Chemistry Reference
In-Depth Information
Scapolite occur as metasomatic replacement of plagioclase in basic igne-
ous rocks (gabbros, dolerites, ophites the Pyrenees) and in some gneisses of
high metamorphic grade.
Some partially altered biotites being shown in their cleavage lenses of
epidote or prehnite.
3.5 ACCESSORY MINERALS
We call here accessory minerals, minerals bearers of elements relatively minor
in the rocks: boron, phosphorus, titanium, zirconium, etc. Although these
elements are often significant, they remain relatively minor to major elements
such as Si, Al, Fe, Mg, Ca, Na, K. Some previously treated minerals fall within
this definition: for example allanite (REE), piemontite (Mn) and beryl (Be).
We also deal here with spinels, which do not fit the above definition,
because they could not conveniently be included in the preceding groups.
3.5.1 Spinel group
3.5.1.1 Chemical composition
Spinels s. l. are cubic minerals with the unit cell formula:
R
2+
8
R
3+
16
O
32
Cations are distributed into eight tetrahedral A sites and 16 octahedral
B sites. There are two structural types, the normal spinels which contain 8
R
2+
in the A site and 16 R
3+
in B site, and inverse spinels with 8 R
3+
in A site
and 8 R
2+
8 R
3+
in B site B.
More simply the formula may be written:
+
R
2
+
R
3
+
2
O
4
ou
R
2
+
O R
3
+
2
O
3
with
R
2+
=
Fe
2+
, Mg, Mn, Zn, Ni, Co, Ca
R
3+
=
Al, Fe
3+
, Cr, V, Mn
3+
Titanium, a tetravalent element, is important in the spinel group; the
Ti- end member has an inverse structure and its formula is
R
2
+
2
Ti
4
+
O
4
with
mostly R
2+
Fe (
ulvospinelle
).
Most end members have been named, some are rare and of limited
interest. There are solubility gaps between these end members. They are
incompletely explored.
The spinel group minerals are classified according to R
3+
.
=
