Isograds and Facies - Lecture 21 Isograds and Facies Metamorphic Isograds Came from the work of Barrow in the Scottish Highlands. He attributed this to systematic increase in temperature and pressure. He used the terms low grade to high grade with grade indicating the degree of metamorphism Index Mineral - metamorphic mineral characteristic of a particular zone. Isograd - line on a map representing the first appearance of an index mineral.
Problems lie in the nature of the reactions.
However, in many cases isograds are not so straightforward. This coupled with Goldschmidt's observation that phase relationships strictly limited the number of phases that could exist lead to the metamorphic facies concept. Eskola originally proposed five facies that have since been added to and modified by others. Note that at the time he proposed these facies he had no idea of their T and P ranges. Those were not worked out until decades later.