254 7 Crystal Faceting
tainer, whether it was their mixture or divided monomineral masses. Furthermore,
regularities of fluorite faceting observed in numerous crystallizing systems do not
depend upon a precipitation technique, be it metasomatic or direct precipitation.
Typical for the crystal habits of water-soluble ammonium dichromate (NH
4
)
2
Cr
2
O
7
(Glikin et al. 1994) is their pronounced susceptibility to influence of a solution super-
saturation and concentrations of components in it. This case can serve as a negative
illustration of quantitative interpretation of Bravais Law and its modifications.
Structural constants of (NH
4
)
2
Cr
2
O
7
are as follows: C2/c or Cc, a = 13.26 Å,
b = 7.54 Å, c = 7.74 Å, b = 93°12´ (Bystrom and Wilhelmi 1951; Wyckoff 1966;
Dalgaard et al. 1974; ICPDF, 1999); they correspond to the following crystal optics
data: +2V = 104°, Nm = b, cNg = 35°, Ng = 1.905, Nm = 1.80, Np = 1.725 (Winchell
and Winchell 1964), and, partially, to following morphology parameters: 2/m, a:b:c
= 1.0277:1:1.766, b = 93°42´; flattening along {001}, {101}, or {100}, elongation
along [010], faceting by 12 pinacoids and prisms; cleavage along {101} (Groth
1906; Winchell and Winchell 1964; Dalgaard et al. 1974). However, symmetry 2
occurs systematically in its faceting (Kozlova et al. 1979; Glikin et al. 1994) and
this is a subject of a special consideration.
To conduct a detailed investigation of crystal symmetry and to discover a cor-
relation between the crystal habit and growth conditions, crystals of (NH
4
)
2
Cr
2
O
7
were grown according to a temperature-reduction technique using seeds under
controlled stationary conditions (Petrov et al. 1983) or spontaneous nucleation in
bulk with approximate estimation of supercooling degree, and including the method
of sphere regeneration. Goniometric investigations (scale interval was equal to 4´)
were carried out according to a standard technique using a two-circle instrument
(Bulakh 1981) and photogoniometer with a parabolic mirror (Glazov 1981).
Orientation and perfection of cleavage were studied according to the known
(Tatarskii 1965) quantitative method.
The results obtained are shown in Tables 7.1 and 7.2 and in Figs. 7.3 and 7.4.
Investigations revealed a wide range of face and habit types reducing symmetry
class to 2, which was earlier defined as 2/m (Winchell and Winchell 1964), but
preserving the basic geometric constants, viz.: a:b:c = 1.77:1:1.03, b = 94°12´. This
allowed the crystal forms to be indexed as monohedrons, pinacoids, and dihedrons.
On the contrary, data obtained for cleavage number and orientations proved to be
widely different from the reported values. The most perfect cleavage with 63–64%
occurrence was found to be parallel to {010}, while occurrences of the other two
{h0l} cleavages were found to amount to 6–10% (the cleavage in the vicinity of the
circular section of indicatrix) and 13–23%. Orientation of optical indicatrix turned
out to be absolutely different: Np = b.
The data obtained reflect clearly a decisive influence of medium upon the devel-
opment of various crystal forms the structure of the crystallizing compound can
allow. Development of the habit types described (Fig. 7.5) is determined by super-
cooling conditions in the solution. It is to be noted that supercooling ranges defin-
ing formation of a particular habit are different for freshly prepared and aged
solutions (Table 7.2). Consequently, a crystal habit is affected by the ratio
NH
4
/Cr
2
O
7
, which varies with time due to evaporation of ammonia.