▁mon oh ydro cal cite ▁and ▁i ka ite . ▁When ▁mixing ▁two ▁su pers atur ated ▁solutions ▁of ▁calci um ▁ch lor ide ▁and ▁sod ium ▁carbon ate ▁( or ▁sod ium ▁b ic ar bon ates ) ▁these ▁polym orph s ▁will ▁precip itate ▁from ▁solution ▁following ▁Ost wald ' s ▁step ▁rule , ▁which ▁states ▁that ▁the ▁least ▁stable ▁polym orph ▁will ▁precip itate ▁first . ▁But ▁while ▁A CC ▁is ▁the ▁first ▁product ▁to ▁precip itate , ▁it ▁rapidly ▁transform s ▁into ▁one ▁of ▁the ▁more ▁stable ▁polym orph s ▁within ▁seconds . ▁When ▁in ▁pure ▁Ca CO 3 , ▁A CC ▁transform s ▁within ▁seconds ▁into ▁one ▁of ▁the ▁cry st all ine ▁calci um ▁carbon ate ▁polym orph s . ▁This ▁transformation ▁from ▁am orph ous ▁to ▁cry st all ine ▁is ▁proposed ▁to ▁be ▁a ▁dissol ution - re pre cip itation ▁mechanism . ▁Despite ▁A CC ' s ▁highly ▁un stable ▁nature , ▁some ▁organ isms ▁are ▁able ▁to ▁produce ▁stable ▁A CC . ▁For ▁example , ▁the ▁American ▁Lob ster ▁Hom arus ▁amer ican us , ▁maintain s ▁stable ▁A CC ▁throughout ▁its ▁year ly ▁molt ▁cycle . ▁Studies ▁of ▁bi ogen ic ▁A CC ▁have ▁also ▁shown ▁that ▁these ▁stable ▁forms ▁of ▁A CC ▁are ▁hyd r ated ▁whereas ▁the ▁trans ient ▁forms ▁are ▁not . ▁From ▁observations ▁of ▁sp ic ule ▁growth ▁in ▁sea ▁ur ch ins , ▁it ▁seems ▁that ▁A CC ▁is ▁depos ited ▁at ▁the ▁location ▁of ▁new ▁min