The boundaries for his zones were based on the first appearance of a particular mineral, called
, which is characteristic of the zone. These boundaries were later called
(equal grade) and likely represent surfaces in a three dimensional sense. He called the
zone of lowest grade rocks the "zone of digested clastic mica," but Tilley, mapping the area in
1925, renamed this zone the chlorite zone.
Mineral Assemblage in Pelitic Rocks
quartz, chlorite, muscovite, albite
biotite begins to replace chlorite, quartz, muscovite, albite
Garnet (phyllites and schists)
quartz, muscovite, biotite, almandine, albite
quartz, biotite, muscovite, almandine, staurolite, oligoclase
quartz, biotite, muscovite, oligoclase, almandine, kyanite
Sillimanite (schists &
quartz, biotite, muscovite, oligoclase, almandine, sillimanite
Mineral assemblages for pelitic rocks of the Barrovian Zones are listed in the table above.
Note the following important points:
The index mineral that defines a zone, does not necessarily disappear when entering the
next higher grade zone.
For example the first appearance of biotite is at the biotite
isograd where chlorite is seen to be reacting to produce biotite.
Biotite does not
disappear at the garnet isograd, and, in fact continues to be seen though the garnet,