Slate to mylonite are part of the Barrovian metamorphic series, also known as the Barrovian zones or Barrovian sequence. This series is named after the Scottish geologist James Barrow, who extensively studied the metamorphic rocks in the region. It is characterised by a sequence of progressive metamorphic changes that occur in response to increasing temperature and pressure as rocks are buried deeper within the Earth's crust. It represents a range of metamorphic conditions and mineral assemblages that form under different pressure-temperature regimes.The sequence includes the following rock types, from lower to higher grades of metamorphism:

• Slate:  is a low-grade metamorphic rock that forms from the metamorphism of shale or mudstone. It typically exhibits a foliation caused by the alignment of fine-grained minerals like mica and chlorite.

• Phyllite: is an intermediate-grade metamorphic rock that forms from the further metamorphism of slate. It has a more pronounced foliation and often contains larger mica flakes.

• Schist: is a medium- to high-grade metamorphic rock that forms from the metamorphism of mudstones, shales, or phyllites. It has a well-developed foliation with visible minerals like mica, chlorite, garnet, and others.

• Gneiss: is a high-grade metamorphic rock that forms from the metamorphism of various rock types, including shale, schist, or granite. It displays a distinct layering of light and dark minerals, often with prominent mineral segregation.

• Mylonite: is a high-grade metamorphic rock that forms along fault zones through the process of shearing. It exhibits a characteristic texture called "mylonitic texture," which results from the intense deformation and recrystallization of minerals due to the movement along the fault.

The progression from slate to mylonite within the Barrovian metamorphic series represents an increase in metamorphic grade due to higher temperatures and pressures. The minerals present in each rock type reflect the changes in the metamorphic environment. As the rocks are buried deeper within the Earth's crust, they experience more heat and pressure, leading to the development of new minerals and the rearrangement of existing ones. This sequence of changes forms the basis for the Barrovian metamorphic series.

What are the main minerals in this series?

The main minerals found in the Barrovian metamorphic series vary as the rocks undergo increasing levels of metamorphism. Here are some of the key minerals associated with each stage of the series:

Slate

• Muscovite: A common mica mineral that contributes to the foliation in slate.

• Chlorite: A green, platy mineral often present in slates, contributing to their color and texture.

• Quartz: grains are usually present in slates, contributing to their overall composition.

Phyllite

• Muscovite: flakes become larger and more prominent in phyllites compared to slates.

• Chlorite:  grains may still be present but can become less dominant as the metamorphic grade increases.

• Quartz: remains a significant mineral in phyllites.

Schist

• Muscovite: can still be present, but other minerals become more important.

• Biotite: A dark-colored mica mineral that becomes more common in schists.

• Garnet: is a common mineral in schists and becomes more prevalent with higher metamorphic grades.

• Staurolite: is often present in schists and can be a key indicator of specific metamorphic conditions.

• Quartz: is still present, but it may become less dominant compared to lower-grade rocks.

Gneiss

• Biotite: remains present but can be joined by other minerals.

• Feldspar: Feldspar minerals (such as plagioclase and orthoclase) become more abundant, contributing to the layered appearance of gneisses.

• Quartz: is often present in gneisses, contributing to their composition.

• Garnet: can continue to be present in certain gneiss varieties.

Mylonite

• Quartz: grains are often elongated and oriented parallel to the fault surface in mylonites.

• Feldspar: grains may be aligned or recrystallized due to the shearing process.

• Muscovite: can occur, but it may have a stretched appearance due to the deformation.

• Chlorite: can be present, and its alignment can contribute to the mylonitic texture.

Keep in mind that the exact mineral assemblages can vary depending on the specific composition of the original rocks and the conditions of metamorphism. The presence of these minerals at different stages of the Barrovian metamorphic series reflects the changing pressure, temperature, and chemical conditions that the rocks experience during their transformation.