3.2 Material Balance and Mechanisms of Replacement 95
interchange of ions between them. This results in changing the film composition,
which, in turn, leads to resuming the salting-out, formation of a new equilibrium
film, and repeating the whole sequence. As a result of this process, the volume of
the crystal increases.
Prima facie, this process should be extremely slow, as diffusion rate in crystal,
especially at low temperatures, is believed to be negligible. However, noticeable
diffusion and replacement rates may be stimulated by crystal defects (strains, dislo-
cations, and point deformations; microcracks; or solution microinclusions), and
localization of autoepitaxial excrescences in vicinity of defects confirms this conclu-
sion. It is clearly seen in Fig. 1.3c that excrescences produced are distributed along
the edges and the scratch, i.e., regions of strained lattice and dislocations. Figure 1.4
shows that replacement proceeds along the cracks. For example, in reaction Ib/24,
the cracks are developed directly during the replacement, probably, due to heterom-
etry effect (Sternberg 1962): nonuniform distribution of isomorphic components
results in development of stresses and cracks along the cleavage. Both cracks and
dislocations developed according to various schemes of heterometry strain removal
(Punin 1994), as well as lattice deformations in strained regions, accelerate diffusion
in the bulk of crystal to the following effect: the greater the number of defects, the
faster is the diffusion, and the faster the diffusion, the more rapid is the replacement
and formation of new strained areas and defects owing to heterometry.
The results of the experiments described in the previous paragraph unambigu-
ously demonstrate importance of the crystal imperfection for volume-excess
replacements. Development of strains and corresponding imperfections during the
process can be shown by formation of secondary cracking in pseudomorphs of
(Mg,Ni)SO
4
.
7H
2
O after NiSO
4
·7H
2
O (reaction Ib/24). Moreover, published data
obtained by Voloshin and collaborators present in situ X-ray topographic observa-
tion of developing the inhomogeneous strain on the sites of heterogeneous replace-
ment of KHC
8
H
4
O
4
crystals with (K,Rb)HC
8
H
4
O
4
(Glikin et al. 2003).
The author has not succeeded in finding any direct data on acceleration of diffu-
sion in imperfect and strained crystals. However, mechanisms suggested for trans-
port phenomena in crystals (Friedel 1964; Geguzin 1970; Shuvalov et al. 1988)
make this process likely to occur.
Hence, both volume-deficit and volume-excess monocrystalline replacements
involve a common physicochemical scheme of salting-out, i.e., via local acts of
dissolution and subsequent growth. However, they have different mechanisms
of the limiting stages. The former replacement is undoubtedly limited by diffusion
of components in the solution. The latter process is assumed with a high degree of
certainty to be limited by solid-phase diffusion of components.
Proposed model of isomorphic replacement interrelates some of the fundamental
conceptions of chemistry and mineralogy. First of all, a quantitative aspect of the
process has been brought to conformity with the principle of thermodynamic equi-
librium. A priory and ambiguous principle “particle for particle” is replaced by exact
quantitative characteristics of the salting-out reaction and mechanisms of conjugated
dissolution and growth, which control the proportion of components in crystal and
solution. At the same time, the volume effect of the process can be exactly expressed