# How much energy does it take to extract water from silicate?

Browsing Asterank, it seems that people believe many asteroids have large amounts of iron or magnesium silicate. These compounds may come in a hydrated form which contains water, but it is chemically fixed into the solids.

For example, the composition of the NEA named Anteros is listed as magnesium silicate, aluminum, iron silicate. Magnesium Silicate is $Mg_3Si_4O_{10}(OH)_2$.

Is it feasible to extract the hydrogen and oxygen for a return trip? Assume sufficient, reliable, automation is in place to perform this complicated task. With energy provided by solar panels, and enough time, could fuel be created? How much time and energy would be required to produce a unit (e.g. kg) of fuel?

• Is there a place where you saw mentioned something about such compounds combined with hydrogen and oxygen in asteroids? Is it somewhere specific on the Asterank site? – kim holder Jan 15 '16 at 19:05
• It is my understanding that magnesium silicate itself contains hydrogen and oxygen. – MattD Jan 15 '16 at 19:19
• I had to google magnesium silicate to look into it, and it seems the hydrated form is talc. You mean cost in terms of energy, i take it. – kim holder Jan 15 '16 at 19:41

Hydrated silicates will give up the trapped water by simply heating to 600C (873K).

A black body at 1 AU would have a temperature of 279K by the Stefan-Boltzmann law. So, heat sufficient to increase temperature by 594 degrees is required. Specific heat of magnesium silicate is 885 J/C/kg, so it takes 525,690 J of energy to bake the water out of 1kg of hydrated magnesium silicate.

Magnesium silicate is $XMgO:YSiO_2$ where X:Y is a molar ratio. Hydrated, it becomes $XMgO:YSiO_2 * H_2O$. Talc, a terrestrial mineral hydrated magnesium silicate is $Mg_3Si_4O_{10}(OH)_2$, or written as a synthetic silicate, $3MgO:4SiO_2*H_2O$.

The ratio of the atomic weight of water to hydrated magnesium silicate is 4.75%, so 1 kg of hydrated magnesium silicate (aka talc) would yield 47.5 grams of water.

A paper by A.S. Kivkin of Massachusetts Institute of Technology suggests that hydrated minerals are present in the mid-asteroid belt. In particular CI, CM, and CR types, but also in M and E-class asteroids.

• You might want to check at what pressure the dehydration reaction occurs. – My Other Head Jan 16 '16 at 6:32
• @MyOtherHead: Since it's a solid, pressure is a bit fuzzy measure anyway. The water molecules are trapped in a crystal structure. The main effect I'd look into would be the grain size, as the water has to migrate from the core of each grain to the surface. – MSalters Jan 18 '16 at 1:06
• These are good comments. I am not sure how to research them. I'm going to accept the answer, but if you can give me any pointers, I'll read up on it and update here. – MattD Jan 19 '16 at 16:20
• You only consider the energy needed to heat it up to 873 K, is not the enthalpy change of the reaction what really matters? – SE - stop firing the good guys Jan 23 '16 at 13:06
• Good point. I'm not even sure if this is endothermic or exothermic. Since it releases a vapor, I guess endothermic. Do you think that would dominate the heat required? I would think that because water is only 5% of the mass it would make little difference. Perhaps with a large amount of time and small granules, less temperature is required, and then enthalpy would dominate. – MattD Jan 23 '16 at 18:31