The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C

Petrogenetic interpretation of granitoid rock series using multicationic parameters

Mullite has achieved outstanding importance as a material for both traditional and advanced ceramics because of its favourable thermal and mechanical properties. Mullite displays various Al to Si ratios referring to the solid solution Al 4+2 x Si 2−2 x O 10− x , with x ranging between about 0.2 and 0.9 (about 55 to 90 mol% Al 2 O 3 ). Depending on the synthesis temperature and atmosphere mullite is able to incorporate a number of transition metal cations and other foreign atoms. The crystal structure of mullite is closely related to that of sillimanite, which is characterized by chains of edge-connected AlO 6 octahedra running parallel to the crystallographic c -axis. These very stiff chains are cross-linked by tetrahedral chains consisting of (Al,Si)O 4 tetrahedra. In more detail: Parallel to a the tetrahedra are linked to the relatively short more stiff Al–O(A, B) bonds, whereas parallel b they are linked parallel to the relatively long more compliant Al–O(D) bonds. In mullite some of the oxygen atoms bridging the tetrahedra are removed for charge compensation. This gives rise to the formation of oxygen vacancies and of T 3 O groups (so-called tetrahedral triclusters). The anisotropy of the bonding system of mullite has a major influence on the anisotropy of its physical properties. For example: • the highest longitudinal elastic stiffness is observed parallel c , but lower ones parallel a and especially parallel b , • the maximum of the thermal conductivity occurs parallel c , but maller ones parallel a and especially parallel b , • large thermal expansion especially parallel b , • fastest crystal growth and highest corrosion parallel c . Heat capacity and thermal expansion measurements of mullite display reversible anomalies in the temperature range between about 1000 and 1200 °C. It is believed that tetrahedral cations, bridging O atoms, and O vacancies undergo dynamical site exchange processes at high temperatures. At lower temperatures the dynamic disorder may transform to a static one. Diffraction experiments revealed that also partially ordered states may exist.

Structure and properties of mullite—A review

New activity–composition (a–x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland & Powell (2011, Journal of Metamorphic Geology, 29, 333–383). The a–x relations include a broader consideration of Fe2O3 in minerals, changes to the formalism of several phases and order–disorder in all ferromagnesian minerals where Fe–Mg mixing occurs on multiple sites. The a–x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the first time, the entire set of a–x relations for the common ferromagnesian minerals in metapelitic rocks is parameterized simultaneously, with attention paid to ensuring that they can be used together to calculate phase diagrams of geologically appropriate topology. The a–x relations developed are for use in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system for both subsolidus and suprasolidus conditions. Petrogenetic grids in KFMASH and KFMASHTO are similar in topology to those produced with earlier end-member datasets and a–x relations, but with some notable differences. In particular, in subsolidus equilibria, the FeO/(FeO + MgO) of garnet is now greater than in coexisting staurolite, bringing a number of key staurolite-bearing equilibria into better agreement with inferences from field and petrographic observations. Furthermore, the addition of Fe3+ and Ti to a number of silicate phases allows more plausible equilibria to be calculated in relevant systems. Pseudosections calculated with the new a–x relations are also topologically similar to equivalent diagrams using earlier a–x relations, although with many low variance fields shifting in P–T space to somewhat lower pressure conditions.

New mineral activity-composition relations for thermodynamic calculations in metapelitic systems

The classification of granitic pegmatites was frequently attempted during the past century, with variable degrees of success and applicability. Internal structure, paragenetic relationships, bulk chemical composition, petrogenetic aspects, nature of parent medium, and geochemical features were applied. However, all schemes were marked by contemporary degrees of understanding of these parameters, and most attempts were hindered by ignoring differences in geological environment. Substantial progress was achieved only since the late 1970s. The classification is approached here from two directions, based on but broadened and refined from earlier works by Ginsburg and Cerný. The first concept deals with geological location, leading to division of granitic pegmatites into five classes (abyssal, muscovite, muscovite – rare-element, rare-element, and miarolitic), most of which are subdivided into subclasses with fundamentally different geochemical (and in part geological) characteristics. Further subdivision of most subclasses into types and subtypes follows more subtle differences in geochemical signatures or P–T conditions of solidification, expressed in variable assemblages of accessory minerals. The second approach is petrogenetic, developed for pegmatites derived by igneous differentiation from plutonic parents. Three families are distinguished: an NYF family with progressive accumulation of Nb, Y and F (besides Be, REE, Sc, Ti, Zr, Th and U), fractionated from subaluminous to metaluminous A- and I-type granites that can be generated by a variety of processes involving depleted crust or mantle contributions; a peraluminous LCT family marked by prominent accumulation of Li, Cs and Ta (besides Rb, Be, Sn, B, P and F), derived mainly from S-type granites, less commonly from I-type granites, and a mixed NYF + LCT family of diverse origins, such as contamination of NYF plutons by digestion of undepleted supracrustal rocks.

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The classification of granitic pegmatites revisited

1. Definition and Types of Metamorphism.- 2. From Diagenesis to Metamorphism.- 3. Factors of Metamorphism.- General Considerations.- The Composition of the Fluid Phase.- Directed Pressure.- 4. Mineral Parageneses: The Building Blocks of Metamorphic Rocks.- 5. Graphical Representation of Metamorphic Mineral Parageneses.- Composition Plotting.- ACF Diagram.- A'FK Diagram.- How Are ACF and A'FK Diagrams Used?.- AFM Diagrams.- 6. Classification Principles: Metamorphic Facies versus Metamorphic Grade.- 7. The Four Divisions of Metamorphic Grade.- General Considerations.- The Terms Isograd and Isoreaction-Grad.- The Division of Very-Low-Grade Metamorphism.- The Division of Low-Grade Metamorphism.- The Change from Low-Grade to Medium-Grade Metamorphism.- The Change from Medium-Grade to High-Grade Metamorphism.- Granulite-High Grade Regional Hypersthene Zone.- Pressure Divisions of the Metamorphic Grades.- Problems with the Al2SiO5 Species.- 8. General Characteristics of Metamorphic Terrains.- Metamorphic Zones in Contact Aureoles.- Metamorphic Zones in Regional Metamorphism.- Paired Metamorphic Belts.- 9. Metamorphic Reactions in Carbonate Rocks.- General Considerations.- Metamorphism of Siliceous Dolomitic Limestones.- Formation of Wollastonite.- Metamorphism of Carbonates at Very High Temperature and Very Low Pressure.- 10. Metamorphism of Marls.- 11. Metamorphism of Ultramafic Rocks: Systems MgO-SiO2-CO2-H2O and MgO-CaO-SiO2-H2O.- 12. Metamorphism of Mafic Rocks.- Transformations Except Those of Very-Low-Grade Metamorphism at Low Pressures.- Very-Low-Grade Metamorphism at Low Pressures.- Evaluation of Metamorphic Changes at Very-Low Grade.- The Role of CO2 in Very-Low-Grade Metamorphism.- 13. Very-Low-Grade Metamorphism of Graywackes.- 14. Metamorphism of Pelites.- General Statement.- Metamorphism of Pelitic Rocks at Very-Low and Low-Grade.- Metamorphism of Pelitic Rocks at Medium- and High-Grade.- 15. A Key to Determine Metamorphic Grades and Major Isoreaction-Grads or Isograds in Common Rocks.- Very-Low-Grade Metamorphism.- Low-Grade.- Medium- and High-Grade.- Geothermometers and Geobarometers.- Sequences of Isoreaction-Grads or Isograds.- 16. Regional Hypersthene Zone (Granolite High Grade).- Nomenclature and Mineralogical Features of "Granulites".- Metamorphism of Granolites and Related Granoblastites.- Petrogenetic Considerations.- 17. Eclogites.- 18. Anatexis, Formation of Migmatites, and Origin of Granitic Magmas.- Anatexis: General Considerations.- Experimental Anatexis of Rocks Composed of Alkali Feldspar, Plagioclase, and Quartz.- Experimental Anatexis of Rocks Composed of Plagioclase and Quartz but Lacking Alkali Feldspar.- Formation of Migmatites.- Formation of Granitic Magmas by Anatexis.- Appendix: Nomenclature of Common Metamorphic Rocks.- Names of Important Rock Groups.- Prefixes.- Classification.

Petrogenesis of metamorphic rocks

Experimentelle Gesteinsmetamorphose—II: Bildung von anatektischen granitischen Schmelzen bei der Metamorphose von NaCl-führenden kalkfreien Tonen

The stability relations between cordierite and almandite in rocks, having a composition of CaO poor argillaceous rocks, were experimentally investigated. The starting material consisted of a mixture of chlorite, muscovite, and quartz. Systems with widely varying Fe2+/Fe2++Mg ratios were investigated by using two different chlorites, thuringite or ripidolite, in the starting mixture.

Helmut G. F. Winkler

Papers

Petrogenesis of metamorphic rocks

Experimentelle Gesteinsmetamorphose—II: Bildung von anatektischen granitischen Schmelzen bei der Metamorphose von NaCl-führenden kalkfreien Tonen

Stabilitätsbeziehungen zwischen Chlorit, Cordierit und Almandin bei der Metamorphose

Experimentelle Gesteinsmetamorphose—IV: Bildung anatektischer Schmelzen aus metamorphisierten Grauwacken

Experimentelle Gesteinsmetamorphose—V: Experimentelle anatektische Schmelzen und ihre petrogenetische Bedeutung