'although impossible' is correct and usable in written English.
It is used to express the idea that something is unlikely despite other evidence to the contrary. For example: Although impossible to immediately quantify the impact of the new policy, it is believed to be significant.
North America and part of the Atlantic Ocean will be in view at the time, although impossible to discern.
Perpetual motion, although impossible to produce, has fascinated both inventors and the general public for hundreds of years.
The images, although impossible to verify independently, left the hellish image of a city devoid of people but plagued by random eruptions of fiery, black smoke.
Although impossible to quantify in PIAAC, these cases would be marked as mismatched by field of study when, in practice, they may not experience such mismatch.
Although impossible to know, history tells us that we might expect more of the same; expansion of evo devo into other areas of biology (ecology, physiology, behavior); absorption of evo devo by evolution or a unification of biology in which evo devo plays a major role.
Although impossible to predict when genetic drift occurs, such drift could result in temporal changes in virulence of circulating strains.
It is hard, although not impossible, for that person's career and reputation to recover.
Although seemingly impossible for the All-Star guard, Bryant played beyond his 21 years in the Lakers' 93-91 victory.
Bleach effectively removes the colour from the cloth, so trying to remove the stain is difficult, although not impossible.
It's surely harder now too (although scarcely impossible), to sustain a government cover-up of any major debacle.
Were he to lose Ohio, Mr. Romney would have a number of undesirable, although not impossible, options.
