Editing Methane clathrate (section)

==Structure and composition==
[[File:Gas_Hydrate_Crystals.jpg|thumb|microscope image]]
The nominal methane clathrate hydrate composition is (CH<sub>4</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>23</sub>, or 1 [[mole (unit)|mole]] of methane for every 5.75 moles of water, corresponding to 13.4% methane by mass, although the actual composition is dependent on how many methane molecules fit into the various cage structures of the water [[Crystal structure|lattice]]. The observed density is around 0.9 g/cm<sup>3</sup>, which means that methane hydrate will float to the surface of the sea or of a lake unless it is bound in place by being formed in or anchored to sediment.<ref name=Max>{{Cite book |last=Max |first=Michael D. |title=Natural Gas Hydrate in Oceanic and Permafrost Environments |publisher=Kluwer Academic Publishers |year=2003 |page=62 |url=https://books.google.com/books?id=fd8QFKwcSskC |isbn=978-0-7923-6606-5}}</ref> One litre of fully saturated methane clathrate solid would therefore contain about 120 grams of methane (or around 169 litres of methane gas at 0&nbsp;°C and 1 atm),{{#tag:ref|The average methane clathrate hydrate composition is 1 [[mole (unit)|mole]] of methane for every 5.75 moles of water. The observed density is around 0.9 g/cm<sup>3</sup>.<ref name=Max/> For one mole of methane, which has a molar mass of about 16.043&nbsp;g (see [[Methane]]), we have 5.75 moles of water, with a molar mass of about 18.015&nbsp;g (see [[Properties of water]]), so together for each mole of methane the clathrate complex has a mass of {{nowrap|16.043 g + 5.75 × 18.015 g}} ≈ 119.631&nbsp;g. The fractional contribution of methane to the mass is then equal to {{nowrap|16.043 g / 119.631 g}} ≈ 0.1341. The density is around 0.9&nbsp;g/cm<sup>3</sup>, so one litre of methane clathrate has a mass of around 0.9&nbsp;kg, and the mass of the methane contained therein is then about {{nowrap|0.1341 × 0.9 kg}} ≈ 0.1207&nbsp;kg. At a density as a gas of 0.716&nbsp;kg/m<sup>3</sup> (at 0&nbsp;°C; see the info box at [[Methane]]), this comes to a volume of {{nowrap|0.1207 / 0.716 m<sup>3</sup>}} = 0.1686&nbsp;m<sup>3</sup> = 168.6&nbsp;L.|group="nb"}} or one cubic metre of methane clathrate releases about 160 cubic metres of gas.<ref name=BBCMay17 />

Methane forms a "structure-I" hydrate with two [[dodecahedron|dodecahedral]] (12 vertices, thus 12 water molecules) and six [[tetradecahedron|tetradecahedral]] (14 water molecules) water cages per unit cell. (Because of sharing of water molecules between cages, there are only 46 water molecules per unit cell.) This compares with a [[hydration number]] of 20 for methane in aqueous solution.<ref name="Dec 2005">{{Cite journal |title=Direct Measure of the Hydration Number of Aqueous Methane |journal=[[Journal of the American Chemical Society|J. Am. Chem. Soc.]] |year=2006 |volume=128 |issue=2 |pages=414–415 |doi=10.1021/ja055283f |pmid=16402820 |last1=Dec |first1=Steven F. |last2=Bowler |first2=Kristin E. |last3=Stadterman |first3=Laura L. |last4=Koh |first4=Carolyn A. |last5=Sloan |first5=E. Dendy}} Note: the number 20 is called a [[magic number (chemistry)|magic number]] equal to the number found for the amount of water molecules surrounding a [[hydronium ion]].</ref> A methane clathrate [[Magic angle spinning|MAS NMR]] spectrum recorded at 275 [[kelvin|K]] and 3.1 [[Pascal (unit)|MPa]] shows a peak for each cage type and a separate peak for [[gas phase]] methane.{{Citation needed|date=September 2010}} In 2003, a clay-methane hydrate intercalate was synthesized in which a methane hydrate complex was introduced at the interlayer of a sodium-rich [[montmorillonite]] clay. The upper temperature stability of this phase is similar to that of structure-I hydrate.<ref name="Guggenheim 2003">{{Cite journal |last=Guggenheim |first=S |author2=Koster van Groos AF |year=2003 |title=New gas-hydrate phase: Synthesis and stability of clay-methane hydrate intercalate |journal=[[Geology (journal)|Geology]] |volume=31 |issue=7 |pages=653–656 |doi=10.1130/0091-7613(2003)031<0653:NGPSAS>2.0.CO;2 |bibcode=2003Geo....31..653G}}</ref>
[[File:Methane Hydrate phase diagram.jpg|thumb|Methane hydrate phase diagram. The horizontal axis shows temperature from -15 to 33 Celsius, the vertical axis shows pressure from 0 to 120,000 kilopascals (0 to 1,184 atmospheres). Hydrate forms above the line. For example, at 4 Celsius hydrate forms above a pressure of about 50 atm/5000 kPa, found at about 500m sea depth.]]