The first thing that identifies metamorphic rocks is their texture. They can be divided into:
non-foliated (or massive) rocks. These are called:
and foliated (leaf-like layers) rocks. These are called:
- slate (
The first thing that identifies metamorphic rocks is their texture. They can be divided into:
non-foliated (or massive) rocks. These are called:
and foliated (leaf-like layers) rocks. These are called:
- slate (having a cleavage (split along a plane))
- phyllite (having a cleavage and micaceous sheen)
- schist (having a parallel arrangement of platy (flat thin planes) minerals)
- knotted schist (similar to schist but with distinctive large non-platy minerals surrounded by the platy minerals)
- gneiss (alternating layers of different composition, usually on the scale of a few millimetres to a centimetre)
The second thing that helps to identify a metamorphic rock is its mineralogy - the minerals that make it up.
To name a metamorphic rock, the mineralogy is listed (in decreasing order of abundance) and then the rock is given a suffix relating to its general texture. For example:
- A rock composed of 60% muscovite and 40% biotite, both occurring as interlocking platy grains - a schistose (schist-like; schists are a group of medium-grade metamorphic rocks) texture - is called a muscovite-biotite schist.
- A massive rock composed of 50% grossular garnet, 40% epidote and 10% wollastonite is called a grossular-epidote-wollastonite skarn (skarns are metamorphically altered limestones).
The Al2SiO5 polymorphs
The minerals andalusite, kyanite and sillimanite are collectively known as the Al2SiO5 polymorphs. They occur in metamorphosed rocks but have different stability fields with respect to temperature and pressure. These minerals are us