Discussion Forum



1)

 If the unit cell of a mineral  has cubic close-packed (ccp) array of oxygen atoms with m fraction of octahedral  holes occupied  by aluminium ions and n fraction of tetrahedral holes  occupied  by magnesium ions, m and n respectively, are 


A) $\frac{1}{2},\frac{1}{8}$

B) $1,\frac{1}{4}$

C) $\frac{1}{2},\frac{1}{2}$

D) $\frac{1}{4},\frac{1}{8}$

Answer:

Option A

Explanation:

 Oxide ions are at ccp position, hence 4O2- ions, Also there are four octahedral voids and eight tetrahedral voids. Since 'm'  fraction of octahedral  voids contain  Al3+   and 'n'  fraction of tetrahedral voids contain Mg2+ ions, to maintain  electroneutrality 2(2Al3+=+6 charge) and (Mg2+= +2 charge ), will make unit cell neutral 

    Hence  : m= $\frac{2}{4}=\frac{1}{2}$, n=$\frac{1}{8}$