234 6 Epitaxy and Quasiepitaxy in Solutions
This position allowed preventing the crystals homogeneously nucleated in a bulk of
the solution from dropping down onto the substrate. Both substances composing
the pairs served as substrates and precipitates.
Temperature of K
2
Cr
2
O
7
–KBr–H
2
O solutions was gradually lowered until pre-
cipitated matter became visible (eightfold magnification) on the substrates and then
it was maintained constant for 15–20 min (the size of the precipitated crystals was
about 0.n–1 mm). Solutions of KCl–NaCl–(MgCl
2
)–H
2
O were supercooled by 1.0
or 2.0°C; the residence times were 40–60 min. Then the samples were isolated,
dried, and distribution of angles of deviation from the epitaxial direction was stud-
ied under an optical or electron microscope. Growth and dissolution rates were also
measured in situ by means of an optical microscope under the above conditions
according to movement of the {010} face for K
2
Cr
2
O
7
, and the {100} faces for KBr,
KCl, and NaCl. Growth rates of K
2
Cr
2
O
7
and KBr were determined independently
in different portions of the same solutions. In experiments with KCl and NaCl first
the growth rates of the substrates had been measured, and after that, the partner-
phase crystals were caused to precipitate on the substrates. Growth rates of the
precipitating crystals were determined independently.
Histograms of orientation distribution were plotted for each experiment. The his-
togram step in each case was 2°. Deviation angles were measured in diapason 0–45°.
Measurements in diapason 45–90° were not conducted, as it would result in mirrored
distribution pattern. Distributions have maxima in the epitaxial direction; when the
angle of deviation from the epitaxial direction increases, the number of oriented crys-
tals diminishes until reaching the background line. Orientation degree was deter-
mined as the value of E = s(h − a)/nh, where s was a number of crystals occupying
the peak domain down to the baseline, h – a number of epitaxially oriented crystals,
a – the average background value, n – the total number of the grown crystals. The
value E takes into account both a fraction (h − a)/n of the precisely oriented crystals,
and the fraction s/n of crystals having various degrees of orientation. Complete diso-
rientation (E = 0) is shown as an absence of the maxima in the histograms and s = a
= h; precise orientation looks like a sharp maximum and absence of the background
with a = 0, s = h = n.
The most valuable data on orientation of adhered crystals were obtained in the
experiments involving precipitation of the cleaved microfragments from a suspension
of CaCO
3
onto horizontal rhombohedral cleaved chips of calcite or onto growing
faces of sodium nitrate rhombohedrons, which had almost similar structures (space
group R3
_
c; for CaCO
3
a
rh
= 6.42, a = 101°55′; for NaNO
3
a
rh
= 6.49, a = 102°40′).
Calcite is insoluble, and, thus, its heterogeneous nucleation is impossible. Therefore,
calcite is useful in the studies of crystal orientation in precipitates which exclusively
adhere to the substrate. Precipitation onto a calcite substrate was conducted in suspen-
sions prepared in different solvents (water, acetone, dioxane, etc.) at ambient tem-
perature. Precipitation onto a sodium nitrate substrate was performed in aqueous
suspensions of NaNO
3
obtained by 3°C supercooling the corresponding aqueous
solutions prepared at 35°C. The experiments were conducted in vigorously stirred
media. The optimum consistence of the suspensions (≈0.02 g/ml) ensuring smooth
precipitation of sufficient amounts of separated and unsticking fragments was deter-