Metamorphism (means changed form) is recrystallization without melting of a previously existing rock at depth in response to a change in the environment of temperature, pressure, and fluids. Common minerals in metamorphic rocks include: quartz and feldspar in siliceous metamorphic rocks, and calcite in marble. These minerals are not mentioned in the name of a metamorphic rock. Minerals that are mentioned in the name of a metamorphic rock are called Index Minerals and include: chlorite, epidote, serpentinite, muscovite, biotite, garnet, hornblende, staurolite, andalusite, kyanite, cordierite, and sillimanite. Index minerals in metamorphic rocks indicate the grade of metamorphism, i.e. how much changed from the original rock the metamorphic rock is.

Contact metamorphism of limestone

metamorphic aureole

Contact metamorphism of limestone

metamorphic aureole

marble

marble

skarn with garnet

Contact metamorphism of shale

metamorphic aureole

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of tiny flakes of clay minerals and lesser amounts of tiny fragments (clay-sized to silt-sized clasts) of quartz.

muscovite (white mica)

(green mica) chlorite

Andalusite

sillimanite

cordierite

kyanite

andalusite

sillimanite

hornfels

Contact metamorphism of quartz sandstone

metamorphic aureole

quartz sandstone

quartzite

oceanic crust in plate tectonic theory

Contact metamorphism of oceanic crust Basalt and Gabbro

Greenschist - Olivine, pyroxene, and plagioclase in an original basalt change to amphiboles and chlorite (both commonly green) as water in the pore spaces reacts with the original minerals at temperatures and pressures of low grade metamorphism.

Pillow lavas are generally basaltic in composition but have been pervasively basalt is altered to mineral assemblages in the zeolite and greenschist metamorphic facies. Thus the plagioclase has been converted to albite, and the mafic minerals have been transformed mainly to chlorite. These basaltic pillow lavas with albitic plagioclase were originally thought to be a special type of magma and were given the name spilite. Today, however, their partricular composition is recognized to be the product of hydrothermal alteration.

spillite

Basalt is altered to mineral assemblages in the zeolite and greenschist metamorphic facies.

Sheeted Dikes

Most of the dikes are less than 5m wide. Many appear to have only one chilled margin. This is due to repeated intrusions in the center of a single open fissure. By studying statistically the asymmetry of chilled margins, the position to main axis of spreading can be determined. The dikes are diabases composed of plagioclase and augite. They preserve an ophitic texture, but most of their minerals have been hydrothermally altered. Although probably related to the underlying gabbros, they are not derived directly from them. The dikes cut the underlying gabbros but pinch out downward. The overlying pillow lavas appear to have been fed by the dikes.

Contact metamorphism of continental Basalt and Gabbro

Amphibolite - As pressure and temperature increase to intermediate grades of metamorphism, only dark colored amphiboles and plagioclase survive and the resulting rock is called an amphibolite.

Gabbro metamorphosed at the amphibolite facies. Rubies had formed when fluid derived from the gabbro magma interacted with the host rocks under a silica unsaturated environment. The rubies predate the metamorphism.

Ruby is found in close association with white feldspar, mica, and dark green amphibole.

Barton Garnets are some of the largest in the world. .

chrysotile amosite crocidolite

tremolite actinolite anthophyllite

“Asbestos” minerals

white asbestos blue asbestos

Contact metamorphism of dunite and peridotite

Chrysotile is not a single mineral, but a group of polymorphous minerals with the same chemical composition but different crystal lattice. Chrysotile polytypes are: Clinochrysotile (monoclinic) Orthochrysotile (orthorhombic) Parachrysotile (orthorhombic) Lizardite (hexagonal)

Asbestos and Vermiculite Vermiculite is a hydrated laminar magnesium-aluminum-iron silicate which resembles mica. It can be used for many industrial applications and has been used as insulation. Some deposits of vermiculite have been found to be contaminated with small amounts of asbestos.

Talc (soapstone rock) results from contact metamorphism (metasomatism): 1) when heated waters carrying dissolved magnesium and silica reacted with dolomitic marbles, 2) when heat and chemically active fluids altered rocks such as dunite and serpentinite.