What are some sheet-like metamorphic rocks

Metamorphic rock formations

The rock address serves as the first aid to characterize rocks in the terrain.

Metamorphic rocks arise through a mineral transformation / new formation in the solid state, so the rock does not melt, but undergoes metamorphosis due to high pressure and / or high temperature. This happens e.g. in the course of mountain formation or in the area of ​​subduction zones. Metamorphic rocks have different characteristics than igneous rocks or sediments, especially in terms of structure. Of course, these should not be missing in a good rock description. The features that should not be forgotten for a detailed rock address in the terrain report are listed below.

Here you will also find the pages Rock Formation Magmatite and Rock Formation Sedimentite

Addressing metamorphic rocks in the field

equipment

Since the rock address usually takes place in the field, some aids must be carried with you for the determination of the rocks. The following equipment is useful for an initial assessment:

  • diluted HCl (10%), for calcareous rocks
  • Geologist's hammer to break out handpieces / for the scratch test
  • Magnifying glass to distinguish / describe minerals more precisely
  • Magnet to identify minerals such as magnetite
  • Knife, window glass, fingernail, nail to determine the hardness

Color and color number

How light or dark is the rock? Is it leucocrat (bright), mesocrat (medium) or melanocrat (dark)? You can find an overview table for the classification of the rock here:

descriptionColor number M
Hololeucocrate 0-10
Leukocrat 10-35
Mesocrat 35-65
Melanocrat 65-90
Holomelanocrat 90-100
Tip:
Since it is often not so easy to determine the exact color number in practice, an approximate classification (leuco, meso, melanocrat) is sufficient.


In addition to the classification according to mineral inventory, migmatites are also classified according to their degree of metamorphosis. The term migmatite describes rocks that consist of two or more different parts, which are based on the partial melting of the original rock. The classification differs between one Neosome (i.e. newly formed) and Paleosome (i.e. unchanged):

Paleosome

Mesosome: More or less unchanged, metamorphic parent rock

Neosome

Leukosome: Light rock (i.e. rich in quartz and feldspar), no preferred orientation of the mineral grains

Melanosome: Dark rock (rich in dark (mafic) minerals), residual material is called restite

Space filling and density

Metamorphic rocks are usually moderate (eng. compact, massive). An absolute specification of the density is not possible when addressing the rock and should therefore not be specified.

Grain size

A distinction is made between the grain size fine-grained (<1 mm; eng. Close-grained, fine [-grained], fine-granular), medium grain (1-5 mm; eng. Medium-grained, medium-granular) and coarse-grained (> 5 mm; eng. Coarse [-grained], coarsely granular, coarse textured):

descriptionGrain size in mm
Fine-grained < 1
Medium grain 1-5
Coarse-grained > 5

structure

structure

Size ratio of the minerals

What is the size ratio of the minerals to each other?

  • homeoblastic newly formed minerals of the same size
  • heteroblastic newly formed minerals of unequal size
  • porphyroblastic large blasts (porphyroblasts) in a fine-grained matrix

Special metamorphic structure

There are also a number of other names for the structure of the rock:

  • granoblastic Minerals are hardly regulated and largely homeoblastic
  • nematoblastic Occurrence of stalky, elongated minerals
  • lepidoblastic leaflet or scaly formation
  • fibroblastic pronounced fine-grained minerals
  • poikiloblastic Crystals that enclose other minerals
  • coronitic Minerals that are enveloped by other minerals

texture

Distribution of minerals

  • homogeneous - There is an even distribution of the minerals present and no areas can be identified in which individual minerals are accumulated.
  • inhomogeneous - There is an uneven distribution of the minerals present and there are areas or locations in which certain minerals dominate.


In addition, a distinction can be made between isotropic (directionless) and anisotropic (adjustment in a certain direction) for the texture.

In addition, the handpiece can have a two-dimensional texture, i.e. a foliated structure, or a linear texture, i.e. a fold, or an eye structure, flaser structure).

Mineral inventory

Monomineral rocks (e.g. quartzite) composed of (almost) only one mineral, polymineral Rocks (e.g. granite) are far more common and are made up of different minerals.

The Main part of the mixture (Eng.Essential constituent, main constituent, major constituent) of a rock make up its largest part, Minor fractions (Eng. accessory constituent, medium constituen) are accordingly less common, but clearly recognizable. The main and secondary portions are given in percent. Accessories (accessory) occur only sporadically in the rock (up to one percent), but can be seen with the naked eye.

Minerals

This is followed by mineral descriptions of the individual components equivalent to the igneous rocks. Do not forget the following features:

training

i.e. how easy are the crystal faces to be seen, can one determine the shape of the crystal?

colour

What color is the mineral? As with the color impression of the rock, a first impression of the mineral can be obtained.

shine

The shine (eng. Brightness; (diamond-like) adamantine luster; (glass-like) subvitreous luster; (metal-like) submetallic luster; (metallic) metallic luster; (mother-of-pearl) pearly luster, pearly sheen) is reflected by the mineral Light determines and can vary depending on the type of mineral:

           Metallic luster: the proportion of reflected light is large; this mainly applies to non-transparent, opaque minerals (e.g. pyrite, hematite)

           Non-metallic luster: these are transparent / translucent minerals, e.g. glass-like (epidote), silky (plaster of paris), mother-of-pearl (albite), greasy (quartz)

           Diamond luster: This refers to transparent and translucent minerals with high refraction, e.g. diamond, zinc blende, zircon

hardness

Another characteristic property of individual minerals is that Mossche hardness (Eng. Mohs ’hardness scale, Moh’s scale), which can be determined with the help of various tools (more on this under Determination of hardness according to Mohs)

Grain / crystal shape (habitus and costume)

The Costume (Eng. sum of crystal faces) of a mineral is determined from the arrangement of a certain combination of surfaces, the Habitus (Eng. habit [us]), on the other hand, describes the proportions of the surfaces to one another (leafy, needle-like, prismatic, columnar, stalky, tabular, cubic).

(Ferro) magnetism

Whether a mineral is magnetic can be easily determined with the help of a small hand magnet; the best-known mineral with this property is magnetite.

Percentage

Estimating the percentage of each mineral can later help you determine what rock it is.

Cleavage / breakage

The Cleavage (more about this under Cleavage, tight. plitting [property]; (clearly recognizable) distinct cleavage, easy cleavage) indicates whether a mineral breaks under mechanical action along certain crystal surfaces. The cleavage can be (very) good as well as (very) bad, or not at all. If individual crystals do not divide along such cleavage surfaces, a crystal breaks (failure pattern, fracture pattern); this can be designed in different ways (e.g. clean-cut fracture), hooked, scalloped (narrow. conchoidal), splintery (hackly fracture).

Line color

If the mineral to be determined can be tested individually (sufficient size, increased relief compared to the rest of the rock), the Line color (Eng. streak color) can be determined with the help of an unglazed porcelain plate. This partly characteristic color can help to determine the mineral.

transparency

Minerals can be defined as transparent / translucent, opaque or opaque (metals).

Twinning

Some minerals are also easy to identify based on their twinning. Well-known examples are, for example, the Carlsbad twin of the alkali feldspar, or the polysynthetic twins of the plagioclase.

Danger:
Bear in mind that some of the rock has been severely deformed metamorphically and an exact description of the minerals will not always be possible here.

Classification and rock name

After the parent rock (protolith)

e.g .: meta-gabbro, meta-conglomerate

This means that the prefix meta is attached to the parent rock (in this case gabbro and conglomerate).

According to metamorphic mineral composition and structure

e.g .: garnet-sillimanite-slate, cordierite-muscovite-gneiss, garnet-pyroxene-rock

Here you can recognize the classification by the designation "slate", "gneiss" or "rock".

Special names (especially after the structure)

Mylonite, cataclasite, kakirite, pseudotachylite

These are special forms of metamorphosis.

Special names (especially according to mineral inventory)

Greenschist, amphibolite, blueschist, eclogite

These are special forms of metamorphosis.

particularities

Finally, special features of the respective rock are described, such as the degree of weathering or the presence of fissures.

Overprinting

The type of overprinting can be physical, chemical, biogenic, etc. (e.g. weathered, altered, compacted, broken). Various degrees of strength can be specified for the strength of the over-embossing, particularly noticeable are changes in weathering (e.g. weathered, completely weathered, ghastly weathered). This feature can also be differentiated with regard to the individual components.

Degree of metamorphosis

Can the degree of metamorphosis be determined using facies minerals and can it be determined in the metamorphic facies types (P-T diagram)?

weathering

Weathering usually occurs secondary and therefore only plays a subordinate role in the description of the rock. Externally, the color usually changes (e.g. reddish-brown coating due to oxidized iron) and the space filling or the density (i.e. the rock is less solid).

Others

Other abnormalities can also be viewed in relation to the rock formation. Fissures, the moisture / water content (e.g. mountain damp, dry (dried out), wet), or a recognizable parent rock (e.g. clay from marl, diaphthorite from mica slate, greisen from granite) are noticeable here.


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Categories

rocks
Terrain methods
Working in the field
Evaluation of terrain data
rocks
Sedimentary rocks
Metamorphic rocks
Igneous rocks

pages

Rock description metamorphic rocks - example
Rock description
Sedimentite rocks
Rock formation magmatites


credentials

Schweizer, V. (2012): Dictionary of Geology - Dictionary of Geology.– 1st edition, 673 p .; Springer Verlag Berlin Heidelberg (Springer Spectrum).

Vinx, R. (2015): Rock determination in the terrain. - 4th edition, 480 p .; Springer Verlag Berlin Heidelberg (Springer Spectrum).

Lecture notes for the Geosciences II exercise (rock course)


Further information and literature


Author: inside

This article was created by:
Friederike Knauss, Julia Holzmüller, Donjá Aßbichler, Lukas Sidorenko, Lena Able