Feldspars and their Reactions

Feldspar minerals make up 60% of the crust of the Earth. They are stable in the upper mantle, and are so abundant in the crust that they form the basis of the classification of igneous rocks. At the surface, feldspars weather to form clay minerals which are the most important mineral constituent of soils. The articles in this book review the chemical reactions of feldspars over the whole sweep of pressure and temperature regimes in the outer Earth, and describe the fundamental aspects of crystal structure which underlie their properties. The book covers intracrystalline reactions, such as order-disorder transformations and exsolution, and transfer of stable and radiogenic isotopes, which can be interpreted to provide insights into the thermal history of rocks. It is suitable for final year undergraduates or research workers.

Inhalt

1. The Crystal Structures of the Aluminium-Silicate Feldspars.- Abstract.- 1. Introduction.- 1.1 Historical Perspective.- 1.2 Recent Progress and Overview.- 2. Chemistry of Feldspars.- 3. Topology of the Feldspar Framework.- 4. Patterns of Al, Si Order-Disorder: Space Groups.- 4.1 Alkali Feldspars.- 4.1.1 Monoclinic (C2/m) K-rich Alkali Feldspars.- 4.1.2 Triclinic ($$C\bar 1$$) K-rich Alkali Feldspars.- 4.1.3 Room-Temperature Phase Transition in Low Microcline?.- 4.1.4 Triclinic ($$C\bar 1$$) Anorthoclases, K-Analbites, and the Transition to C2/m Na-Sanidines.- 4.1.5 Triclinic ($$C\bar 1$$) Low, Intermediate, and High Albite.- 4.1.6 Analbite and the Transition to C2/m Monalbite.- 4.2 Celsian and Hyalophane.- 4.3 Anorthite and Intermediate Plagioclases.- 4.3.1 The 'Aluminium Avoidance' Principle and Al, Si order/disorder.- 4.3.2 Primitive Anorthite.- 4.3.3 Pseudo-symmetry and the Average Structure Concept.- 4.3.4 Body-centered Anorthite.- 4.3.5 Intermediate Plagioclases.- 5. The M Cations in Feldspar Structures.- 5.1 Potassium in Sanidines and Microclines.- 5.2 Sodium in Albites.- 5.2.1 Sodium in Low Albite.- 5.2.2 Sodium in High Albite and Monalbite.- 5.2.3 Sodium in Intermediate Albite (IA) and Na/Ca in Sodic Plagioclase.- 5.2.4 Ca/Na in Calcic Plagioclase.- 6. Average Structures of Feldspars Represented by Cell Parameters: the Temperature-Pressure-Composition Analogies.- 6.1 Changes in Volume with P, T, X.- 6.1.1 Compressibility and Phase Changes with Pressure.- 6.1.2 Thermal Expansion.- 6.1.3 Variation of Composition in 'Solid-Solution' Series.- 6.2 Changes in Cell Dimensions with P,T,X.- 6.2.1 Series of Na- and K- Mixed with Ca-, Sr-, and Ba-feldspars.- 6.2.2 The M2+-feldspars.- 6.2.3 Alkali Feldspars.- 6.3 Changes in T-O-T Angles and M-O Distances with P,T, X.- 6.3.1 Disordered Alkali Feldspars.- 6.3.2 Ordered Alkali Feldspars and the Structure of Low Albite at 5 GPa.- 7. Future Work.- References.- 2. Aspects of Alkali Feldspar Characterization: Prospects and Relevance to Problems Outstanding.- Abstract.- 1. Introduction.- 2 Light Optics.- 3. Composition and Lattice Strain.- 3.1 Composition of Unstrained Feldspars.- 3.2 Strained Feldspars.- 3.2.1 Background.- 3.2.2 Determination of Lattice Strain.- 3.2.3 Determination of Composition.- 3.3 Composition from K, Na Site Refinements of X-Ray Intensity Data.- 4. Determination of Tetrahedral Al Contents in Alkali Feldspars.- 4.1 T-Site Occupancies from Lattice Parameters.- 4.2 T-Site Occupancies from T-O Distances.- 4.3 T-Site Occupancies from Si, Al Site Refinements of X-Ray Intensity Data.- 5. Single-Crystal X-ray Diffraction Patterns and Relationships with TEM Techniques.- 5.1 The Diagonal Association.- 5.2 The Antidiagonal Association.- 5.3 P21/a Structural Arrangement.- 6. Unanswered questions and suggestions for future work.- Acknowledgements.- References.- 3. Phase Transitions and Vibrational Spectroscopy in Feldspars.- Abstract.- 1. Introduction.- 2. Phase Transitions and Order Parameters.- 2.1 The order Parameter.- 2.2 Coupling between order Parameters.- 2.3 Some Typical Landau Potentials.- 3. Kinetic and Other Non-Equilibrium Behaviour.- 4. Hard Mode Phonon Spectroscopy.- 4.1 Introductory Remarks on Optical Spectroscopy.- 4.2 Soft Modes Versus Hard Modes.- 4.3 The Characteristic Length Scale.- 4.4 How to Do It.- 5. Applications and Examples.- 5.1 Temperature Dependence of Hard Modes in Albite.- 5.2 Effect of the C2/m-$$C\bar 1$$ Transition in Anorthoclase.- 5.3 A Displacive Phase Transition with Some Fluctuations: $$P\bar 1 - I\bar 1$$ in Anorthite.- 5.3.1 Space Average Model.- 5.3.2 The Ca-jumping Model.- 5.3.3 Soft-mode Model.- 5.4 Determination of the Degree of Al, Si order in Na Feldspar.- 5.5 Structural Modulations in Natural Potassium Feldspar.- 5.6 On the Effect of Na-K Mixing and the Study of Exsolution: an Outlook.- Conclusion.- Acknowledgements.- Appendix 1.- Deviation of the Gibbs Free Energy for a Phase Transition with Continuous Local Potentials.- Appendix 2.- On the Intercorrelation of Al, Si Ordering and the Generation of Spontaneous Strain.- References.- 4. Partial Melting Reactions of Plagioclases and Plagioclase-Bearing Systems.- Abstract.- 1. Introduction.- 2. Previous Work on Plagioclase-melt Interaction.- 3. Starting Materials and Experimental Procedure.- 4. Experimental Results and Discussion.- 4.1 Dry Melting of Plagioclase An68 At 1420°C.- 4.2 Wet Melting of Plagioclase An P = 5 Kbar.- 4.3 The Influence of Additional Components and Phases.- 4.3.1 Melting Reactions in the System Qz-Ab-An-H2O.- 4.3.2 Melting Reactions in the System Qz-Ab-An-Al2O3-H2O.- 4.3.3 Melting Reactions in the System Qz-Or-Ab-An-H2O.- 4.3.4 Kinetics of partial melting in systems with additional SiO2-containing phases beside feldspars.- 5. Summary.- Acknowledgements.- References.- 5. Ternary Feldspar/Melt Equilibria: A Review.- Abstract.- 1. Introduction.- 2. Ternary Feldspar Geothermometry.- 2.1 Activity Models for Ternary Feldspar Solid Solutions.- 2.2 Geothermometry: Calculation Methodology and Results.- 3. Ternary Felspar/Melt Equilibria.- 3.1 Liquidus Relations in the Feldspar System.- 3.2 Compositional Evolution of Feldspars during Crystallization.- 3.2.1 Equilibrium Crystallization.- 3.2.2 Fractional Crystallization.- 4. Conclusions.- 5. Acknowledgements.- 6. References.- 6. Subsolidus Phase Relations of the Plagioclase FeldsparSold) Solution.- Abstract.- 1. Introduction.- 2. A Possible Phase Diagram.- 3. Lines of Evidence.- 3.1 Order/disorder Experiments.- 3.1.1 C2/m = $$C\bar 1$$ Transition.- 3.1.2 $$C\bar 1 - I\bar 1$$ Transition.- 3.1.3 $$I\bar 1$$ = e1 Transition.- 3.1.4 $$C\bar 1$$ = e2 Transition.- 3.1.5 Ordering within the C1 stability field.- 3.1.6 $$I\bar 1 = P\bar 1$$ Transition.- 3.1.7 Summary.- 3.2 Microstructures.- 3.2.1 Peristerite gap.- 3.2.2 Bøggild gap.- 3.2.3 Huttenlocher gap.- 3.3 Compositions of Coexisting Crystals.- 3.3.1 Coexisting Phases across the Peristerite Gap.- 3.3.2 Coexisting Plagioclases with Intermediate Compositions.- 3.3.3 The Low Temperature Assemblage Albite + Anorthite.- 3.4 Properties of the Incommensurate e Structure.- 3.4.1 Mestable Incommensurate Structure of Anorthite.- 3.4.2 Structure of e Plagioclase.- 3.4.3 Influences of Temperature and Composition.- 3.4.4 Distinction between e1 and e2 Structures.- 3.5 Thermodynamic Data.- 3.5.1 Enthalpies of Ordering and Mixing.- 3.5.2 Energetics of Antiphase Boundaries.- 3.5.3 Ion-exchange Data.- 3.6 Kinetic Constraints.- 4. The Rudiments of a Thermodynamic Model.- 4.1 Schematic G-X Curves.- 4.2 Landau Free Energy Expansions.- 5. Conclusion.- 5.1 Al/Si Ordering At An0 - An15.- 5.2 Local Ordering in the Cl Solid Solution.- 5.3 Microstructural Evidence for Split Behaviour At ~An52.- 5.4 $$C\bar 1$$ = e2 Transition Temperatures.- 5.5 The $$I\bar 1$$ = e1 Ordering Transition.- 5.6 Thermodynamics of Incommensurate Ordering.- 5.7 Incommensurate Structures and Differences between e1 And e2 Ordering.- 5.8 Phase Equilibrium and Ion-Exchange Experiments.- Acknowledgements.- References.- 7. Feldspars at High Pressure.- Abstract.- 1. Introduction.- 2. Stability of Feldspars at High…