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.

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 Feldspar Sold) 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 Pressure.- 2.1 Albite.- 2.2 K-Feldspar.- 2.3 Anorthite.- 3. Compressional Behaviour.- 3.1 Bulk Moduli.- 3.2 Unit Cell Parameter Variation with Pressure.- 3.3 Strain Tensors.- 4. Structural Phase Transitions.- 4.1 Phase Transitions in Alkali Feldspars.- 4.2 $$P\bar 1$$ to $$I\bar 1$$ Transition in Anorthite.- 4.2.1 $$P\bar 1$$ = $$I\bar 1$$ in End-Member Anorthite.- 4.2.2 Effect of Al, Si Disorder on $$P\bar 1$$ = $$I\bar 1$$.- 4.2.3 Effect of Albite Substitution on $$P\bar 1$$ = $$I\bar 1$$ Transition.- 4.2.4 Phase Diagram for Anorthite $$P\bar 1$$ = $$I\bar 1$$ Transition.- 4.2.5 Thermodynamic Analysis.- 4.3 Phase Transitions in Other Feldspars.- 4.4 Conclusions.- 5 Acknowledgements.- 6. References.- 8. Recent Work on Oscillatory Zoning in Plagioclase.- Abstract.- 1. The Problem.- 2. Empirical Observations of Zoning.- 2.1 Hypotheses and Assumptions.- 2.1.1 Crystal Growth.- 2.1.2 Analysis of Zoning Patterns.- 2.2 Trace Elements and Disequilibrium.- 2.3 Discussion of Plates.- 2.3.1 Plate 1.- 2.3.2 Plate 2 “Corners”.- 2.3.2 Plate 3 “Interfaces”.- 2.4 Conclusion from Empirical Observations.- 2.4.1 The Consequences of Saw-toothed Patterns.- 2.4.2 Significance of Dissolution.- 2.4.3 Constraints on Growth Mechanisms and Laws.- 3. Spatiotemporal Dynamics in Oscillatory Zoning.- 3.1 Introduction.- 3.2 Theoretical Rationale.- 3.3 Empirical Textural Evidence.- 3.4 Identification of Chaotic Growth.- 3.4.1 General Comments.- 3.4.2 Random Zoning Patterns.- 3.4.3 Real Crystal Zoning Patterns.- 4. Attempts at Modelling Zonation.- 4.1 Theoretical and Empirical Models.- 4.2 Details of Models.- 4.2.1 Sibley et al (1976).- 4.2.2 Haase et al (1980).- 4.2.3 Allègre et al (1981).- 4.2.4 Lasaga (1982).- 4.2.5 Loomis (1979, 1982).- 4.2.6 Simakin (1983).- 4.2.7 Ghiorso (1987).- 4.2.8 Pearce (1993).- 4.2.9 Wang and Merino (1993).- 5. Discussion and Conclusions.- 5.1 General Comments.- 5.2 Preferred Model.- 5.3 Suggestions for Further Work.- 6. Acknowledgements.- 7. References.- 9. Isotopic Equilibrium/disequilibrium and Diffusion Kinetics in Feldspars.- Abstract.- 1. Introduction.- 1.1 Isotope Fractionation.- 1.2 Diffusion Measurement.- 1.3 Terminology.- 1.4 Applications.- 2. Part 1. Experiments and Theory.- 2.1 Oxygen Diffusion Kinetics.- 2.1.1 Hydrothermal Experiments.- 2.1.2 Dry Experiments.- 2.2 Cation Diffusion Kinetics.- 2.2.1 Strontium Diffusion.- 2.2.2 Other Cations.- 2.2.3 K-feldspars.- 2.2.4 Systematics of Cation behaviour.- 2.3 Prediction of D Values.- 3. Part 2. Applications of Diffusion Kinetics to Tectono-Thermal Histories.- 3.1 Oxygen Isotopes and the Rates of Cooling from High Temperatures.- 3.2 Rb-Sr Systematics and Cooling Rates from High Temperature.- 4. Conclusions.- Acknowledgements.- References.- 10. Hydrogen in Feldspars and Related Silicates.- Abstract.- 1. Introduction.- 2. Abundances and Substitution Mechanisms of Structural “Water”.- 2.1 Chemical Analysis.- 2.2 Spectroscopic Constraints.- 3. Early Studies and Models.- 3.1 Oxygen Isotope Exchange Experiments.- 3.2 Al-Si order-Disorder and Interdiffusion Experiments.- 4. The Question of Solution-Reprecipitation vs Solid-State Diffusion.- 5. Al-Si Order-Disorder and Interdiffusion in Feldspars as a Guide to the Role of Hydrogen.- 6. The Role of Hydrogen in Oxygen Diffusion in Felspars.- 6.1 Introduction.- 6.2 First order Effects: Wet versus Dry Oxygen Diffusion.- 6.3 Second Order Effects: P(H2O)?.- 6.3.1 Experimental Problems.- 6.3.2 Interdependence of Variables in H2O-bearing Systems.- 6.3.3 Discussion.- 7. The Effect of Water on Cation Diffusion.- 8. Properties of Eifel Sanidine: A Natural Example of the Catalytic Role of Hydrogen in Feldspars?.- 9. Hydrogen in Feldspars: Conclusions and Prospects of Future Progress.- 10. Footnote: Water in Feldspars at High P(H2O), and Feldspar Hydrates.- Acknowledgements.- References.- 11. Argon Diffusion in Feldspars.- Abstract.- 1. Introduction.- 1.1 Perspective.- 1.2 Scope.- 2. Background.- 2.1 40Ar/39Ar Technique.- 2.1.1 Some Basics.- 2.1.2 Dating of Feldspars.- 2.1.3 Effects of Neutron Irradiation.- 2.2 Diffusion of Argon.- 2.2.1 General.- 2.2.2 Temperature Dependence.- 2.2.3 Measurements and Models.- 2.3 Diffusion and the 40Ar/39Ar Techniques.- 2.3.1 Closing Temperature.- 2.3.2 Argon Gradients.- 3. Brief Review of Selected Feldspar Argon Studies.- 3.1 Overview.- 3.2 Early Diffusion Studies.- 3.3 40Ar/sAr Studies.- 4. Defining the Parameters of Ar Diffusive Transport in Feldspar.- 4.1 Behavior in Benson Mines Orthoclase.- 4.1.1 Specimen.- 4.1.2 Behavior of Ar in Natural Material.- 4.1.3 Behavior of Ar in Irradiated Material.- 4.1.4 Behavior of Ar in Incremental-Heating Experiments.- 4.2 Generalizations.- 4.2.1 Diffusion Following “Fickian” Behavior.- 4.2.2 Anisotropy of Ar Transport.- 4.2.3 Effect of External Atmospheres.- 4.2.4 Effects of Neutron Irradiation.- 4.2.5 Diffusion Mechanisms.- 4.2.6 Sites of Argon.- 4.2.7 Effect of Twin and Lamellar Boundaries.- 4.2.8 Intrinsic versus Extrinsic Control.- 4.2.9 Argon Trapping.- 4.2.10 Applicability to Nature.- 5. References.- 12. Feldspars in Igneous Rocks.- Abstract.- 1. Introduction.- 2. Conditions of Crystallization and Alteration of Feldspars in Rocks.- 2.1 The Feldspathic Component of Magmas.- 2.2 The Effect of Emplacement Conditions on Nucleation and Growth.- 2.2.1 Nucleation.- 2.2.2 Growth.- 2.3 Intracrystalline Textures and Chemical Zoning.- 2.4 Equilibrium Phase Relationships.- 2.5 Equilibrium and Fractional Crystallization.- 2.6 Phase-Transformation and Exsolution Micro- Textures.- 2.7 Deuteric and Hydrothermal Alteration.- 2.8 Deformation and Uplift.- 3. Feldspar in the Classification of Igneous Rocks.- 3.1 The Modal Iugs Classification Scheme.- 3.2 The Chemical Tas Diagram.- 3.3 Normative Plagioclase Compositions and Classification Schemes.- 4. Feldspars in Volcanic and Hypabyssal Rocks.- 4.1 Structures and Textures in Volcanic Rocks.- 4.1.1 Structures in Volcanic Rocks.- 4.1.2 Volcanic Intercrystalline Textures.- 4.1.3 Volcanic Intracrystalline Textures.- 4.2 Zoning Paths in Volcanic Feldspars.- 4.2.1 One-Feldspar Paths--Plagioclase.- 4.2.2 One-Feldspar Paths — Sanidine.- 4.2.3 Simultaneous Two-Feldspar Paths.- 4.2.4 Sequential Two-Feldspar Paths.- 4.2.5 Critical Solution Line.- 4.3 Volcanic Subsolidus Transformation and Exsolution Microtextures.- 4.3.1 Anorthoclase.- 4.3.2 Cryptoperthites.- 5. Feldspars in Plutonic Rocks.- 5.1 Structures and Textures in Plutonic Rocks.- 5.1.1 Structures in Plutonic Rocks.- 5.1.2 Plutonic Intercrystalline Textures.- 5.1.3 Plutonic Intracrystalline Textures.- 5.2 Zoning Paths in Plutonic Feldspars.- 5.3 Plutonic Subsolidus Transformation and Exsolution Microtextures.- 5.3.1 Plagioclase.- 5.3.2 Ternary and Alkali Feldspars in Hypersolvus Rocks.- 5.3.3 Alkali Feldspars in Subsolvus Rocks.- 6. Deuteric Alteration of Feldspars and Low-Temperature Reactivity.- 6.1 Deuteric Microtextural Changes in Alkali Feldspars.- 6.2 Geochemical Implications and Temperatures of Deuteric Reactions.- 7. Acknowledgements.- 8. References.- 13. Evolution of Feldspars in Granitic Pegmatites.- Abstract.- 1. Introduction.- 2. Terms of Reference.- 3. Paragenetic and Textural Relationships.- 3.1 Zoned Pegmatites.- 3.2 Other Structural Types.- 3.2.1 Layered Pegmatites.- 3.2.2 Quasi-homogeneous Pegmatites.- 3.2.3 The Albite Type.- 3.2.4 Miarolitic Pegmatites.- 4. Compositional Evolution at the Magmatic Stage.- 4.1 Trends in Bulk Composition.- 4.2 Minor Elements in K-Feldspar.- 4.3 Minor Elements in Plagioclase.- 4.4 Fractionation Trends of Minor Elements.- 5. Isotopic Data.- 6. Subsolidus Processes.- 6.1 Evolution of K-Feldspar.- 6.1.1 Ordering - Structural and Textural Aspects.- 6.1.2 Perthitic Exsolution and Coarsening.- 6.2 Evolution of Plagioclase.- 6.2.1 Ordering.- 6.2.2 Exsolution.- 6.3 Low-Temperature Hydrothermal Phases.- 7. Replacements involving Feldspars.- 7.1 Albite-Rich Units.- 7.2 Micaceous Units.- 8. Concluding Remarks.- Acknowledgements and References.- 14. Surface Chemistry of Feldspars.- Abstract.- 1. Introduction.- 2. Quantitative Chemical and Physical Theories.- 2.1 Electronic Theories of Bulk Crystal Structure.- 2.2 Thermodynamic Concepts.- 2.3 Acid-Base Behavior.- 2.4 Defects.- 3. General Ideas on Surfaces.- 3.1 Growth Morphology and Surface Properties.- 3.2 Hydroxyls, Terminal Oxygen Atoms and Water Molecules.- 3.3 Inorganic Ions and Complexes.- 3.4 Organic Ions and Complexes.- 3.5 Ultra-High Vacuum.- 3.6 High- and Low-Temperature; High Pressure.- 4. Atomic Packing at Feldspar Surfaces.- 4.1 Hydrogen-Free Termination of Regular Structure.- 4.2 Hydrogenated Surface.- 4.3 Inorganic Adsorb Ates.- 4.4 Organic Adsorbates.- 5. Analytical Techniques and Results on Feldspars, Clays, Zeolites, etc..- 5.1 Introduction.- 5.2 Microscopy.- 5.2.1 General.- 5.2.2 Atomic force microscopy.- 5.2.3 Electron...