Could it be that the pollen filter at some time has been left out and the casing filled with dust, replaced after wards and the sand is coming from the housing?
Have you actually had the filter out or just established that it is there?

Have you had the engine top cover and fuel injector cover off?
If it is fine sand due to some previous exposure you will see evidence of that in the numerous nooks and crannies that hide under the covers.
If there is no "sand" there then your mystery dust must be coming from elsewhere - slow process of elimination is the only way to eventually track the source.
Good luck
Jim No Longer the owner of a 2013MY Fuji White GS

"Fiat Lux" a philosophy not a car

Hi Taz, yes I replaced the pollen filter about 6 months ago and the deposits are slightly away from the filter location.
Westville, I will take the covers off (when it stops raining) and have a look, although I am not convinced it is coming from an external source.
I will have a bit more of a rummage as I cleaned the car yesterday and have some more deposits after driving for a couple of hours.

It's so hard to look down there and under with a torch and see what's happening??...
Maybe a little dismantling?
Or do we get a prize for the one who guesses right the source of the debris?

Dried up antifreeze ? from a slight leak ( Heater matrix ? ) Jaguar x-type sport gone
Jaguar x-type 2.2d sport one of my best cars ever sadly gone
Freelander 2 GS auto 2008 most unreliable ever ! gladly gone
Freelander 2 GS manual 2013 only time will tell !

I have sand in mine - from Thailand or Laos. Sorry no idea where yours is from. G4 Freelander 2005 V6 HSE Stage 1 Thailand/Laos. HQ Support YZT
Grey Freelander 2 2011 SD4 HSE - Works Van - gone to a new home.

Thanks for all the input. So there is a heater matrix in that area? Is there anything else I need to be looking for?

Ok had another look and the only place I can see where this would come from is the recirculation vent at the top of the footwell behind the glovebox, although whatever I set the blower to I can't seem to replicate it. The smaller deposits can be rubbed to dust in the fingers but the larger stuff is more like eggshell.
Any thoughts?

Sand and small stones from gritting lorry, or do they use rock salt now. Did you ever drive a bit close to one?.

Nope not that, it is not that sort of material.
Anyone know about the heater/cooler/ventilation system? Is there anything that could produce a 'scale' or something similar that could be blown through?
It seldomly happens but I would like to get to the bottom of it just in case something is on the way out. The only other thing that might be linked is on very rare occasions, when I would not expect it the front screen misting up but I can clear that very quickly with the blower.
Any other thoughts anyone?

Judging by the pattern, density, colour and small amount and position in vechile

HAVE YOU BEEN TO TAL-ACRE BEACH (as the scoucers call it )in north wales ..???

 Wales
Were the mud is thicker, and the ruts are deeper.
(And we have the water to wash it off, and more on the way..! )
Sd4 and chrome Ar*e piece, Auto box for best fun
09 Xs Gone
12 xs Gone
HSE with Leather seats and 4 years warrenty.. staying for now !

Hmmm let me think about that one!

It could be "scale" deposits from a heater matrix that are getting into the blower somehow - I'm afraid I don't know the LR stuff well enough yet to be confident but it seems likely that some sort of deposit (or could it be a component part) in the heater/ventilation system is breaking down and depositing itself.
What made me think of this was memories of the sandy gunk you used to get on older car cooling pipes etc. where hose connectors were loose or there had been a slow leak - that was pale and crumbly but with uneven lumps and some fine dust if you disturbed it.
How you strip it down to find the source? I'm afraid I don't know the specifics - would a Haynes manual help? - Perhaps others on the Forum can assist?
HTH  No Longer the owner of a 2013MY Fuji White GS

"Fiat Lux" a philosophy not a car

Westville, many thanks for the reply, it sounds like a start to do some more investigation.
Will do some digging! (Excuse the pun )

Cant be a general cause as no one else has or knows of anyone else having the same problem so it must be something unique to your car, unless everyone elses is suddenly due start doing the same thing, I suppose there has to be one to start it all off??