Geoscience Reference
In-Depth Information
5.8.1 Crystal Structure of Apatite
Most of the studies concerning the crystallochemical, geochemical and phase
stability-related aspects of apatite have been carried out on natural or hydrothermally
synthesized large single crystals. The crystal structure of FAp was first determined
by Naray-Szabo (1930)
[213]
and by Mehmel (1930)
[214]
independently. Followed
by these, several publications appeared on the structure of various forms of apatite
[215
217]
. The general agreement on the crystallographic aspects is as follows:
i. Apatite shows ionic bonding character, and it is made up of a close packing of large oxy-
gen ions, resulting in the hexagonal crystal system
[218]
.
ii. The space group of FAp is P6
3/m
[213
217]
.
iii. Although the space group of HAp is believed to be P6
3/m
, HAp prepared at high temperature
is P2
1/b
(monoclinic) at room temperature
[213]
, nearly identical with chlorapatite (ClAp).
iv. A phase transition in HAp is suggested to occur at approximately 200
C, probably due
to the order
disorder orientation at (OH)
2
ions along the c-axis
[219,220]
.
5.8.2 Phase Equilibria
The phase equilibria in the system CaO
H
2
O have been extensively studied by
the solid-state reactions method under the atmospheric pressure of water vapor by Van
Wazer (1958)
[221]
, and in aqueous systems at temperatures lower than 100
Cby
Brown et al. (1991, 1992)
[222,223]
. Biggar (1966)
[224]
has studied the phase equilib-
ria in the system CaO
P
2
O
5
950
C, and P of
1 kbar. Feng and Rockett
[225]
(1979) have studied the system CaO
P
2
O
5
H
2
O in the temperature range 700
P
2
O
5
H
2
Oat
200
C.
Figure 5.58
shows the Ca(OH)
2
Ca
3
(PO
4
)
2
H
2
O at 1000 bar with 50 wt%
°
C
900
HA+L+V
L+V
850
Δ
T
=
130
°
C
HA+V
800
750
x
= 12% HA
Δ
X
r
= 1/2
x
= 24% HA
Δ
X
r
= 1/4
x
= 40%
Δ
X
r
= 1/7
Δ
X
=60%
735
°
CH+L+V
CH+HA+V
700
0
Ca(OH)
2
10
20
30
40
50
90
100
HA
Ca
3
(PO
4
)
2
wt%
Figure 5.58 Phase diagram of Ca(OH)
2
Ca
3
(PO
4
)
2
H
2
O system.
