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This note was uploaded on 07/06/2011 for the course GLY 5245 taught by Professor Staff during the Spring '11 term at University of Florida.

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