Chemistry Reference
In-Depth Information
position)-Ca (7-fold coordination)-OH. These sheets determine the perfect
(001) cleavage. The composition of prehnite varies little, the only significant
substitution is that of octahedral alumina by ferric iron.
Prehnite is a mineral of low temperature (less than 380-400°C), charac-
teristic of zeolites and prehnite-pumpellyite facies.
Scapolite group
Scapolite are tectosilicates forming a series between the marialite end mem-
ber 3(NaAlSi
3
O
8
), NaCl and meionite end member 3(CaAl
2
Si
2
O
8
), Ca(CO
3
,
SO
4
). Aluminum is in tetrahedral sites. These are the equivalents of plagi-
oclase but with a looser structure permitting the introduction of large anions
Cl
-
, CO
3
2-
, SO
4
2-
.
Terms close to marialite are sometimes called dipyre. Intermediate terms
were called wernerites.
Scapolites are minerals of medium (550-600°C) at high (850-1050°C)
temperature. The meionite end member is stable at only temperatures above
800°C. They are also stable at relatively high pressures (up to 20 kb). Their
stability depends, of course, on the fluid fugacity and especially that of
CO
2
.
Calcic garnets
Calcci garnets are characterized by calcium as the main cation in X site
(§3.3.1.2). The three end members («ougrandites») are:
Ouvarovite Ca
3
Cr
2
Si
3
O
12
Grossular Ca
3
Al
2
Si
3
O
12
Andradite Ca
3
Fe
3+
2
Si
3
O
12
Ouvarovite
is a rare garnet limited to the chromite deposits associated
with ophiolitic complexes or with mafic-ultramafic complexes of Bushveld
type.
Garnets of the calcic metamophic rocks (marbles, calc-silicate-gneiss,
skarns) belong to the
grossular-andradite
series (“grandites”).
Andradite from alkaline igneous rocks frequently contain titanium,
Ca
3
(Fe
3+
, Fe
2+
, Ti)
2
Si
3
O
12.
This element enters the lattice in Y site by
substitution:
2Fe
3+
Fe
2+
Ti
Some authors consider that titanium may also enter into the tetrahedral
site as a substitution to silica. Titaniferous garnets are called melanite if
Fe
3+
>
Ti and schorlomite Fe
3+
<
Ti.
