SiO2 IS 'sand' as in deserts AND (a major component of) the dust coming out of the volcano, as well as being the 'sand' in some types of sandpaper. Key differences are that the grit in sandpaper is 'new' and deliberately left with sharp edges. Volcanic dust is also newly formed. Desert sand particles have mostly been around long enough to have had their edges smoothed off by contact with other particles. Given that all sorts of different 'rock' get melted into the magma under a volcano, who knows what the particles will comprise, anyway. Suffice it that to some extent geography determines what will come out of each one. Doesn't really matter... they're sharp-edged and relatively low melting-point, and therefore bad for engines, airframes and anything else they hit at high speed.
BUT that doesn't address the key issue. The industry and Europe generally simply cannot afford to remain grounded indefinitely, until there is NO detectable ash still floating around, anywhere. At some point, someone (ICAO?) is going to have to develop some cojones and come up with comprehensive and time-critical RISK-BASED advice, not just a simple 100% ban that serves mostly to cover their own backsides.
The key questions are:
- how much ash per cubic metre of intake air, and for how long, will a given turbine accept without any significant risk of ANY type of failure, due to dust solidifying in the turbine hot section OR ANY OTHER REASON. There's must be a 'no fly' ash density that could be calculated from that. The answer seems likely NOT to be zero particles per cube.
- then the area of ash-cloud declared off-limits can be calculated, taking into account the source volcano and (especially) the time the various zones of the ash cloud have been airborne. (Other factors: humidity, temperature, probability of rising air masses, ... . Only physics and metereology!). The key answer is the sink-rates of particles of various sizes and the actual threat from each size.
Once a safety-floor has been set, then operators can get their own advice about probable exposure on specific routes, and calculate the ECONOMICS of flying them, taking into account increased maintenance costs.