Chemin
As you wrote...
"What I should have said was "ash" of course. Silly me. I was going to mention, but didn't, soot from the Kuwait oilfields that were ignited during the gulf war, which along with blowing sand, caused huge amounts of "clag" in the area, but did not stop operations.
However, if you think we are not dealing with "grit" and "silicon" what do you suggest it is that has been deposited on my car this morning?"
The danger is of the silicates, not silicon, melting and solidifying as they pass through a jet engine. The point is that if you are at the level where you think the precise composition of the stuff is unimportant, and that the effects of volcanic silicates are similar to those of desert sands or silicon and those of volcanic ash are similar to those of hydrocarbon ash, that suggests to me that you're not really engaging meaningfully with the problem.
My experience is that the melting behaviour of silicates is hugely variable, and that it would be essential to know what exactly was up there before trying to predict its effects on an engine.