Chemistry Reference
In-Depth Information
red, from dolomite, which remains colorless; potassium ferrocyanide distin-
guishes iron minerals: ankerite is colored in blue, siderite in black.
Aragonite
is the only common orthorhombic carbonate in the rocks. It
is a high pressure polymorph of calcite but can exist in the metastable state
at low pressure and at ordinary temperature under specific conditions: shells
of mollusks, some hydrothermal deposits and sedimentary environments.
Aragonite, either formed in metastable state or from high-pressure meta-
morphism, is more or less rapidly transformed into calcite.
3.4.1.2 Aluminous, anhydrous and hydrated, calcic
silicates
These minerals have compositions close to that of anorthite (Table 3.2)
and play a similar role in conditions where it is unstable (Figure 3.4 and
ยง3.1.2.2).
At low temperatures, the hydrated Ca-Al minerals (epidote, prehnite,
lawsonite) play the role of anorthite under conditions of low temperature
metamorphism at low or high pressure. Scapolite plays the role of feldspar
in the presence of volatile elements (NaCl, CO
3
, SO
4
). Gehlenite of the
melilite group plays the role of a calcic plagioclase in rocks under-saturated
in silica.
Epidote, scapolite, calcic garnet and melilite calcium form families larger
than the above Ca-Al end members.
Epidote group
Epidotes are sorosilicates made of chains of octahedra sharing an edge in
b direction (M site). These chains are linked by SiO
4
tetrahedra and Si
2
O
7
double tetrahedra. Y octahedral sites are attached to the chains. There are
also between the chains the 9-fold and 10-fold coordinated sites occupied by
large cations (X site). The general formula of epidote is:
X
2
Y M
2
SiO
4
Si
2
O
7
.O OH
X
=
Ca, Ce, La, Y, Th, Fe
2+
, Mn
2+
, Mn
3+
Y
=
Al, Fe
3+
, Mn
3+
, Fe
2+
, Mn
2+
, Ti
M
=
Al, Fe
3+
, Mn
3+
Epidotes
s. s.
form a series of general formula Ca
2
(Al, Fe
3+
)Al
2
Si
3
O
12
(OH)
between a purely aluminous end member Ca
2
Al Al
2
Si
3
O
12
(OH) and a ferric-
iron Ca
2
Fe
3+
Al
2
Si
3
O
12
(OH). The iron-bearing end member,
pistachite
, is
monoclinic. The aluminous end member has two polymorphs: a monoclinic
form of low-pressure,
clinozoisite
and an orthorhombic high pressure form
zoisite
. Fe
2
O
3
content of zoisite does not exceed 2 wt%. Thulite is a man-
ganesiferous variety of zoisite containing up to 0.5 wt% of MnO.
