I'm currently writing a science fiction story set on a small ship inhabited by a motley collection of various races, one of these is a silicon based crystalline humanoid. Now in the bit that i'm stuck with he's in charge of repairing the ships engines after they got hit in a battle, various sections of the ship are on fire so the crew decide to vent the atmosphere and hope that the lack of oxygen extinguishes the fire. Now all of the crew have a life support canister on them for events like this, so I was wondering what kind of gasses would the critter have in this emergency canister?
Silicon behaves chemically extremely like Carbon, so in theory you could use equivalent gases - have them breathing in an air that's similar proportions of oxygen and nitrogen. However, for an exact equivalent, that would have them eating silicon based food and exhaling air containing silicon dioxide. Now we run into problems, because Silicon Dioxide is only a gas at temperatures above 2230 celsius (more than 4000 fahrenheit). This probably isn't practical to fit in the confines of your story. If they do eat silicon-based food and produce silicon dioxide as a waste product, then their respiratory waste might have to carry the silicon dioxide as a solution in a suitable liquid (water would do). So, they could breathe in oxygen, but somewhere from their body would need to excrete liquid respiratory waste. In order for that approach to be practical, they would also need to take in quite a lot of water to use for excreting the silicon dioxide solution. Or, they'd have to excrete powder. There are other problems - silicon is less chemically 'flexible' than carbon - it can't form many of the same shapes that carbon does easily, making it hard to create equivalents of many of the metabolic pathways used by carbon-based life. That said, they're still discovering crazy biochemistries in carbon-based life, so we may just be suffering from lack of imagination in this regard. I'm not much of a chemist, so these are the best guesses I can take off the top of my head.
A crystalline being is going to have some issues moving. Crystalline structure isn't very good for flexibility, but that's not a problem with silicon which creates many quite flexible compounds like silicone. The issue with silicon life is excretion. There are plenty of articles online that boil down to the one issue. Silicon dioxide (the hypothetical waste product if the active element in respiration is oxygen) is not easy to get out of the body as [MENTION=8918]iolair[/MENTION] points out quite well. It's not an impossible life form, but it's not likely to be a neat crystalline creature. You are more likely to get a rubbery, leaky, drippy creature.
Silicon tetrafluoride (SiF4) is a gas. They could breath an atmosphere high in fluorine. Since Fluorine is next to Oxygen on the periodic table, you could probably get away with passing it of as having comparable chemical properties to your non-chemist audience members.
Silicon-based life has so many issues, that the less you reveal about the technicalities of it, the better, probably, for you. As mentioned, oxygen is possible, but has the awkwardness attached that your character would be exhaling sand (or choking on it). SiF4 could be a way to go. It sounds exotic enough. Flourine is highly reactive, though, so it comes with all sorts of trouble attached, and I guess "normally" your character would be working in an air (i.e. oxygen) atmosphere? You could try silane (SiH4), with H2O and SiO2 as the combustion products. But again, your character will be exhaling sand in water. And you need to get Oxygen to the character as well (via food? via the skin?). Maybe you don't have to tell your readers? Just describe the canister as bright red, with large yellow letters "silicon life forms only" and it's acrid smell and bitter taste...
