7.2 Metamorphic Textures and Rocks

Texture

refers to the size, shape, and arrangement of grains within a rock

Foliation

any planar arrangement of mineral grains or structural features within a rock

• Parallel alignment of platy and/or elongated minerals

• Parallel alignment of flattened mineral grains and pebbles

• Compositional banding

• Slaty cleavage where rocks can be easily split into thin, tabular sheets

Examples of foliation

• Rotation of platy and/or elongated minerals

• Recrystallization of minerals in the direction of preferred orientation

• Changing the shape of equidimensional grains into elongated shapes that are aligned

Ways where foliation can form

Compositional layering

defined by alternating layers composed of different mineral composition and/or different grain sizes. Easily recognized by differences in color of layers

Gneissosity

defined by compositional layering of equant crystals alternate with platy or elongate mineral layers.

Gneissic rocks

They exhibit a distinctive banded appearance

Schistosity

defined by alignment of play or inequant minerals

slaty cleavage

Schistosity surface along which the rock may break in very fine-grained mica and/or chlorite in slate and phyllite

crenulation cleavage

Schistosity surface along which the rock may break in alignments with cm- to mm-scale periodic folding

Slaty cleavage

closely spaced planar surfaces along which rocks split

Mylonite layering

defined by layers of highly strained rock with elongated grains due to grain size reduction and dynamic recrystallization during shearing

Lineation

parallelism or alignment of linear elements in the rock

• Preferred orientation of elongated mineral aggregates (e.g. quartz pebbles in metaconglomerates)

• Preferred orientation of elongate minerals (feldspars & Hb)

• Lineation defined by platy minerals

• Fold axes (especially of crenulations)

• Intersecting planar elements.

Types of lineations

Nonfoliated

They lack foliation, develop in environments where deformation is minimal. Typically composed of minerals that exhibit equidimensional crystals

Hornfelsic textures

random orientation of fine-grained rocks, due to lack of stresses

Granofelsic textures

random orientation of medium- to coarse-grained rocks

Granoblastic texture

A mosaic of fine to coarse grained anhedral grains, such as marble and granulites

Porphyroblastic texture

A relatively large crystal in smaller fine grained matrix

• Idioblast (euhedral)

• Subidioblast (subhedral)

• Xenoblast (anhedral)

types of porphyroblastic texture

Porphyroclastic texture

A large strained grain in fine grained matrix

Blastoporphyritic texture

A relict of porphyritic volcanic texture in metamorphic rocks

Augen texture

Porphyroblast of feldspars with eye-shape cross section in fine graine gneissic matrix

Poikiloblastic or sieve texture

Porphyroblast containing numerous inclusions of one or more fine grains

Corona or reaction rim

A zone consisting of grains of a new minerals that have formed at rim around mineral

Slate

Very fine-grained, excellent rock cleavage. Most often generated from low-grade metamorphisms of shale, mudstone, or siltstone

Phyllite

Gradation in the degree of metamorphism between slate and schist. Platy minerals not large enough to be identified with unaided eye. Has glossy sheen and wavy surface and exhibits rock cleavage

Schist

Medium to coarse grained, platy minerals predominate commonly micas.

Gneiss

Medium to coarse grained, banded appearance, high-grade metamorphism. Often composed of light colored feldspar rich layers with bands of dark ferromagnesian minerals

Marble

Coarse, crystalline and exhibits a variety of colors. Composed of calcite or dolomite crystals

Quartzite

Formed from quartz-rich sandstone where quartz grains are fused together

Slate-Phyllite-Schist-Gneiss

order of foliated metamorphic rocks from low grade to high grade metamorphism

Coarse grained

Metamorphic rocks with crystals more than 1 mm in diameter

Medium grained

Metamorphic rocks with crystals from 1 mm to 0.1 mm in diameter

Fine grained

Metamorphic rocks with crystals less than 0.1 mm in diameter

Metamorphic grade

For metamorphic rocks from one metamorphic complex which have undergone metamorphism for approximately the same length of time, the grain size increases with temperature, so that fine-grained rocks have probably been metamorphosed at low temperatures, coarse grained ones at higher temperatures.