The basics of crystallography and diffraction (3rd Edition)

1.11 Some more complex crystal structures 39
tetrahedron being made up of four oxygen anions with the silicon cation in the tetrahedral
interstice in the centre (Fig. 1.30). Silicate chemistry is based on the linking of the SiO 4
tetrahedra, i.e. whether they occur separately, or whether they are linked by common
oxygen anions to form chains, rings, sheets or complete frameworks. This provides the
initial basis for the classification of silicate structures. If the Si–O bond is considered
to be purely ionic, there are four positive charges associated with each silicon cation
and two negative charges associated with each oxygen anion; hence there are four net
negative charges associated with each SiO 4 tetrahedron. The ‘charge balance’in silicates
may be achieved in seven possible ways (see Fig. 1.31):
(a) Separate SiO 4 tetrahedra (nesosilicates): the charge balance (four net negative
charges) is achieved with metal cations, e.g. Mg 2+ ,Fe 2+ , which also link the tetrahedra
together. Typical minerals are forsterite, Mg 2 (SiO 4 ), or fayalite, Fe 2 (SiO 4 ),
the end-members of the olivine group, MgFe(SiO 4 ).
(b) Two tetrahedra linked together sharing one oxygen anion (sorosilicates): the Si:O
ratio is now Si 2 O 7 giving six net negative charges which are balanced with
metal cations. Typical minerals are melilite, Ca 2 Mg(Si 2 O7), or hemimorphite,
Zn 4 (OH)H 2 O(Si 2 O 7 ).
(c) Three or more tetrahedra linked together to form rings, each tetrahedron sharing
two oxygen anions (cyclosilicates): the Si:O ratio is Si n O 3n , where n is the number
of tetrahedra in the ring. A typical mineral is beryl, with a ring of six tetrahedra,
Be 3 Al 2 (Si 6 O 18 ).
(d) Many tetrahedra linked together to form single chains (inosilicates): each tetrahedron
shares two oxygen anions, as in the ring structures above, and which therefore give
rise to the same Si:O ratio. This is the basis of the group of minerals called the
pyroxenes. Typical examples are enstatite, Mg 2 (Si 2 O 6 ), or diopside, CaMg(Si 2 O 6 ).
(e) Tetrahedra linked together to form double chains (inosilicates), each tetrahedron
sharing alternately two and three oxygen anions, giving the Si:O ratio Si 4 O 11 . This
is the basis of the group of minerals called the amphiboles. Typical examples are
anthophyllite, Mg 7 (OH) 2 (Si 4 O 11 ) 2 , or tremolite, Ca 3 Mg 5 (OH) 2 (Si 4 O 11 ) 2 .
(f) Tetrahedra linked together to form sheets (phyllosilicates), each tetrahedron sharing
three oxygen anions giving the Si: O ratio Si 2 O 5 . This is the basis of the micas,
chlorites and the clay minerals.
(g) Tetrahedra linked together such that all the oxygen anions are shared giving a threedimensional
framework (tectosilicates). The Si:O ratio is now SiO 2 and there is an
overall charge balance without the necessity of any linking cations.
Figure 1.31 shows the arrangements of SiO 4 tetrahedra in these seven silicate
structures. However, having established the basic pattern there are very important complications
both in the chemistry and the arrangements of the tetrahedra which must not
be overlooked. In all the silicates, and in the chain, sheet and framework silicates in particular,
the silicon in the centre of the tetrahedron can be substituted by aluminium—a
trivalent rather than a tetravalent ion. For each such substitution an additional positive
charge by way of a ‘linking cation’ is required. All sheet silicates or phyllosilicates show