1.5 Genetic Nature of Replacement Products 35
morphology of the products and rate of their formation, especially in the course of
pseudomorph replacements.
At the initial stage of volume-excess replacements, a new formation tightly
envelops the protocrystal, and thereafter reactions Ia/10, Ib/24, 26, 28, 30 (Fig. 1.3),
and 34, III/46 proceed very slowly or terminate. As a result, the process appears
uncompleted and the system remains in nonequilibrium but in a stable state for an
indefinite period of time; the size of the blocked relic can vary up to the initial size
of protocrystal enveloped with very fine (tenths and hundredths of a millimeter)
tight rim of the reaction products. Volume-excess automorph replacement produces
products formed some way apart from the protocrystal, leaving it free for the access
of the solution. However, even in this case the replacement may slow down, like in
system III/43, where a very thin dense bag of copper chromates is formed (Fig.
1.11), the process accelerating abruptly after the bag breaks. On the contrary, the
new formation obtained via replacements with volume deficit always has consider-
able friability, and solution, penetrating through the pores, rapidly and freely reacts
with the protocrystal causing its complete replacement as, for example, in systems
Ia/22 (Fig. 1.9) and Ib/25, 27 (Figs. 1.5 and 1.6).
The sharpest difference in the replacement mechanisms with volume excess and
deficit was observed in monocrystalline isomorphic replacements in system of Type
Ib (Glikin and Sinai 1983; Glikin et al. 1994a, 2003; Glikin 1996a, 2007) accom-
panied by totally new effects, which appeared to be beyond traditional concepts of
crystallization. Detailed analysis is given in the following sections, but the essence
of the process is briefly stated below.
Monocrystalline pseudomorphs are divided into two modifications correspond-
ing to reactions proceeding with excess and deficit of volume.
Pseudomorphs of the first type were observed to be formed in the reactions
Ib/24, 26, 28, 30, 32, 33, and 34 (Fig. 1.3). In this case, a continuous envelope of
the new formation forms a permanent autoepitaxial prolongation of the relic, there-
fore shielding the relic completely. Reactions of this type are very slow; for exam-
ple, it took about 2–3 months to replace the entire volume of a crystal with the size
up to 0.3–0.5 mm, while the large crystals were observed to be surrounded only by
a thin autoepitaxial rim.
The second modification was found in corresponding reversed reactions Ib/25,
27, 29, 31, 32, 33, and 35. The new formations also grew autoepitaxially on the
initial crystal, but they did not cover its whole surface. The volume deficit was
compensated by inclusions formed in the protocrystal that allowed easy detection
of the effect (Fig. 1.6). The reactions were very fast and the total volume of 4–5 mm
crystal was replaced within 2–7 days.
It should be noted that nonlinear isotherms occur more frequently, and due to
this fact the volume effect of the reaction, as a rule, varies with time. This vari-
ation is a reason for the cavities to occur in pseudomorphs obtained in reactions
Ia/11, 14, 22 (Fig. 1.9) and some other effects discussed in Chapters 3–5.
Thus, Fig. 1.16 (diagram Ia) shows that dissolving the peripheral zones of the
protocrystal makes the reaction proceed at the relatively flat bottom sector of
isotherm F
1
–F
2
and, therefore, the volume of the new formation is relatively