Serpentinite

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Banded serpentinite handpiece from the northern Carpathians, ground on one side .
abandoned serpentinite quarry Ciampono in Val di Gressoney , former mining work with a wire saw

Serpentinites are metamorphic rocks that are formed from the transformation of ultramafic rocks (mainly peridotites ) through interaction with aqueous fluids and under increased pressure and temperature in the lithospheric mantle or shallow crustal valley. These prerequisites have recently been met especially on the mid-ocean ridges during ocean floor metamorphosis , and the deposits on land today are therefore mostly interpreted as the former sea floor ( ophiolite ). The rock is named after its main hydrous mineral components, the serpentine minerals , including chrysotile , clinochrysotile , orthochrysotile , parachrysotile , lizardite or antigorite .

terminology

In everyday language, the term serpentine is often used for serpentinite . However, this also stands for any mineral of the serpentine group . Although the two terms are closely related, they do not mean the same thing. Therefore, when referring to the rock, one should always speak of serpentinite .

Petrology

Beginning serpentinization in a vulcanite in a thin section with crossed polarizers : three neighboring olivine crystals (colored interference colors) are crossed by veins of chrysotile (gray). A pyroxene crystal (yellow) can be seen in the lower part of the picture, which partially encloses the olivine and appears completely unaffected.

Serpentine minerals arise from the transformation of olivine , pyroxenes and amphiboles in the peridotitic parent rocks under certain pressure and temperature conditions (300 to 500 ° C) and with the participation of aqueous fluids. This process is known as serpentinization. One tectonic scenario in which serpentinization occurs are oceanic spreading zones (see also →  ocean floor metamorphosis ).

various texture images of serpentinites (left ophiolithic, right brecciated)

education

The serpentinization begins within microscopic cracks in the olivine grains of the parent rock and thin serpentine membranes from chrysotile fibers form. These fibrous crystals grow further into the surrounding grain structure . The network of crystal fibers that is formed in this way creates cavities that are filled again with young (smaller) chrysotile fibers and lizardite that is formed. If higher temperatures occur, antigorite is also formed. Fine-grained magnetite is produced in parallel with these processes. In the further sequence, the orthopyroxene is converted after the olivine, which begins after a similar process with the initial formation of veins in the crystal aggregates. The minerals clinopyroxene, anthophyllite and cummingtonite are less affected by the transformation processes and suffer them at most at a later point in time during rock formation. This complex process is referred to as serpentinization and leads to differentiated silicate paragenesis . Through further processes ( metasomatosis ) new minerals ( e.g. carbonates ) or corresponding accompanying rocks can arise (e.g. ophicalcite due to CO 2 metasomatosis, up to pure magnesite and dolomite rocks).

During serpentinization, large amounts of water (mainly in the serpentine minerals) are bound in the rock, and this process is assumed to play an important role in the earth's water cycle .

structure

The structure of serpentinites can be very different depending on the previous rock-forming and tectonic-metamorphic processes. The structural images are very differentiated from deposit to deposit and are causally linked to their complex forms of formation. As the name of this rock group from the Latin derivation refers to the snake ( serpens ), wavy-banded textures are referred to as an ophiolithic structure (Greek ophítēs , snake-like). Because of their sometimes striking texture, serpentinites were also known as snake stones in the past. Serpentinite masses subject to severe tectonic stress often show a breccia structure .

Often there are two images:

  • an ophiolithic structure that shows serpentine coiled bands or strips and encircled, knot-like inclusions (popularly also called serpentine band );
  • the texture of a tectonic breccia with cementation from serpentine minerals and / or calcite and the like. a. Minerals (partly transition facies to ophicalcite ). It is typical in the millimeter to decimeter range.

Play of colors

The colors of serpentinite rocks can be very different. They are generally known as strong green materials in various shades. Some of them are burgundy to reddish brown and even dark brown. There are also black, black-green and gradations up to light green varieties. The play of colors is particularly large in the Zöblitz serpentine (Zöblitz, in the Saxon Ore Mountains). In Ligurian and Turkish varieties it can happen that the color changes from burgundy red to green within individual fragments of the breccia.

The brecciated texture can be visually enhanced if the spaces between the rock rubble are not filled with serpentinite masses of similar color, but with calcite or other light-colored minerals (chlorite, magnesite, chrysotile, etc.).

Accompanying rocks

Accompanying rocks, caused by the very complex transformations in the formation of serpentinites and subsequent mixing with contact rocks, occur:

  • Chlorite shale
  • Talc rocks
  • Talc actinolite rocks
  • Amphibole rocks

Mineral composition

Serpentinite thin sections under the
polarizing microscope
Uncrossed polarizers: the mesh structure is only hinted at, strings of magnetite-xx (black) can be seen
Crossed polarizers: the mesh structure of chrysotile dominates the image
Crossed polarizers: Leafy antigorite in a serpentinite from the Variscan crystalline from Erbendorf / Upper Palatinate. On the right still chlorite (colorful interference colors) and magnetite (black).

In addition to the main minerals mentioned, magnetite or hematite are often found in considerable proportions in serpentinites . In dark serpentinites, the magnetite content can lead to a magnet in the immediate vicinity of the rock noticeably responding. If other minerals than those listed above and typical of the rock occur, the rocks are z. B. referred to as garnet serpentinite or bronzite serpentinite. In the case of serpentinites containing chrysotile, there is an acute risk of asbestos during processing .

A specific phenomenon of serpentinites associated with metamorphosis is the appearance of minerals in fissures. These include talc, actinolite , nephrite , amianth, andradite and various carbonates. Some vein-shaped formations of these fissure minerals represent physical-mechanical weak points in the rock. This phenomenon is of considerable importance for rock-mechanical / engineering-geological considerations and technical applications ( natural stone ).

An exhaustive statement about the complex mineral composition of all serpentinite rocks cannot be given. The diverse sub-processes in their formation, subsequent transformations and reactions with contact rocks create an almost unmanageable variety of the respective mineral associations. For this reason and the alternating structural features, serpentinites are differentiated according to type. The dense green serpentinite rocks from the border area of ​​Italy, France and Switzerland are classified by some authors as alpinotype serpentinites.

The serpentine minerals can be seen under the microscope. differentiate: While antigorite has a leafy structure, chrysotile forms fibers that are mostly perpendicular to former cracks and crevices in the rock from which the serpentinization reactions originated. The result is a very typical mesh structure.

Occurrence of serpentinite rocks

Basically, serpentinites occur on the earth's surface in areas that were previously subject to significant tectonic impacts with medium to high-grade degrees of metamorphism and were raised from greater depths. For this reason, they can only be found in a relatively small area and, compared to sedimentary rocks, they are only of limited size. Typical sectors are old subduction zones along the edges of the continental plate as well as fracture zones and fold mountains . They are also part of the oceanic crust in the mid-ocean ridges and plate margins.

Some selected and known occurrences are named in the following list.

Europe

Africa

  • South Africa, as part of the Barberton Greenstone Belt
  • Zimbabwe, crevice-like extensions of Greenstone Belt structures
  • Ethiopia, along Precambrian formations

America and the Caribbean

  • Cuba, along the Atlantic coast
  • California, USA, Coast Ranges , etc. a. in the Bay Area (as parts of ophiolites, i.e. with igneous origin, but partly probably also with sedimentary origin as deposits of so-called serpentinite mud volcanoes in the Forearc basin)

Asia

  • Russia, flank areas of the Urals, West Sayan, Tuva
  • India, in the Rajasthan region
  • Turkey, Anatolia, represented in the Alpidic unfolding
  • Georgia, in the Caucasus (small outcrops)
  • Taiwan

Economic use

Baroque fountain in the Stockalper Palace in Brig , serpentinite of the southern Alpine zone

The types of natural stone represented in international trade are not adequately addressed under the entry "serpentinite" alone. In petrographic sense, it is also about serpentinite breccias and Ophicalcite .

The natural stones combined under the trade name “Verde Alpi” below have characteristics of both stone groups. In the Aosta Valley near Châtillon, the varieties Verde Issoire (Cret Blanc quarry) and Verde San Denis (Blavesse quarry) are extracted. A few kilometers to the south is the mining site of the Verde Issogne variety (Issogne Fleurant quarry). All three show properties that correspond to the ophicalcite type but also to a serpentinite breccia. Not far from Châtillon, above the village of Verrayes , there is a very large quarry on the Aver massif (Becca d'Aver), in which a considerable amount of serpentine breccia is extracted from underground and above ground (as of 2007). This carries the trade name Verde Aver . To the east of Verrayes, another company extracts serpentinite breccia in the Raffort quarry under the trade names Verde Chiesa and Verde Antico . Other serpentinite stones come from the neighboring Val di Gressoney .

In Germany, serpentine stone types from the Aosta Valley are mostly traded under the general name Verde Alpi and are only rarely differentiated more finely. Some variety names are protected, others are not.

The naming of types in the international natural stone trade does not always follow obvious connections at first glance. The variety Verde Guatemala available today comes from India and is also traded under its regional name (see below). The name probably refers to a previously used occurrence in Guatemala with a similar texture.

Natural stone names (selection)

Hohenstein serpentine

Common natural stone names for serpentinite stones are:

Web links

Commons : Serpentinite  - collection of images, videos and audio files

literature

Individual evidence

  1. ^ W. Wimmenauer: Petrography of igneous and metamorphic rocks. (see literature ), pp. 286-289.
  2. ↑ Roland Vinx: Rock determination in the field. Munich 2005, ISBN 3-8274-1513-6 , p. 78.
  3. ↑ Max W. Schmidt, Stefano Poli: Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation. Earth and Planetary Science Letters. Vol. 163, No. 1–4, 1998, pp. 361–379, doi : 10.1016 / S0012-821X (98) 00142-3 (alternative full text access : Michigan Technology University )
  4. ↑ Zheng-Xue Anser Li, Cin-Ty Aeolus Lee: Geochemical investigation of serpentinized oceanic lithospheric mantle in the Feather River Ophiolite, California: Implications for the recycling rate of water by subduction. Chemical Geology. Vol. 235, No. 1–2, 2006, pp. 161–185, doi : 10.1016 / j.chemgeo.2006.06.011
  5. ↑ WE Troger: Optical determination of the rock-forming minerals . 2nd Edition. tape 2 . Swiss beard, Stuttgart 1969, p. 610-622 .
  6. ↑ John Wakabayashi: Contrasting Settings of Serpentinite Bodies, San Francisco Bay Area, California: Derivation from the Subducting Plate vs. Mantle Hanging Wall? International Geology Review. Vol. 46, 2004, pp. 1103–1118, doi: 10.2747 / 0020-6814.46.12.1103 (alternative full text access : CSU Fresno ).