Elevator Pitch
It's a bit unclear what you're pitching here. As a reader I'm not sure if this is an actual lifeform or just a large structure/statue until the narrative section, so that would be a bit of formatting I would clean up and elaborate on. Thay being said, I think silicon-based lifeforms are an interesting concept, although I believe they ARE used in several other SCPs, such as SCP-553, as it's a semi-common trope in speculative biology sci-fi. However, because of this, it would need more anomalous properties than its silicate makeup to stand out, which brings me to the next part.
I don't really find the properties of the SCP to be very anomalous, as a lot of these effects are found in nature. Of course, not every property of an SCP needs to be anomalous, but does need to be clear to the reader which parts are and aren't anomalous.
1. The inability to melt certain silicate material isn't super uncommon. Fulgurites, for example, melt at 3000°F (for reference, the sun is 5000°F) Complete inability to be melted is of course anomalous, but is a super common trait in SCPs, so doesn't really make the snail stand out on it's own.
2. The gaseous "semi-fluid" substance. Gases are a fluid, so I'm not sure what this really means. If you meant liquid, I'm having a difficult time imagining what that means either. Melting down proteins isn't anomalous at all, in all honesty. The main reason life isn't able to survive at high temperatures is just that: high heat causes proteins and DNA to unravel and lose shape. It seems like the only real anomalous property of the gas is that it transforms human possessions into glass. However, I'm not really sure how this fits in. What is the through line from the snail to human possessions? Is it thematically relevant?
3. It's unclear what you mean by "critical" here. If you mean it in the chemistry sense, I'm going go assume you mean it sublimes, and that you don't intend that to be an anomalous trait, but a natural side effect.
Narrative
I actually really like the narrative idea here! I think the concept of an SCP being exploited for resources and subsequently killed is super interesting, and I think that keeping its "remains" anomalous is a clever way to get around having to classify it as neutralized. Although there are plenty of SCPs where it is outlined how they are/can be used to the Foundation's benefit, I don't think there are many that are treated as a potential resource to be fought over for the purpose of making something out of the material (as opposed to something like a weapon or a strong person).
Overall
I think you have a great idea for a story, but as far as anomalous properties go, I think the ones displayed here are either explainable, or so widely used that they don't stand out as interesting by themselves (giant size, kills humans specifically, etc). I think you should work more with the concept of it changing possessions to glass: find a thematic through line, and it might be interesting enough to the carry the SCP.