Metamorphic Rocks

features of each metamorphic rock

Spotted rock

• low/ medium grade contact

• Porphyroblastic texture - large chiastolite needles up to 2cm long

• Needles show random orientation, groundmass is mainly mica

• No relic structures are present.

Hornfels

• high grade contact

• Coarser grained

• tough, fibrous and splintery looking rock

• Andalusite porphyroblasts are sometimes present

Marbles

• Contact metamorphism of limestone

• All fossils and older structures are destroyed during recrystallisation

Slate

• low grade regional

• clay minerals recrystallize into chlorite and b. mica; these are coarser than original minerals

• Slaty cleavage: chlorite and b. mica align perpendicular to the main stress direction.

• Some relic sed. structures maybe preserved (v. low grade)

• Distorted fossils (compressed or stretched)

Schist

• Medium grade regional

• larger crystals, 1-2mm than slate

• Garnet (main mineral) along with quartz and micas

• Garnet appears as porphyroblasts (5mm in diameter)

Gneiss

• High grade regional metamorphism

• Crystals over 2mm

• Similar composition to granite - feldspar, quartz and micas

schistose texture

distortion of foliation due to garnet porphyroblasts

Granoblastic texture

all crystals are roughly the same size. 3 crystals meet at 120 degrees

• Also can be described as sugary texture

Gneissose banding

minerals are divided into discontinuous layers

How does the organisation of minerals in Gneiss differ to that in Granite?

The minerals are banded.