Chemically pure sodium nitrate can be obtained by repeated recrystallization of Chile saltpetre or by synthesis.
It may be purified by recrystallization from hot water.
Schists in the common acceptance of that term are really highly crystalline rocks; fissile slates, shales or sandstones, in which the original sedimentary structures are little modified by recrystallization, are not included in this group by English petrologists, though the French schistes and the German Schiefer are used to designate also rocks of these types.
The chief impurities are copper and ferric sulphates; the former may be removed by adding scrap iron, which precipitates the copper; the latter is eliminated by recrystallization.
The crystals are purified by recrystallization from water.
The potassium salt, after recrystallization from warm water, separates in large tabular crystals.
The precipitated alumina is filtered off, the filtrate evaporated and the ammonium salt of the acid purified by recrystallization.
The recrystallization induced by pressure is probably of still greater importance.
They are rarely metamorphosed to the point of recrystallization, though locally shales are altered to roofing slates, sandstones are indurated, limestones slightly marblized, and coals, originally bituminous, are changed to anthracite in northern Pennsylvania, and to graphite in Rhode Island.
This is recrystallized and roasted to vanadium pentoxide, which is then suspended in water into which ammonia is passed, when ammonium metavanadate is again formed and may be purified by recrystallization.