Chemistry Reference
In-Depth Information
3.3.1.7 Sapphirine
Sapphirine is an inosilicate composed of complex chains of tetrahedra T
6
O
18
connected by bands of octahedra M
7
O
2
, an additional octahedral site con-
nects these bands. The general formula of sapphirine is M
7
(M)O
2
T
6
O
18
,
where site T is the occupied by Si or Al and M site by Mg, Al, Fe
2+
, Fe
3+
.
Sapphirines therefore derive from the formula Mg
8
Si
6
O
20
by substitu-
tions Si Mg
Al
IV
Al
VI
, and to a lesser extend Mg
Fe
2+
and Al
VI
Fe
3+
.
Sapphirine is deficient in silica relatively to cordierite.
Sapphirine is a high temperature mineral (750-800°C) which is replaced
at very high pressures (17-25 kb) by pyrope.
3.3.1.8 Topaz
Al
2
SiO
4
(F, OH)
2
is a mineral close to andalusite (especially in its optical
properties). It is made of double chains of octahedra linked by isolated
(SiO
4
)
4-
tetrahedra. The octahedral sites are occupied by aluminum. Two of
the six anions occupying the apexes of the octahedra are occupied by F or
OH, the others by oxygen. The chemical composition of topaz is very uni-
form. The ratio F/F
OH seems always higher than 70%.
Experimental data suggest that topaz is stable within a temperature
range of about 500-850°C.
+
3.3.1.9 Beryl
Be
3
Al
2
Si
6
O
18
has a structure similar to that of cordierite: Si
6
O
18
rings con-
nected by octahedra containing Al and distorted tetrahedra containing Be.
As in cordierite, there are channels that can contain water and alkali (Na,
Cs). The presence of trace elements either by substitution of Al or in the
channels induces remarkable colorations (in macroscopic samples) of some
beryls: Cr
3+
and/or V
3+
(emerald), Fe
3+
(heliodor, golden yellow), Fe
2+
(aqua-
marine, light blue green), Mn
2+
(morganite, pink). Under the microscope,
beryl is colorless.
3.3.1.10 Corundum
Al
2
O
3
is made of layers of oxygen in an approximately hexagonal close
packing, that provide intermediate sites occupied by aluminum; only 2/3
of these sites are occupied. The red color of ruby is due to the presence of
chromium in substitution to aluminum, the blue color of sapphires of that of
iron and titanium. Minute inclusions of rutile oriented at 60° (sagenite twin)
produce the effect of asterism in star sapphires.
Corundum is under-saturated in silica and alkalis. Its high melting point
(2000-2050°C) denies that it can be of magmatic origin.
