Chemistry Reference
In-Depth Information
Several formulas have been proposed:
(K, Na, Ca)
0,6
(Fe
3+
, Fe
2+
, Mg, Mn)
6
Si
8
Al (O, OH)
27
· 2-4 H
2
O
(K, Na, Ca)
0,7
(Fe
3+
, Fe
2+
, Mg, Mn, Al)
8
(Si,Al)
12
(O, OH)
27
· n H
2
O
Stilpnomelane is essentially an iron-rich mineral. The iron may be in the
ferrous state (ferrostilpnomelane) or ferric (ferristilpnomelane). The transi-
tion from one to another occurs by the substitution:
Fe
2+
(OH)
-
Fe
3+
O
2-
Stilpnomelane is a rather common mineral in metamorphic metapelites
(and basic rocks) of greenschist facies (with chlorite, muscovite, albite,
eventually epidote, actinolite) and blueschist facies (with phengite, garnet,
glaucophane).
It is also a major constituent of iron ore of banded iron formation (BIF)
type with other iron-bearing minerals: magnetite, minnesotaïte (ferroan
equivalent of talc) grunerite (orthorhombic amphibole) chamosite, greenal-
ite (oxychlorites).
3.2.1.9 Talc
Talc is a phyllosilicate close to the micas, but its occurrences are different. It
will be treated with the magnesian minerals (2.4.1d).
3.2.1.10 Zussmanite - Howieite - Deerite
Zussmanite, K(Fe
2+
, Mg, Mn)
13
AlSi
17
O
42
(OH)
14
(trigonal) is a phyllosilicate,
somewhat similar to trioctahedral micas: a continous octahedral layer con-
taining iron, with tetrahedral (Si,Al) attached on both sides, linked by potas-
sium atoms and tree-member rings of tetrahedra.
Howieite, Na(Fe
2+
, Mn
2+
)
10
(Fe
3+
, Al)Si
12
O
31
(OH)
3
(triclinic) and deer-
ite ((Fe
2+
, Mn
2+
)(Fe
3+
, Al)
3
Si
6
O
20
(OH)
5
(monoclinic) are inosilicates, with 4
pediodic single chains. In howeite ribbons of octahedral layer, similar to the
ones of micas, are sandwiched between two [Si
6
O
17
] chains.
Experimental data (Dempsey, 1981) indicate that zussmanite is sta-
ble between 10-30 kb, at temperatures up to 600°C. The lower pressure
range of deerite is estimated to 4 kb at 200°C, 6 kb at 300°C (Muir Wood,
1972).
These are rare minerals of metamorphism of iron-rich formations in
blue schist facies. They are commonly associated, and occur with such iron-
rich (and manganese-rich) minerals as stilpnomelane, spessatine, riebeckite,
aegyrine, grunerite.
