Homework 1 - Myweb @ CW Post

---! Next, list bonds e-k, in order from most covalent to most ionic. Notice that these
are the major elements bonding with oxygen as in silicate minerals.
coordination polyhedra & ionic substitution
6)! Using Figures 2.7-2.8 and Table 2.5 in the textbook, determine the coordination
number and type for the following ions.
! Note: the values listed in Figs 2.7 & 2.8 are for general use but not necessarily the
specific radius in a given compound. The radius listed for Si +4 in b) below is specific
for its bonding with oxygen and differs from the general radius given in Fig 2.7.
! a) Na + relative to Cl - in halite (Cl- ionic radius: 1.72 Å)
! b) Si +4 relative to O -2 in silicates (Si +4 ionic radius here is ~0.34 Å; Faure, 1998)
! c) Al +3 relative to O -2 in silicates
! d) Na + relative to O -2 in silicates
! e) Fe +2 relative to O -2 in silicates
7)! Atomic and ionic radii vary with valence state but also with the type and number of
surrounding ions (for example, see Table 2.6). Determine the coordination number
and type for carbon in carbon dioxide and in carbonate ions:
! a) C +4 relative to O -2 in CO2 (radius of C +4 in CO2: 0.16 Å)
! b) C +4 relative to O -2 in CO3 -2 (radius of C +4 in CO3 -2 : 0.29 Å)
!
8)! Determine whether the following substitutions will occur. Show why or why not, and
if coupled substitution is required. Use Figure 2.7 and Table 2.6 for cation radii (note
correct coordination in Table 2.6).
a) will Mg +2 substitute for Fe +2 bonded to O -2 in silicates
! b) will Ca +2 substitute for Na + bonded to O -2 in silicates
! c) will K + substitute for Na + bonded to O -2 in silicates