See also Firing talc for notes on magnesium-silicate ceramics processing, Glass foam for notes on using magnesium chloride to raise the softening point of waterglass foam, and Synthesizing reactive magnesia? on the oxide.
In a crude kitchen experiment, I mixed 136 g of a waterglass solution (35%?) with 136 g of tap water in a cut-off Monster can, then took 72 g of kinda gummy magnesium chloride from the sack where it had been deliquescing and melted it on the stove. (If this were the hexahydrate that would be 34 g of anhydrous.) It stubbornly refused to dissolve completely, so I added 48 g of tap water, which did the trick. Upon dumping this into the diluted waterglass, it immediately (<500 ms) formed a solid white slushy material that I couldn’t really stir with the chopstick, with only a little bit of milky liquid to be squeezed out of it. I was able to pour off 62.7 g of this syrupy liquid with a few chunks, so there’s about 330 g of solid “gel” in the can, of which only about 14% is waterglass and 22% is the hydrated magnesium chloride, so probably something like 10% is actual anhydrous magnesium chloride. So it’s 76% water, 24% solid matrix. It feels gritty, but it’s easily crumbled by hand. The stuff at the base of the can, where probably less magnesium penetrated, was even grittier.
Rather than any sort of glassy material, this seems more like a crystalline precipitate that’s got a lot of water locked up inside its crystalline structure (a fairly well-defined amount, not the kind of loosey-goosey anything-goes relationship you see with a gel) and some more water in the interstices between the numerous sand-sized crystals. The obvious candidate would be talc, but although the crystals are soft, they don’t feel as soft as talc. More gritty, like baking soda. I can’t identify the crystal habit, and I wonder if maybe the hard substance formed along a mixing boundary between the two solutions rather than in any sort of crystalline way. It doesn’t get less gritty when I rub it around on my hand for a while, and it doesn’t dissolve in (96%) ethanol. Heating it at 500° for half an hour leaves it whiter, and it’s still super crumbly, but tells me nothing else useful.
Apparently this is the recipe for synthetic magnesium silicate, which forms porous, amorphous masses.
Roughly guessing at the stoichiometry, 136 g of 35% waterglass would be 47 g of waterglass. If it’s 3:1 SiO₂:Na₂O by weight then that’s 35 g of silica, which is 60.08 g/mol, so 0.58 mol of silica (and of silicon). Anhydrous MgCl₂ is 95.211 g/mol so 34 g of it would be 0.36 mol of MgCl₂ (and of magnesium). So that’s about 1.6 silicons per magnesium (Mg/Si molar ratio 0.62). Forsterite has 2 magnesiums per silicon (0.5 silicons per magnesium), serpentines have 1.5 magnesiums (or irons) per silicon, enstatite (the eutectic) has 1 silicon per magnesium, and talc has 1⅓ silicons per magnesium. So on average this material is a little closer to silica gel than talc is. Synthetic magnesium silicate for food has 2.5 silicons per magnesium.
It gets a little crunchier when I wash it, maybe because a little residual sodium silicate was lubricating it.
Apparently at low enough Mg/Si ratios you get crystalline talc but only above 200°. The paper suggests that maybe I have some silica mixed in with my magnesium silicate because I didn’t add enough magnesium.
I stuck a chunk of the stuff in a bowl with a little sunflower oil in it, and it provided a pretty usable oil-lamp wick, so I guess it must be pretty porous. A deposit of porous black carbon built up on the top surface, and the flame is pretty smoky once it gets going, a process that often involves a certain amount of spluttering. There’s a bit of an acid smell to it, like a lit match or the exhaust from high-sulfur diesel. I tried sand and vermiculite as alternative wick materials, which worked much more poorly. The amount of smoke is high enough to discourage me from using it in practice, but cutting it to a better shape might solve that problem, and of course blowing the flame into a combustion chamber with extra air would solve it.
The microporous nature of the material suggests that it might be useful for filtering; a major commercial use of the stuff is adsorbing polar radicals from used frying oil.
One paper describing the synthesis of a magnesium silicate with a particular molar ratio added the silicate and the magnesium salt dropwise to a continuously stirred solution of ethanol thickened with PEG. I guess the idea was to keep the concentration of unreacted feedstocks low enough that they would produce a mineral of a consistent composition.