3.3. Comparison of [CO3]-containing

3.3. Comparison of [CO3]-containing silicates with carbonates and
silicates
Our fully relaxed calculations show no obvious discontinuities
occur during the structure compression of these minerals, except
for subtle abnormities in the evolution of lattice parameters. We
explain the abnormalities in terms of the existence of complicated
coordination polyhedra in the structures and the complex rotations
of polyhedra under pressure. Particularly, the Ca-polyhedra are the
most responsible for the volume compression. As there are no such
complicated polyhedra in carbonate (calcite) or silicate (larnite),
their axial ratio curves display no observable changes. The axial
ratios with pressure in spurrite and galuskinite present anisotropy
with b-axis being more compressible than the ac-plane. It can be
structurally interpreted through the orientation of the [CO3] units,
which are parallel to the ac-plane, and exhibit pressure-resistance
to a-axis and c-axis. It corresponds to the anisotropy in axial
compressibility of carbonates, whose the coplanar [CO3] groups are
parallel to the ab-plane, and thus the c-axis is distinctly more
compressible. The pressure coefficients (b) of CeO, SieO and CaeO
bonds are averaged to be w0.5 103
, w0.8 103 and
w2.0 103
, respectively. Comparing with the data of calcite (Brik,
2011), which has the compressibility of 0.82 103 Å GPa1 for
CeO and 5.3 103 Å GPa1 for CaeO, it is easy to find that both the
rigid [CO3] units and the compressible Ca polyhedra in minerals are
stiffer than in carbonates.
In comparison with larnite, these minerals also present much
more compressibility irrespective of the structural similarities. The
Figure 7. bulk modulus associated with larnite was estimated to be
K0 ¼ 166(15) GPa with K0
0 being fixed at 4 (Remy et al., 1997). But
the bulk modulus of spurrite, galuskinite, and tilleyite are generally
calculated to be w70 GPa. The distinction in structure compressibility
is mainly due to the [CO3] groups. As seen in Fig. 4a, the
larnite layers are directly interconnected through [SiO4] tetrahedra
and [CaO7] polyhedra. In contrast in minerals, the SieCa aggregations
are separated by [CO3] groups, leading to a relatively looser
structure. Thus the compressibility in these compounds should be
more than in larnite due to the existence of voids. Nonetheless, the
K0 values of these minerals are close to those measured for calcite
with K0 ¼ 69.5(2.1) GPa and K0
0 ¼ 4(fixed) (Fiquet et al., 1994). To
understand the correlation between the structural compressibility
and the [CO3] proportion, we summarized the K0 values in minerals,
larnite and calcite, and drew them out as a function of the
[CO3] wt.% in Fig. 8. Obviously, the K0 value has a negative correlation
with the [CO3] wt.%. That is, the existence of the [CO3] units
could improve the compressibility of the structure. And conversely,
the higher Si polyhedra proportion, the more incompressible of the
mineral.