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Camosun College GEOS 250 Lectures: 9:30-10:20 M T Th F300 Lab: 9:30-12:20 W F300

Introduction to Mineralogy Dr. Tark Hamilton Chapter 4: Lecture 14 The Chemical Basis of Minerals (isostructural minerals and phase transformations). Camosun College GEOS 250 Lectures: 9:30-10:20 M T Th F300 Lab: 9:30-12:20 W F300. Isostructural Minerals.

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Camosun College GEOS 250 Lectures: 9:30-10:20 M T Th F300 Lab: 9:30-12:20 W F300

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  1. Introduction to MineralogyDr. Tark HamiltonChapter 4: Lecture 14The Chemical Basis of Minerals(isostructural minerals and phase transformations) Camosun College GEOS 250 Lectures: 9:30-10:20 M T Th F300 Lab: 9:30-12:20 W F300

  2. Isostructural Minerals • UO2 & CaF2 (Uraninite & Fluorite, Oct/Tet) (different in solutions, Linear & Oct/Cubic, different energy or they wouldn’t dissolve!) • NaClHalite & MgOPericlase & PbSGalena & MnSAlabandite & AgClChlorargyrite & TiNOsbornite (named after Donny & Marie?) • StishoviteSiO2 & RutileTiO2 are both octahedral, for Silicon this takes GPa’s of P • Isostructural Groups: Barite (sulphates), Calcite (Carbonates) Aragonite (Carbonates)

  3. Periclase MgO 4/m 3 2/m Manganosite MnO inclusions Nordmark Sweden K. Gatedal Alabandite MnS perched on Pyrrhotite Fe0.83-1.0S & Calcite with soft asphaltum Garpenberg Sweden, Osterlof photo Ag(Cl,Br) Azurite Broken Hill NSW O. Meyer 2004

  4. Aragonite Group:Isostructural Carbonates

  5. Aragonite CaCO3 Orthorhombic 2/m 2/m 2/m Dalesford, Victoria Australia Judy Rowe Witherite BaCO3 on Barytocalcite 2mm BaCa(CO3)2 Blagill Mine, Alston Moor UK, Steve Rust Strontianite SrCO3 on Barite BaSO4 Dreislar Mine Westphalia Germany fluorescent (Dipyramidal) Peter Haas

  6. 2: Electrostatic Valency PrincipleBond Strength (e.b.s. or e.v.) • For a cation Mm+ surrounded by n anions Xx- the electrostatic bond strength of the cation is defined as:- • e.b.s. = m/n • For each anion (cation) the sum of the electrostatic bond strengths of the surrounding cations (anions) must balance the negative (positive) charge on the anion (cation) • Σm/n = x • For a binary compound AxBy the coordination numbers of A and B are in the ratio y:x • e.g. Fluorite, CaF2 Ca2+ (8-coordinate), F- (4-coordinate) or 8/4 = 2/1, stoichiometry hints at possible structure

  7. C.N. x Bond Strength = Ionic Charge • InPerovskite, CaTiO3 • Ca2+ is 12-coordinated by O2- Ca-O bond has e.b.s. = 2/12 = 1/6 • Ti4+ is 6-coordinated by O2- Ti-O bond has e.b.s. = 4/6 = 2/3 • {Green = Ca; Blue = Ti; Red = O} • O2- has a total valency of 2 satisfied by {4 x Ca2+(1/6)} +{2 x Ti4+(2/3)} • Each Oxygen must be common to 4 CaO12 cuboctahedra & 2 TiO6 octahedra • This information suffices to define the idealized structural arrangementUniquely 12 x 1/6 = 2+ 6 x 2/3 = 4+ A cube of Ti+4 ions with a Ca+2 ion at the body Center & O-2 ions at all of the edge center positions 2/3 + 4/3 = 2- Perovskite CaTiO3 Pseudocubic 90.67° (MgSiO3 Lower Mantle Structure)

  8. Pauling Rule 4: Cation Evasion in >Binaries"In a crystal containing different cations those of high valency and small coordination number tend not to share polyhedron elements with each other"e.g. In Perovskite, CaTiO3 • Ca+2 12 C.N. in CaO12 share faces OK for low valence, large cations Ti4+ 6 C.N. in TiO6 share vertices

  9. Pauling Rule 5: Environmental Homogeneity • "The number of essentially different kinds of constituent in a crystal tend to be small" • i.e.as far as possible, similar environments for chemically similar atoms

  10. Treating the mineral Garnet Ca3Al2Si3O12 as an ionic crystal • O2- bond strength of 2 is satisfied by a number of alternative combination of bonds • Pauling Rule 5 Each O2- would prefer the same environment • Only one possible arrangement defines the garnet structure uniquely Rule 5 is, however, often not obeyed

  11. Andradite? Tyrol, Austria Karl Volkman 2006 Minas, Gerias, Brazil Pedro Gonzales Jericho Contwoyto Lake NWT G-10 Garnet (Hi Cr, Low Ca) (Eclogite=Jadeite+Omphacite) J. Nimitz Andradite-Grossular? Black L. Que. Schuster Pyrope, Altay #3 Xingiang, China J.S.S.

  12. Coordination Change from 6 to 8: Alkali Halides Octahedral Cubic Rb/Cl = 0.732 Polymorphs

  13. The Phase Rule & Heterogeneous Equilibria • Freedom = Phases – Components + 2 • e.g. for SiO2 there is 1 component • So at a fixed value of Pressure & Temperature, 3 polymorphs could coexist • Under normal conditions at some random P & T, only 1 phase would occur • Similar to Ice-Water-Steam phase diagram

  14. β- to α-Displacive Phase Transitions • The α- and β-forms of quartz (and Tridymite and Cristobalite) are special polymorph pairs, • Their structures have all the same bonds (they’re topologically identical) but the atoms are in shifted positions (they’re geometrically distinct). • They are low and high temperature polymorphs of one another. • At 1 bar pressure, the change from α-quartz to β-quartz occurs very • rapidly and reversibly at 573°C. Indeed, it is not possible to “quench” β-quartz • β-quartz exists only at temperatures above 573°C. Because the change from α- to β-quartz occurs without the breaking of any bonds, this change is called a displacive transformation. • The β- to α- phase transition occurs spontaneously on cooling, as the mineral loses volume. • This is also true for Cristobalite & Tridymiteβ- to α- phase transitions

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