Magnesium chloride is very hygroscopic and tends to react with atmospheric CO2 (carbonation). Seemingly, a combined carbonation/hydration process might have occurred, perhaps prior to the dissolution of the salt in water, possibly accordingly: MgCl2 + 2H2O ⇌ Mg(OH)2 + 2HCl; Mg(OH)2 + CO2 ⇌ MgCO3 + H2O. Also apparently possible, might be carbonation occurring at the solution, particularly if the salt solution is kept for prolonged time under vigorous agitation (e.g. with swirling). Please note that either magnesium hydroxide, carbonate, or bicarbonate, have a very limited solubility in water. Without contamination, the salt should dissolve readily.

Dear Carlos,

Thanks for the info. I will try using sonication.

Daniel, I am quite curious whether you found a way of dissolving it. Let me know!