Sub-Antarctic Mode Water (SAMW) is an important water mass in Earth's oceans. It is formed near the Sub-Antarctic Front on the northern flank of the Antarctic Circumpolar Current . The surface density of Sub-Antarctic Mode Water ranges between about 1026.0 and 1027.0 kg/m, and the core of this water mass is often identified as a region of particularly low stratification.
21-397: Another important facet of SAMW is that silicate (an important nutrient for diatoms ) is depleted relative to nitrate . This depletion can be tracked over much of the globe, suggesting that SAMW helps set the blend of nutrients delivered to low-latitude ocean ecosystems and thus determines the balance of species within these ecosystems. SAMW is a very homogeneous layer that forms north of
42-407: A framework silicate, known as a tectosilicate , each tetrahedron shares all 4 oxygen atoms with its neighbours, forming a 3D structure. Quartz and feldspars are in this group. Although the tetrahedron is a common coordination geometry for silicon(IV) compounds, silicon may also occur with higher coordination numbers. For example, in the anion hexafluorosilicate SiF 6 , the silicon atom
63-401: A typical preparation, monomeric orthosilicate was found to react completely in 75 seconds; dimeric pyrosilicate in 10 minutes; and higher oligomers in considerably longer time. In particular, the reaction is not observed with suspensions of colloidal silica . The nature of soluble silicates is relevant to understanding biomineralization and the synthesis of aluminosilicates , such as
84-833: Is a hexamer of SiO 3 . Polymeric silicate anions of can exist also as long chains. In single-chain silicates, which are a type of inosilicate , tetrahedra link to form a chain by sharing two oxygen atoms each. A common mineral in this group is pyroxene . Double-chain silicates, the other category of inosilicates, occur when tetrahedra form a double chain (not always but mostly) by sharing two or three oxygen atoms each. Common minerals for this group are amphiboles . In this group, known as phyllosilicates , tetrahedra all share three oxygen atoms each and in turn link to form two-dimensional sheets. This structure does lead to minerals in this group having one strong cleavage plane. Micas fall into this group. Both muscovite and biotite have very weak layers that can be peeled off in sheets. In
105-411: Is a major cause of cathode poisoning in vacuum tubes . Although very important in inorganic chemistry and geochemistry, the orthosilicate ion is rarely seen in organic chemistry. Two silicate compounds, however, are used in organic synthesis : tetraethyl orthosilicate or TEOS is used to link polymers, and is especially important in the manufacture of aerogels. Tetramethyl orthosilicate or TMOS
126-535: Is sometimes extended to any anions containing silicon, even if they do not fit the general formula or contain other atoms besides oxygen; such as hexafluorosilicate [SiF 6 ] . Most commonly, silicates are encountered as silicate minerals . For diverse manufacturing, technological, and artistic needs, silicates are versatile materials, both natural (such as granite , gravel , and garnet ) and artificial (such as Portland cement , ceramics , glass , and waterglass ). In most silicates, silicon atom occupies
147-477: Is surrounded by six fluorine atoms in an octahedral arrangement. This structure is also seen in the hexahydroxysilicate anion Si(OH) 6 that occurs in thaumasite , a mineral found rarely in nature but sometimes observed among other calcium silicate hydrates artificially formed in cement and concrete structures submitted to a severe sulfate attack in argillaceous grounds containing oxidized pyrite . At very high pressure, such as exists in
168-740: Is the very low solubility of SiO 4 and its various protonated forms. Such equilibria are relevant to the processes occurring on geological time scales. Some plants excrete ligands that dissolve silicates, a step in biomineralization . Catechols can depolymerize SiO₂—a component of silicates with ionic structures like orthosilicate (SiO₄⁴⁻), metasilicate (SiO₂³⁻), and pyrosilicate (Si₂O₆⁷⁻)—by forming bis- and tris(catecholate)silicate dianions through coordination. This complexes can be further coated on various substrates for applications such as drug delivery systems, antibacterial and antifouling applications. Silicate anions in solution react with molybdate anions yielding yellow silicomolybdate complexes. In
189-403: The global warming caused by this greenhouse gas . Orthosilicate In chemistry, orthosilicate is the anion SiO 4 , or any of its salts and esters . It is one of the silicate anions. It is occasionally called the silicon tetroxide anion or group . Orthosilicate salts, like sodium orthosilicate , are stable, and occur widely in nature as silicate minerals , being
210-634: The Pacific Ocean. From west to east, the density increases from 1026.9 kg/m to 1027.1 kg/m, the temperature decreases from 8.5 °C to 5.5 °C, and the salinity decreases from 34.62 ppt to 34.25 ppt (psu) In the region where the Peru-Chile Undercurrent flows above the SAMW, the SAMW can be distinguished as having locally-characteristic low phosphorus, silicate and other nutrient concentrations in comparison. It moves by
231-686: The Southern oceans. Near the surface it picks up atmospheric oxygen and carbon dioxide and then sinks, or subducts near the Indian Ocean, contributing to the Indian subtropical gyre and cooling and contributing to the Antarctic Circumpolar Current (ACC). The Sub-Antarctic Mode Water acts as a carbon sink , absorbing atmospheric carbon dioxide and storing it in solution. In the event of global heating due to climate change,
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#1732772384054252-492: The Sub-Antarctic Front and is also referred to as a pycnostad. Its uniformity can be attributed to convective overturning that also serves to ventilate it, resulting in the high dissolved oxygen value of >6mL/L. It has slightly less dissolved oxygen than the surface water layer above it, but greater dissolved oxygen than the water masses below it. It has some variability in temperature, salinity and density in
273-473: The amount of carbon dioxide that the SAMW is able to absorb will lessen. Downes et al. (2009) found that through climate modeling, in the event of a doubling of atmospheric carbon dioxide concentration the Subantarctic Mode water will decrease in density and salinity. Silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen , usually with
294-485: The anion, each oxygen carries a unit negative charge. The Si–O bond is 162 pm long. In organic compounds like tetramethyl orthosilicate , each oxygen is formally neutral and is connected to the rest of the molecule by a single covalent bond . Europium doped barium orthosilicate (Ba 2 SiO 4 ) is a common phosphor used in green light-emitting diodes (LEDs). Phosphor for blue LEDs can be made with strontium doped barium orthosilicate. Barium orthosilicate
315-416: The center of an idealized tetrahedron whose corners are four oxygen atoms, connected to it by single covalent bonds according to the octet rule . The oxygen atoms, which bears some negative charge, link to other cations (M ). This Si-O-M-O-Si linkage is strong and rigid, which properties are manifested in the rock-like silicates. The silicates can be classified according to the length and crosslinking of
336-546: The defining feature of the nesosilicates . Olivine , a magnesium or iron(II) orthosilicate, is the most abundant mineral in the upper mantle . The orthosilicate anion is a strong base , the conjugate base of the extremely weak orthosilicic acid H 4 SiO 4 (p K a2 = 13.2 at 25 °C). This equilibrium is difficult to study since the acid tends to decompose into a hydrated silica condensate. The orthosilicate ion or group has tetrahedral shape, with one silicon atom surrounded by four oxygen atoms. In
357-468: The general formula [SiO 4− x ] n , where 0 ≤ x < 2 . The family includes orthosilicate SiO 4− 4 ( x = 0 ), metasilicate SiO 2− 3 ( x = 1 ), and pyrosilicate Si 2 O 6− 7 ( x = 0.5 , n = 2 ). The name is also used for any salt of such anions, such as sodium metasilicate ; or any ester containing the corresponding chemical group , such as tetramethyl orthosilicate . The name "silicate"
378-519: The industrially important catalysts called zeolites . Along with aluminate anions , soluble silicate anions also play a major role in the polymerization mechanism of geopolymers . Geopolymers are amorphous aluminosilicates whose production requires less energy than that of ordinary Portland cement . So, geopolymer cements could contribute to limiting the CO 2 emissions in the Earth atmosphere and
399-1059: The majority of the Earth's rock, even SiO 2 adopts the six-coordinated octahedral geometry in the mineral stishovite , a dense polymorph of silica found in the lower mantle of the Earth and also formed by shock during meteorite impacts. Silicates with alkali cations and small or chain-like anions, such as sodium ortho- and metasilicate , are fairly soluble in water. They form several solid hydrates when crystallized from solution. Soluble sodium silicates and mixtures thereof, known as waterglass are important industrial and household chemicals. Silicates of non-alkali cations, or with sheet and tridimensional polymeric anions, generally have negligible solubility in water at normal conditions. Silicates are generally inert chemically. Hence they are common minerals. Their resiliency also recommends their use as building materials. When treated with calcium oxides and water, silicate minerals form Portland cement . Equilibria involving hydrolysis of silicate minerals are difficult to study. The chief challenge
420-448: The silicate anions. Isolated orthosilicate anions have the formula SiO 4 . A common mineral in this group is olivine ( (Mg,Fe) 2 SiO 4 ). Two or more silicon atoms can share oxygen atoms in various ways, to form more complex anions, such as pyrosilicate Si 2 O 7 . With two shared oxides bound to each silicon, cyclic or polymeric structures can result. The cyclic metasilicate ring Si 6 O 18
441-470: The transference of heat energy via the Subtropical anticyclonic gyre and retains its individuality as differentiated with the less-salty Antarctic Intermediate Water below it and the more highly oxygenated surface water above it. The oxygen maximum portion of SAMW sinks at 28˚S to 700m and lifts back to 500m around 15˚S after oxygen levels decreased. SAMW acts as an oxygenator for mid oceanic depths in
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