7.4 Structure-Chemical Model of Crystal Faceting 277
with this conclusion and can be explained by maximal effect of the salt components
and their equal influence upon the faceting pattern in these systems.
However, this regularity was not displayed by systems containing CaCl
2
or
K
2
SO
4
. Here, fluorite has octahedral faceting under hydrothermal conditions, while
in dry media the faceting is cubic. In general, therefore, molecular complexes
responsible for faceting pattern differ in binary and ternary systems, so, the resulting
crystal habit is also affected by interdependent action of various impurities in ternary
and more complex systems.
Cubic faceting of fluorite in low-temperature aqueous media containing salts
and having pH, which is close to neutrality (Fig. 7.9, Table 7.5), is believed to be a
result of forming a two-layer adsorption film. The film consists of Ca(OH)
2
and
H
2
O, which constitute the first and the following layers, respectively. The size of
component stability region for such systems is difficult to estimate; however, we
can assume with a high degree of reliability that in the majority of almost neutral
systems the least soluble compound, among all the possible structures that can be
construed by combining each positive and negative ion present in the solution, is
portlandite Ca(OH)
2
. The portlandite layer connects fluorite with adsorption water
film, corresponding to one of the principal stability regions in the system that is
most likely to be the closest region to the experimental figurative points.
Indeed, in most cases, addition of an acid results in formation of octahedral faceting
that can be explained in our opinion by disruption of Ca(OH)
2
layer due to elevation of
portlandite solubility in acidic media. Appearance of a new undiagnosed phase in asso-
ciation with the octahedral fluorite in acidic medium indicates formation of new stable
compounds forming adsorption layers with new composition and structure. Unfortunately,
insufficient data prevented identification of these compounds. Formation of the octahe-
dron in alkaline media is evidently associated with a change in the proceeding of
adsorption process, which also cannot be detailed at present. Nevertheless, it should be
mentioned that cubic faceting of fluorite in alkaline medium is always accompanied by
formation of portlandite that is surely to play a certain part in the adsorptive processes.
Attention should also be drawn to the fact that diminishing stability of the cubic crystals
in acidic media was predicted (Glikin and Glazov 1979) by the proposed structural-
chemical model, and then proved by experiments (Kiryanova et al. 1984).
Diminishing stability of cubic faceting at elevated temperatures (Fig. 7.9) can be
attributed to increased morphogenetic influence of the solution salt components
owing to disruption of the top water adsorption layers due to dehydration of the
surface of fluorite and the salt components. Some idea about the processes taking
place in the top adsorption layers, which contain compounds represented by the
regions located near the figurative point, can be derived from the fact that morpho-
drome isolines of CaF
2
–CaCl
2
–NaCl–NaF system have pronounced characteristic
properties in the vicinity of the eutectic points (Fig. 7.10m) that account for the
differences in morphogenetic processes characterized by the adjacent regions situ-
ated at the both sides of the corresponding eutectic points.
Figure 7.12a shows the supposed adsorption structure formed on fluorite, which
forms in conformity to epitaxial conditions and can be presented as follows: substrate/
first adsorption layer/subsequent adsorption layers. Differences between interatomic