Metamorphic Rocks

Contact Metamorphism

• Changes that take place when magma is injected into the surrounding solid rock
• The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it

Regional Metamorphism

• Also known as dynamic metamorphism,
• Is the name given to changes in great masses of rock over a wide area.
• Tends to make the rock more indurated and at the same time to give it a foliated, schistose or gneissic texture, consisting of a planar arrangement of the minerals, so that platy or prismatic minerals like mica and hornblende have their longest axes arranged parallel to one another.

Foliated Metamorphic Rock

• Means the parallel arrangement of certain mineral grains that gives the rock a striped appearance.
• Forms when pressure squeezes the flat or elongate minerals within a rock so they become aligned.
• These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.
• Some kinds of metamorphic rocks -- granite gneiss and biotite schist are two examples -- are strongly banded or foliated.

Granite Gneiss

• Is a foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains.
• It typically contains abundant quartz or feldspar minerals. The specimen shown above is about two inches (five centimeters) across.
• Granite gneiss can also form through the metamorphism of sedimentary rocks.

Biotite Schist

It’s foliated texture (= crystals aligned into bands or stripes or layers) is often only seen when specimens are viewed on edge.
• Are dominated by biotite mica, but often contain a significant muscovite mica component.

Chlorite Schist

Is a metamorphic rock with well- developed foliation.
• It often contains significant amounts of mica which allow the rock to split into thin pieces.
• It is a rock of intermediate metamorphic grade between phyllite and gneiss.
• The specimen shown is a "chlorite schist" because it contains a significant amount of chlorite. It is about two inches (five centimeters) across.

Non-foliated Metamorphic Rock

• Do not have a platy or sheet-like structure.
• There are several ways that they can be produced. Some rocks, such as limestone are made of minerals that are not flat or elongate.
• Another type of metamorphism, contact metamorphism, occurs when hot igneous rock intrudes into some pre-existing rock.
• The pre-existing rock is essentially baked by the heat, changing the mineral structure of the rock without addition of pressure.

Quartzite

Is a non-foliated metamorphic rock that is produced by the metamorphism of sandstone.
• It is composed primarily of quartz.
• The specimen is about two inches (five centimeters) across.

Marble

Is a non-foliated metamorphic rock that is produced from the Metamorphism of limestone or dolostone.
• It is composed primarily of calcium carbonate.
• The specimen shown is about two inches (five centimeters) across.

Lapis Lazuli

The famous blue gem material, is actually a metamorphic rock.
• As a general rule, solid blue lapis or solid blue with a few grains of gold pyrite are the most desirable colors. In the photo, the bottom two cabochons approach that ideal.
• The large cabochon on the top right has a few thin veins of calcite and some calcite mottling.
• The top left cabochon has large patches of calcite that are inter-grown with blue lazurite to yield a faded denim color. It also contains many visible grains of pyrite.

Slaty

Includes slate and phyllite; the foliation is called "slaty cleavage”

Schistose

Includes schist; the foliation is called "schistosity"

Gneissose

Gneiss; the foliation is called"gneissosity"

Granoblastic

Include granulite, some marbles and quartzite

Hornfelsic

Includes hornfels and skarn

Parthenon

Around 22,000 tonnes of marble were used to build it, an important temple in Ancient Greece. It is still used as a building and sculpture material today.