Index Minerals

Index Minerals

1. an index mineral is a mineral that is stable over a range of certain PT conditions

   • if we are able to identify these minerals we know what PT conditions the rock has undergone
     • this is dependant on protolith also
   • most common index minerals are garnet and Al2 SiO5 polymorphs
     • presence of these minerals depend on protoliths, as their chemical composition cannot be changed with just P and T
     • Al2 SiO5 polymorphs can only be formed from a protolith such as shale, that has a high abundance of Al and Si
     • majority of basic igneous rocks composition is dominated by Al and Si

2. Pelites

   • George Barrow 1993 undertook a study understanding index minerals in the dal radian supergroup in SE scotland
     • identifying metamorphic zones and mineral isograds
       • mainly the Barrovian Index Minerals
         • if a shale were to undergo it would follow this sequence
     • sucessive metamorphism zones shown above, are recognised by appearance of new minerals, not present at a lower grade.
   • the mineralogy of pelitic sediments is dominated by Al K rich phyliosilicates composition 50% and quartz composition 10-30%
     • distinguishing features- high Al2+K20 but low CaCO3
       • high clay and mica content that has a tendency to foliate

3. basic igneous rocks

   • basic igneous rocks that undergo metamorphism enter a multitude of metamorphic facies, each with definitive mineral assemblages
   • contain a high number of different minerals, namely plagioclase feldspar +clinopyroxene +- olivine, quartz and clinopyroxene

4. Calcareous Rocks

   • predominatley carbonates, usually limestones or dolostones, they typically form in a stable shelf environment
     • dolostone is included as it shows variability, however small it is. Dolostone becomes unstable over increasing depth
     • carbonates become metamorphosed when the passive margin becomes part of an orogenic belt
   • CaCO3 or CaMg(CO3)2
   • may be pure carbonate (very rare) or contain variable amounts of other precipitates, from possible a river flowing through a passive margin, that contains mud and Al2 SiO5

5. compare and contrast

   • very clear to see that influence that the protolith has in the presence of index minerals, Basic and Pelitic rocks contain a very large range of phases the rock could enter under different P and T
     • due to different chemical compositions
     • pelitic rocks contain lots of mucas which are very sensitive to changing PT conditions
       • calcareous rocks have very limited metamorphic facies due to lack of elements present in protolith

Imágenes y archivos del mapa