Download Introduction to Mineralogy Dr. Tark Hamilton Chapter 3

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
• UO2 & CaF2 (Uraninite & Fluorite, Oct/Tet)
(different in solutions, Linear & Oct/Cubic,
different energy or they wouldn’t dissolve!)
• NaCl Halite & MgO Periclase & PbS Galena &
MnS Alabandite & AgCl Chlorargyrite & TiN
Osbornite (named after Donny & Marie?)
• Stishovite SiO2 & Rutile TiO2 are both
octahedral, for Silicon this takes GPa’s of P
• Isostructural Groups: Barite (sulphates),
Calcite (Carbonates) Aragonite (Carbonates)
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
Aragonite Group: Isostructural Carbonates
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
2: Electrostatic Valency Principle
Bond 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
C.N. x Bond Strength = Ionic Charge
•In Perovskite, CaTiO
•Ca2+ is 12-coordinated by O2Ca-O bond has e.b.s. = 2/12 = 1/6
•Ti4+ is 6-coordinated by O2Ti-O bond has e.b.s. = 4/6 = 2/3
12 x 1/6 = 2+
3
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
{Green = Ca; Blue = Ti; Red = O}
4+ 2
•O2- has a total valency of 2
satisfied by {4 x
6)} +{2 x Ti ( /3)}
Each Oxygen must be common to 4 CaO12 cuboctahedra & 2 TiO6 octahedra
Ca2+(1/
2/3 + 4/3 = 2-
•This information suffices to define the idealized structural arrangement Uniquely
Perovskite CaTiO3 Pseudocubic 90.67°
(MgSiO3 Lower Mantle Structure)
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
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
Treating the mineral Garnet
Ca3Al2Si3O12 as an ionic crystal
Ca2+
Al3+
Si4+
coordination
8
6
4
e.b.s.
1
/4
/2
1
1
•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
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) Andradite-Grossular?
J. Nimitz
Black L. Que. Schuster
Pyrope, Altay #3
Xingiang, China
J.S.S.
Coordination Change from 6 to 8:
Alkali Halides
Octahedral
Cubic
Rb/Cl = 0.732
Polymorphs
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
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 & Tridymite β- to α- phase
transitions