Red Slate. Metamorphosed shale sample. Alignment of tiny mica and chlorite grains in this rock causes it to break parallel to the alignment plane (foliation). Click on the image to see a larger version with more information.
Sillimanite Porphyroblasts. Metamorphosed shale sample. White sillimanite porphyroblasts 2-3 cm in length, Rangely Fm., Walpole, NH, USA. Click on the image to see a larger version with more information.
Metamorphosed Shale Information
Introduction
Most shales are composed principally (>90 weight %) of clay minerals and quartz, with feldspar and carbonate minerals (~8 weight %) comprising much of the rest (see The Hutton Institute). Therefore, the whole rock chemistry of metamorphosed shales is peraluminous and the rocks tend to be rich in muscovite, until muscovite breaks down at high temperatures. Alignment of platy musovite grains is the principal source of the foliation of metamorphosed shales. As the metamorphic grade increases, the grain size of the muscovite increases, changing the rock from slate to phyllite to schist.
Mineralogy of Metamorphosed Shales
Metamorphosed Shale Minerals . Click on the image to visit the list of minerals and links to their properties.
The aluminum-rich compositions of metamorphosed shales lead to the growth of a number of distinctive, aluminum-rich minerals that may grow as porphyroblasts visible in the field. Chemical reactions that occur among these minerals provide constraints on the conditions of metamorphism and P-T paths of metamorphosed shales. A list of minerals commonly observed in metamorphosed shales and links to mineral properties can be found here.
Samples of Metamorphosed Shales
Kyanite-Staurolite-Garnet Schist. A sample of a metamorphosed shale. Click on the image to see a list of metmaorphosed shale images in the Rock Library, including this and other schists.
Rocks and thin sections of metamorphosed shales can be striking in appearance because of the abundance of muscovite and the distinctive character of porphyroblasts of minerals like garnet and kyanite. Links to images of hand samples and thin sections of metamorphosed shales in the Rock Library can be found on this list.
Thompson AFM Projection
Mineral assemblages and chemical reactions for most metamorphosed shales can be clearly shown on a Thompson AFM Diagram. This diagram is a projection from quartz and muscovite or K-feldspar
Thompson AFM Diagram . Click on the image to show compositions of metamorphosed shale minerals or to plot bulk compositions, both on a Thompson diagram.
with H2O treated as a mobile component. In most cases, at equilibrium there will be only two or three major minerals in addition to quartz and muscovite or K-spar. Accessory minerals, including minerals that are opaque in thin section may also be present. More information about the Thompson AFM Diagram can be found here. You can plot bulk rock compositions from your own chemical data in a .csv file on an AFM diagram here.
Chemical Reactions in Metamorphosed Shales
Chemical reactions that may occur during the metamorphism of shale have been studied experimentally and modeled themodyanmically. An example model is
AFM Reactions. Click on the image to see how the equilibrium mineral assemblages change on an AFM diagram as T-P conditions are changed for a metamorphosed shale.
shown in this interactive T-P diagram. Click on the diagram and mouseover the reactions on the T-P diagram to see the mineral assemblages change on an AFM diagram as discontinuous chemical reactions are crossed. Which reactions might affect a particular metamorphic rock or be responsible for an isograd depends on the bulk chemical composition of the rock. Reactions for particular bulk compositions are shown on in this Mineral Assemblage Diagram (MAD also called pseudosection). Details of a continuous chemical reaction are shown on an AFM diagram here.
