Chemistry Reference
In-Depth Information
Nepheline
Na
3
(K, Na)(AlSiO
4
)
4
[NaAlSiO
4
] There are three
polymorphs: a low temperature hexagonal polymorph (ordered,
stable up to about 900°C), a high-temperature hexagonal pol-
ymorph (disordered, up to 1254-1280°C) and a cubic poly-
morph, carnegieite (unknown in nature).
Kalsilite
KAlSiO
4
With numerous poplymorphs, and among
them kaliophilite.
with additional salts:
sodalite group (cubic minerals)
Sodalite
Na
8
(AlSiO
4
)
6
Cl
2
Noseane
Na
8
(AlSiO
4
)
6
SO
4
Haüyne
(Na, Ca)
4-8
(AlSiO
4
)
6
(SO
4
, S)
1-2
series
Cancrinite
(CaCO
3
end member) -
vishnevite
(Na
2
SO
4
end
member) series (hexagonal minerals) (Na, Ca, K)
6-8
(AlSiO
4
)
6
(CO
3
, SO
4
, Cl)
1-2
(H
2
O)
1-5
Most natural nephelines have compositions close to Na
3
K(AlSiO
4
)
4
.
Substitution K Na is generally limited because these elements occupy dif-
ferent sites: Na is 8-fold coordinated, K is 12-fold coordinated. Experimen-
tally, there are solid solutions (in both nepheline and carnegieite) up to a
pure sodic end member NaAlSiO
4
. The nepheline from some ultra-potassic
lavas of high temperature show solid solutions between nepheline and kal-
silite up to Na
2,3
K
1,7
compositions.
Substitution Na Al Si (the symbol means vacant site) occurs in
low proportion in the high-temperature nepheline, indicating a limited solid
solution of alkali feldspar in nepheline.
Calcium enters in very low proportion in the nepheline by Na Si
Ca
Al substitution.
Substitution K
=>
Na occurs only in very rarely in kalsilite; practically
never in leucite.
3.1.3.2 Occurrences
Feldspathoids are minerals of very deficient in silica rocks, mostly igneous
rocks.
Nepheline is stable under relatively high water pressures: it appears as
well in volcanic rocks (high temperature polymorph) as in plutonic rocks
(low temperature polymorph). It coexists with albite and acidic plagioclase
and thus appears in basic as well as evolved rocks.
Sodalite is a mineral of plutonic and volcanic rocks, but rather appears
in differentiated rocks: nepheline syenites and their volcanic equivalents.
Nosean and haüyne appear only in volcanic and hypabyssal rocks of the
phonolites family.
Leucite occurs exclusively in very potassic, mostly basic volcanic
(and hypabyssal) rocks. In the presence of albite it is transformed into
