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Water in the Universe | Part 2

< Water in the Universe Intro | Part 1

How is water locked up in rock and minerals?

Any water vapor in the early Solar System would have been blown out of the inner Solar System by the solar wind.  It is only the water that was bound in certain minerals that would have been stable, incorporated within the rocky material that became the terrestrial planets.  One simple example of a hydrated (it has water) mineral is gypsum, CaSO4•2H2O.  Under the right conditions, calcium (Ca++) and sulphate (SO4=) dissolved in water can precipitate as the mineral gypsum.  The formula clearly shows that water is part of the mineral structure even though gypsum appears to be completely dry.  If the depositional environment happens to be exceptionally hot as in near the shore of the Red Sea or you heat the gypsum, the water can be driven out, converting the gypsum to anhydrite (without water), CaSO4.  If a sedimentary rock contains anhydrite instead of gypsum, it indicates that the climate in which the calcium and sulphate precipitated must have been quite hot.  

A similar example can be seen when dissolved iron oxidizes and precipitates.  Iron can be in several oxidation states.  Uncombined with anything else, it will have an oxidation state of zero (same number of protons and electrons).  In reducing environments (very few electron acceptors; low oxygen) the iron will form compounds in which the iron has an oxidation state of +2, Fe++, otherwise known as ferrous iron.  A good example of that is pyrite, FeS2.  Since the charges in a compound must balance, each sulfur has an oxidation state of –1 whose sum balances the positive charge of the Fe++.  Pyrite is associated with coal deposits and in a flooded underground coal with limited oxygen available in the water, will oxidize to produce considerable dissolved ferrous iron:  2 FeS2 + 7 O2 + 2 H2O ––>  2 Fe++ + 4 SO4= + 4 H+.  The mine water often becomes saturated with dissolved ferrous iron.  

Should that mine water come to the surface in a mine outfall where it is exposed to the air, the ferrous iron will quickly oxidize to ferric iron, Fe+++.  As it happens, ferric iron is much less soluble in water and so it immediately combines with water to produce a reddish-yellowish-orange, gelatinous ‘solid’ known as ferric hydroxide, Fe(OH)3 (locally known as yellow boy) which can sometimes be found staining stream beds for miles.  The same thing can happen if your well water happens to contain too much dissolved ferrous iron, leading to colorful stains on your sinks, toilets, and laundry. 

The chemical formula for this compound doesn’t show water as such, it is hidden in the hydroxide (–OH) part.  Although the formula is Fe(OH)3, it could be rewritten as Fe2O3•3H2O which does show the water (like in the gypsum) although no chemist would write the formula that way.  Ferric hydroxide is not a mineral because it is unstable; it is held together by weak hydrogen bonds as shown below.

Outlined in blue are what could become separate water molecules.  If one of those blue-outlined waters is removed, there would be an oxygen bridge left between the two irons (Fe), creating a strong link between them.  The result would be a mineral generally known as limonite, Fe2O3•2H2O.  Should all of the water be removed, you would have hematite, Fe2O3.  

Yellowish-brown limonite is stable in temperate climates whereas warmer climates will dewater the compound to the reddish hematite, making them great indicators of past and present climate.  Soils in Pennsylvania are often brownish because of limonite in contrast to the red soils of warmer Georgia.  There are red rocks in Pennsylvania, such as the red shale of the Mauch Chunk Formation, but it is red because at the time the clay was deposited hundreds of millions of years ago, what is now Pennsylvania was very close to the equator and had a very hot climate.  Now exposed to the current cool climate, over decades of weathering, the red hematite will fade into the brownish limonite.  

In a hot climate (Atlanta and the Mauch Chunk Shale at the time of its formation) iron oxidized to red hematite (Fe2O3).  In a cool climate (current weathering of the Pocono Sandstone and Pittston) iron oxidized to yellow-brown limonite (Fe2O3•2H2O).  Note the weathering rim on the Pocono sandstone; the un-weathered interior of the sandstone remains its original gray color; given more time and weathering, the red will fade.  

The first two examples of hydrated minerals, the gypsum and the hydrated iron oxides, are found at or near the surface of the Earth and they can be dewatered at relatively low temperatures.  More complicated minerals hold more tightly onto their hydroxides and require much higher temperatures to ‘dewater.’  One example is a mineral group known as the amphiboles, one member of which is hornblende, NaCa2(Mg,Fe,Al)5(Si,Al)8O2(OH)2, a common mineral in continental crustal rocks like granite.  

The formula is certainly complicated, in part because it allows for considerable variation in the composition.  That first bit in parentheses, for example, just means that it could be magnesium or iron or aluminum in that part of the formula.  The important part for this discussion is the OH at the end.  Under the right conditions (very hot) the OH from one part of the hornblende could combine with the OH from another part of the hornblende to free a water molecule, leaving behind an oxygen bridge.  

The minerals found in oceanic crustal rocks such as basalt crystallize out at higher temperatures, temperatures too high to include an OH group, forming an analogue to the amphiboles known as the pyroxenes, essentially amphiboles without the OH.  Oceanic crustal rocks and upper mantle rocks are rich in pyroxenes.  

Yet other minerals hold even more tightly onto OH groups, minerals such as bridgmanite and ringwoodite which include small amounts of OH.  So tightly do these minerals hang onto their OH groups that it is only the temperature and pressure of the transition from the upper to the lower mantle that are enough to separate out the water.  There are no minerals with OH groups in the lower mantle and it is, indeed, their absence, that largely differentiates the less viscous upper mantle from the more viscous lower mantle.  The water equivalent of the OH groups in the minerals of the upper mantle is so large that it is many times the volume of all of the water on or near the surface of the Earth.  

All of this mantle ‘water’ is also very important for plate tectonics because it substantially lowers the melting point of the mantle rock and makes it less viscous; it makes subduction possible.  Unlike Earth, the mantle of Venus is bone dry and this is one of several reasons Venus shows little evidence of plate tectonics.  

Hydrothermal Solutions

Deep within the Earth, the water does not always remain locked up in the rock.  Should conditions become such that the rock becomes molten, it will rise.  That molten rock (magma – if it reaches the surface of the Earth it’s called lava) contains a small amount of water dissolved in the magma.  It may happen that the magma, cooling as it rises, doesn’t make it to the surface, instead crystallizing out various minerals that become intrusive (plutonic) igneous rocks such as granite.  But the story doesn’t end there.  That small amount of water in the magma remains fluid as the silicate minerals crystallize out.  This very hot water under considerable pressure is the hydrothermal (literally hot water) solution that is left after the magma solidifies.  

Because the hydrothermal solution is so hot and under so much pressure, it can contain in solution much greater amounts of dissolved minerals than could the much colder water under much less pressure found at or near the surface of the Earth.  One example is silica, SiO2.  

Quartz is a large, clear or milky, crystallized version of silica.  Most of glass is a noncrystalline ‘solid’ form of silica.  You might not think that something like quartz or glass is soluble in water but a tiny amount of dissolved silica can be found even in tap water.  The tap water of Wilkes-Barre, Pennsylvania typically contains about 20 ppm dissolved silica which is not unusual.  Hydrothermal solutions contain many orders of magnitude more dissolved silica.  

As the hydrothermal solution rises to the surface, it cools and loses pressure which reduces its capacity to hold in solution things like silica.  Unlike magma which can melt its way through overlying rock, hydrothermal solutions rise through fractures in the rock.  At some point, the hydrothermal solution may become saturated with silica at which time some silica starts to crystallize out to form quartz veins in the rock, a very common occurrence.

Should the hydrothermal solution also contain some dissolved gold, gold may be included in the quartz veins (the mother lode).  Many other economic mineral deposits can be attributed to hydrothermal deposits such as the native copper in the Keweenaw Peninsula of Michigan, the silver of Potosi, Bolivia, and the tin of Cornwall, England.  Should a hydrothermal solution reach the surface at the bottom of an ocean as often happens at oceanic ridges and rises, various minerals may precipitate out and form manganese nodules which can be found littering some ocean floor areas.  These nodules are rich in many important metals beyond manganese including: iron, cobalt, copper, nickel, and zinc.  

Although it is generally true that the warmer the water the more dissolved something it can hold, there are a few exceptions.  In a reversal and on a much humbler scale, the solubility of calcium in water (Ca++) decreases as the temperature rises, sometimes leading to scale deposits in hot water heaters and pipes.  

The importance of hydrothermal solutions is that in their interaction with rock, they can create economically important concentrations of many metals and even valuable trace elements.  But hydrothermal solutions and the undersea vents from which they emerge may also be important to life.  

Deep within an ocean, hydrothermal vents can provide the energy and nutrients to sustain an ecosystem (tube worms et al) independent of the energy of the sun.  An ever-expanding list of outer Solar System bodies appear to have subsurface oceans which may include systems of hydrothermal vents.  If there is life beyond the Earth, it may be found in the subsurface oceans of bodies like Europa, a major moon of Jupiter, and Enceladus, a medium-sized moon of Saturn.  Astonishingly, even Ceres (the biggest asteroid) and Pluto may have subsurface oceans.  

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