Chemistry Reference
In-Depth Information
Bushveld Igneous complex, chromite form layers (up to 1-2 m of thickness)
associated with pyroxenites and anorthosites in the Critical Zone, 21 layers
have been counted in the district of Lydenburg. That is a metallurgical chr-
omite rich in iron, low in aluminum and magnesium, sometimes containing
PGMs. The chromium contents ranges from 49 wt% Cr
2
O
3
at the base of
the critical zone to 43 wt% on top. The upper part of the Bushveld (Upper
Zone: gabbros, ferro-gabbro) contains layers of magnetite mined for iron
and vanadium. The contents of titanium and vanadium of this magnetite
range from 12 wt% TiO
2
, 2 wt% V
2
O
5
at the base of the Upper Zone, to
respectively 18 wt% and 0.3 wt% at the top.
Titaniferous magnetite is a ubiquitous mineral in igneous rocks. In
the alkaline series of Velay (France), content in TiO
2
of the magnetites is
21-19 wt% in basalts/basanites, 23.5-23 wt% in hawaiites, 23-15 wt%
in mugearites, 21-14 wt% in benmoreites, 20-15 wt% in trachyte and
quartz trachyte, 14 wt% in phonolites. The content in Al
2
O
3
decrease
from 5.2-5.5 wt% in basalts/basanites to 2-0.5 wt% in the trachytes
and quartz trachytes and 0.10 wt% in phonolites (unpublished data of
E. Berger).
In plutonic rocks magnetite is easily re-equilibrated when the tempera-
ture drops, sometimes with exsolution of ilmenite and/or hematite. Ishihara
(1977) distinguishes M magnetite (and ilmenite) granites and I granites with
only ilmenite. These types are often identified to respectively I (igneous)
granites and S (sedimentary) granites of Chappell and White (1974).
3.5.2 Boron minerals
3.5.2.1 Tourmaline
Tourmaline is a cyclosilicate formed of Si
6
O
18
rings alternating with layers
of complex structure. These layers consist of three central octahedral Y sites
surrounded by six Z sites (distorted octahedra), themselves connected by
three boron ions. These layers are connected by tetrahedral X sites. OH can
substitute for oxygen. The general formula of tourmaline is:
X Y
3
Z
6
B
3
Si
6
(O, OH)
30
(OH, F)
X
=
Na most times, sometimes, Ca
Y
=
Fe
2+
, Mg, Li, Al, Li, Fe
3+
Z
=
Al and rarely Mg, Fe
3+
, Mn, Cr
Most tourmalines belong to the following series:
-
iron-tourmaline (schorl) Na Fe
2+
3
Al
6
- magnesian-tourmaline (dravite)
NaMg
3
Al
6 ;
-
iron-tourmaline - lithium-tourmaline (elbaite) Na(Li, Al)
3
Al
6
