Chemistry Reference
In-Depth Information
Table 3.2
(Continued)
Aluminous, hydrated Ca Fe, Mg minerals
hornblende
(Na, K)
0-1
Ca
2
(Fe, Mg, Al)
5
(Si,
Al)
8
O
22
(OH)
2
Common hornblende
Ca
2
(Mg, Fe)
4
Al
2
Si
7
O
22
(OH)
2
pargasite
NaCa
2
Mg
4
Al
3
Si
6
O
22
(OH)
2
pistachite
Ca
2
(Al, Fe
3
+
)Al
2
Si
3
O
12
(OH)
pumpellyite
Ca
4
(Mg, Fe
2
+
)(Al,
Fe
3
+
)
5
O(OH)
3
(Si
2
O
7
)
2
(SiO
4
)
2
2 H
2
O
vésuvianite
Ca
10
(Fe, Mg)
2
Al
4
(Si
2
O
7
)
2
(SiO
4
)
5
(OH, F)
4
esseneÏte
CaAlFeSiO
6
merwinite
Ca
3
MgSi
2
O
6
Trigonal carbonates form three series:
calcite CaCO
3
Substitution of Ca
++
by other divalent ions is very limited due to the dif-
ference in ionic radius between these ions. At equilibrium, the molar per-
centage of MgCO
3
in calcite at ordinary temperatures does not exceed a
few %. It may be higher, in metastable state, in some sedimentary condi-
tions, or in some organisms. This magnesium-rich calcite recrystallizes
into magnesian calcite
dolomite. The term magnesian calcite is used
when this percentage exceeds 4%. This percentage of MgCO
3
in calcite
in equilibrium with dolomite increases with temperature reaching 20%
around 800°C. Iron, manganese and, to a lesser extent, strontium also
enter in limited quantities in the lattice of calcite;
+
dolomite CaMg(CO
3
)
2
- ankerite Ca(Fe, Mn)(CO
3
)
2
series
Dolomite has a structure similar to the one of calcite but half the Ca
ions are replaced in an ordered manner by Mg ions. There is a con-
tinuous solid solution between the CaMg(CO
3
)
2
end member and the
CaFe(CO
3
)
2
end member. The word dolomite is restricted to minerals
where the ratio Mg/Fe is higher than 4. It may come up about 3 wt%
MnO in the lattice:
-
magnesite MgCO
3
-siderite FeCO
3
-
rhodocrosite MnCO
3
continuous
series
Although the various trigonal carbonates have some differences in color
and indexes (siderite may be brownish in thin section), the most convenient
way to distinguish these minerals in thin section is staining: the attack by a
weak acid solution of alizarin distinguishes calcite that is stained in pink to
