Page 303 - Handbook Of Multiphase Flow Assurance

302 10. Research methods in flow assurance

FIG. 10.61 Crystal of air hydrate inclusion in Antarctic ice (Nakahara et al., 1988).

for X-ray determination of its structure. The shapes of the single crystals were not reported
though. The initial growth of single crystals of sH (methane + methylcyclopentane) hydrate
is due to Smelik and King (1997). sH crystals grew as hexagonal prisms.
By definition, a cubic crystal must have four three-fold rotational axes of symmetry. The
regular octahedral crystal can have four unique axes drawn, as shown in Fig. 10.62, around
which the crystal can be rotated 3 times through angles of 120°; the axes of rotation are la-
beled with the sign . After each rotation the crystal would be congruent to the initial
orientation.
An octahedral THF hydrate crystal shown in Fig. 10.59 had eight triangular faces. The
triangular faces of the octahedron were identified as the {111}-type crystallographic planes
by similarity with other octahedral crystals. No single crystal X-ray measurements were
made specifically to locate the {111} plane on THF hydrate crystals. A {111} plane is defined
by making a unit step from the crystal center along each of the three crystallographic axes
(〈100〉, 〈010〉, and 〈001〉 directions) and drawing a plane through the three reached points
(Fig. 10.63). This plane produces an equilateral triangle when it crosses a cube represent-
ing a cubic unit cell. The other seven triangular faces in an octahedral crystal represent the
{111}, {111}, {111}, {111}, {111}, {111}, and {111} planes. The observed {111} planes exhibited
the slowest growth rate.