28 1 Replacement of Monocrystals
matter. The reactants are a pair of polymorphic modifications, but the reaction itself
is not a solid-phase transformation despite the fact that the phase ratio between the
educts and the products is similar to that observed for polymorphic transformations.
Also, it should be pointed out that replacement processes in the system under dis-
cussion are unsymmetrical in respect to the eutonic point, i.e., substitution of nick-
elhexahydrite results in formation of faceted pseudomorphs, while replacement of
retgersite leads to morphologically diffused formations.
1.4 Structural-Morphological Classification
of Replacement Products
The above morphological correlations between protocrystals and their replacement
products are shown in Fig. 1.15. It can be seen that new formations may have
monocrystalline, polycrystalline, and amorphous structure.
In other words, our systematic description of the results includes a basic
two-coordinate classification of replacement products (Table 1.2) according to
the extent to which a new formation loses the shape and details of the corresponding
protocrystal structure.
Preservation of shape details of the protocrystal deteriorates in downward direc-
tion of Table 1.2, from embossed pseudomorphs to dissipated and translocated
automorphs. Dissipated automorphs can be considered to contain a little more
information about the protocrystal compared to translocated automorphs: the
former have real finite dimensions limiting to some extent the potential location of
the protocrystal, while the latter, on the contrary, can provide false information
about the protocrystal location.
In general, preservation of structural details of the protocrystal regresses when mov-
ing from left to right in Table 1.2, from monocrystalline products to amorphous ones.
Monocrystalline products are isomorphic to their protocrystals, i.e., they directly
and to the maximum extent inherit and preserve integrity of the initial atomic struc-
ture, its pattern, and orientation (Glikin and Sinai 1983, 1991).
Polycrystalline products may indirectly reflect certain peculiarities of the pro-
tocrystal atomic structure. If they are isostructural to the primary crystal, the inher-
ited information is mediated by epitaxial metasomatic textures (Glikin and Kaulina
1988). If the protocrystal has nonuniform distribution of impurities, some part of
this information may be preserved in the product in a similar way of distribution of
initial inclusions (Chesnokov 1974, p. 67; Sinai and Glikin 1989). Non-isomorphic
monocrystal products (Kulkov and Glikin 2007; see Sects. 1.3 and 4.2 of the present
monograph) are of spontaneous origin and should be included in this category.
Amorphous products are cavities filled up with a solution (Sinai and Glikin
1989). They do not practically preserve any peculiarities of the protocrystal struc-
ture apart from insoluble solid inclusions, which should be accumulated in the
bottom parts of the cavities formed at the sites of the protocrystals.
It is interesting to point out a nonuniform distribution of new formations accord-
ing to taxons of the classification (Table 1.2), considering that the systems had been