Serpentinite is ametamorphic rock composed predominantly ofserpentine group minerals formed byserpentinization ofmafic orultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color to snake skin.[1] Greek pharmacologistDioscorides (AD 50) recommended this rock to prevent snakebite.[2]
Serpentinite has been calledserpentine orserpentine rock, particularly in older geological texts and in wider cultural settings.[3][4][5][6][7]
Most of the chemical reactions necessary to synthesizeacetyl-CoA, essential to basic biochemical pathways of life, take place during serpentinization. Serpentinite thermal vents are therefore considered a candidate for the origin of life on Earth.
Serpentinite is formed by near to completeserpentinization ofmafic orultramafic rocks.[8] Serpentinite is formed from mafic rock that ishydrated bycarbon dioxide-deficientsea water that is pressed into the rock at great depths below the ocean floor.[9] This occurs atmid-ocean ridges and in theforearc mantle ofsubduction zones.[10][11]
The final mineral composition of serpentinite is usually dominated byantigorite,lizardite,chrysotile (minerals of theserpentine subgroup), andmagnetite (Fe3O4), withbrucite (Mg(OH)2) less commonly present. Lizardite, chrysotile, and antigorite all have approximately the formulaMg3(Si2O5)(OH)4 or(Mg2+, Fe2+)3Si2O5(OH)4, but differ in minor components and in form.[10] Accessory minerals, present in small quantities, includeawaruite, other native metal minerals, andsulfide minerals.[12]
Theserpentinization reaction involving the transformation offayalite (Fe-end member ofolivine) by water intomagnetite andquartz also produces molecularhydrogenH2 according to the following reaction:
This reaction closely resembles theSchikorr reaction also producing hydrogen gas byoxidation of Fe2+ ions into Fe3+ ions by the protons H+ of water. Two H+ are then reduced intoH2.
In the Schikorr reaction, the two H+ reduced intoH2 are these from twoOH−anions, then transformed into two oxide anions (O2−) directly incorporated into the magnetitecrystal lattice while the water in excess is liberated as a reaction by-product.
Hydrogen produced by the serpentinization reaction is important because it can fuelmicrobial activity in the deep subsurface environment.[citation needed]
Deep seahydrothermal vents located on serpentinite close to the axis ofmid-ocean ridges generally resembleblack smokers located onbasalt, but emit complexhydrocarbon molecules. The Rainbow field of theMid-Atlantic Ridge is an example of such hydrothermal vents. Serpentinization alone cannot provide the heat supply for these vents, which must be driven mostly bymagmatism. However, theLost City Hydrothermal Field, located off the axis of the Mid-Atlantic Ridge, may be driven solely by heat of serpentinization. Its vents are unlike black smokers, emitting relatively cool fluids (40 to 75 °C (104 to 167 °F)) that are highlyalkaline, high inmagnesium, and low inhydrogen sulfide. The vents build up very large chimneys, up to 60 meters (200 ft) in height, composed ofcarbonate minerals and brucite. Lushmicrobial communities are associated with the vents. Though the vents themselves are not composed of serpentinite, they are hosted in serpentinite estimated to have formed at a temperature of about 200 °C (392 °F).[13]Sepiolite deposits on mid-ocean ridges may have formed through serpentinite-drivenhydrothermal activity.[14] However, geologists continue to debate whether serpentinization alone can account for theheat flux from the Lost City field.[13]
Theforearc of theMarianassubduction zone hosts large serpentinitemud volcanoes, which erupt serpentinite mud that rises throughfaults from the underlying serpentinized forearcmantle. Study of these mud volcanoes gives insights into subduction processes, and the highpH fluids emitted at the volcanoes support amicrobial community.[15][11]Experimental drilling into thegabbro layer ofoceanic crust near mid-ocean ridges has demonstrated the presence of a sparse population ofhydrocarbon-degradingbacteria. These may feed on hydrocarbons produced by serpentinization of the underlyingultramafic rock.[16][17]
Serpentinite thermal vents are a candidate for the environment in which life on Earth originated.[15] Most of the chemical reactions necessary to synthesizeacetyl-CoA, essential to basic biochemical pathways of life, take place during serpentinization.[18] The sulfide-metal clusters that activate manyenzymes resemble sulfide minerals formed during serpentinization.[19]
Soil cover over serpentinitebedrock tends to be thin or absent.Soil with serpentine is poor incalcium and other major plantnutrients, but rich in elements toxic to plants such aschromium andnickel.[20] Some species of plants, such asClarkia franciscana and certain species ofmanzanita, are adapted to living on serpentiniteoutcrops. However, because serpentinite outcrops are few and isolated, their plant communities areecological islands and these distinctive species are often highly endangered.[21] On the other hand, plant communities adapted to living on the serpentine outcrops ofNew Caledonia resist displacement byintroduced species that are poorly adapted to this environment.[22]
Serpentine soils are widely distributed on Earth, in part mirroring the distribution ofophiolites and other serpentine bearing rocks.[23] There are outcroppings of serpentine soils in theBalkan Peninsula,Turkey, the island ofCyprus, theAlps,Cuba, andNew Caledonia. In North America, serpentine soils also are present in small but widely distributed areas on the eastern slope of theAppalachian Mountains in the eastern United States, and in the Pacific Ranges of Oregon and California.[citation needed]
Notable occurrences of serpentinite are found atThetford Mines,Quebec; Lake Valhalla,New Jersey;Gila County, Arizona;Lizard complex,Lizard Point, Cornwall; and in localities in Greece, Italy, and other parts of Europe.[24] Notable ophiolites containing serpentinite include theSemail Ophiolite ofOman, theTroodos Ophiolite ofCyprus, theNewfoundland ophiolites, and the Main Ophiolite Belt ofNew Guinea.[25]Another occurrence of serpentinite is in Chester County, Pennsylvania.
Serpentine group minerals have aMohs hardness of 2.5 to 3.5, so serpentinite is easilycarved.[26] Grades of serpentinite higher incalcite, along with theverd antique (breccia form of serpentinite), have historically been used as decorative stones for their marble-like qualities.College Hall at theUniversity of Pennsylvania, for example, is constructed out of serpentine. Popular sources in Europe before contact with the Americas were the mountainousPiedmont region of Italy andLarissa, Greece.[27]Serpentinites are used in many ways in the arts and crafts. For example, the rock has been turned inZöblitz inSaxony for several hundred years.[28]
TheInuit and other indigenous people of theArctic areas (and less so of more southern areas) used a carved lamp made of serpentinite, called aqulliq or kudlik, to burn oil or fat to heat, to make light and to cook with. TheInuit also used serpentinite to make tools, and more recently carvings of animals for commerce.[29]
A variety ofchloritetalcschist associated with Alpine serpentinite is found inVal d'Anniviers,Switzerland and was used for making "ovenstones" (German:Ofenstein), a carved stone base beneath acast iron stove.[30]
Serpentinite has a significant amount ofbound water, so it contains abundanthydrogen atoms able to slow downneutrons byelastic collision (neutronthermalization process). Because of this, serpentinite can be used as dry filler insidesteel jackets in some designs ofnuclear reactors. For example, in theRBMK series, as atChernobyl, it was used for topradiation shielding to protect operators from escaping neutrons.[31] Serpentine can also be added asaggregate to specialconcrete used in nuclear reactor shielding to increase the concrete density (2.6 g/cm3 (0.094 lb/cu in)) and itsneutron capturecross section.[32][33]
Because it readily absorbscarbon dioxide, serpentinite may be of use forsequestering atmospheric carbon dioxide.[34] To speed up the reaction, serpentinite may be reacted with carbon dioxide at elevated temperature in carbonation reactors. Carbon dioxide may also be reacted withalkaline mine waste from serpentine deposits, or carbon dioxide may be injected directly into underground serpentinite formations.[35] Serpentinite may also be used as a source ofmagnesium in conjunction with electrolytic cells for CO2 scrubbing.[36]
It is the state rock ofCalifornia, USA and the California Legislature specified that serpentine was "the official State Rock and lithologic emblem."[4] In 2010, a bill was introduced which would have removed serpentine's special status as state rock due to it potentially containingchrysotileasbestos.[37] The bill met with resistance from some California geologists, who noted that the chrysotile present is not hazardous unless it is mobilized in the air asdust.[38][needs update]
The Term Serpentine
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