Geoscience Reference
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No fundamental differences were found by X-ray topography between the hori-
zontal gradient, vertical gradient, and slow heating methods, when good seeds and
optimized conditions were used [99] . The two principal imperfections readily
observable are crevice flawing and cracks. When growth is on a nonequilibrium
face, there is a tendency for higher growth rates and the formation of hillocks
which tend to protrude across the solute-depleted diffusion layer near the growing
face. The tips of such hillocks experience a high supersaturation and tend to grow
faster than the regions between hillocks. This phenomenon is the hydrothermal
analogue of constitutional supercooling and may be viewed as a kind of dendritic
growth [103,104] . Entrapment of solution between hillocks leads to bubbles of
solutions in the grown material, which are observed as viels. Thus, growth
at slower rates and on equilibrium faces produces a greatly reduced tendency for
crevice flawing.
Cracks are often observed in hydrothermally grown material. In the case of
quartz [27,105] , we found that they were associated with strain caused either by
strain in the seed propagating into the grown crystal or by strain associated with
dislocations arising at inclusions in the new growth. It was further observed in
quartz that disorder regions in seeds propagated into crystals and strain-free seeds
were much less likely to produce cracked growth.
At an annealing temperature which is high enough, berlinite becomes milky,
and in TEM pictures one observes a large density of bubbles with a mean size of
about 1000 ˚ ; while in untreated, as-grown berlinite, no such bubbles are visible.
Furthermore, the growth of these bubbles is accompanied by a severe increase in
the dislocation density of the order of 10 5 cm 2 2 or less, but in the heat-treated crys-
tals this density reaches 10 9 cm 2 2 . An increase of at least six orders of magnitude
is produced, similar to wet quartz, which exhibits the same dramatic dislocation
multiplication. This situation can be observed in berlinite, where bubbles with a
mean diameter of 250 ˚ can be observed together with a very high density of dislo-
cation [100,105] . Morris and Chai (1998) [106] have studied the imperfect low-
angle boundaries and fracture in hydrothermally grown berlinite crystals obtained
on multiple-seed plate arrays.
5.3.5 Thermal Behavior
The stability range of any material with a device potential is an important parame-
ter. Byrappa et al. (1986) [107] have reported the differential thermal analysis for
containing various admixtures ( Figure 5.26 ) [108] . For pure berlinite, the inversion
temperature was reported to be at 586 C and this is similar to that reported by
Beck (1949) [79] . However, there are reports fixing this transition temperature at
581 C [108] and at 579 C [109] . Such a variation in the
inversion temperature
can be explained by the effect of the starting materials, the presence of admixtures
in the final products, and even the metals from the mineralizers which also act as
admixtures.
Studies on the thermal expansion of berlinite have not been reported much in the
literature. Byrappa and Prahallad (1989) [110] have reported the thermal expansion of
αβ
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