A crystal-chemical study of Mg,Al and Ni,Al hydroxy-perchlorates and hydroxy-carbonates

Synthetic products of the form M,_,A1.(OH)rR;| . fHrO, with M : Mg,Ni and R : COr and/or ClOo have been prepared, with x ranging from about 0.33 to 0.20, by precipitation from mixed perchlorate solutions by addition of NaOH; the carbonate ion is present owing to solution of atmospheric CO2. The two kinds of anions show a strong tend€ncy to segregate into separate crystals or, under particular conditions, into separate layers within the same crystals, forming an alternating sequenoe of carbonate and perchlorate layers. Pure carbonate and pure perchlorate phases were obtained by anion exchange reactions. The parameter C, layer thickness, diminished with increase in Al3* ions due to increase in electrostatic attraction between positive layers and negative interlayers. The parameter a for a hexagonal cell also diminished with increase of Al3* ions because of their smaller size. The observed change Aa/L,x agrees semi-quantitatively with calculated values. Reasons are suggested for the range of Al3* ions which can substitute in mixed hydroxide phases and for the commsaly observed value of x = 0.25 in natural minerals, based on possible supercell arrangements of the Al3* and M2* ions.