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Formula: NaAlSiO4
Tectosilicate (framework silicate), feldspathoid
Nepheline forms partial solid solutions with both
albite and anorthite.
Crystal System: Hexagonal
Specific gravity: 2.55 to 2.66 measured, 2.64 calculated
Hardness: 5 to 6
Streak: White
Colour: Colourless, white, grey, yellow to brownish, reddish and greenish
Solubility: Readily soluble in hydrochloric acid
Common impurities: Mg,Ca,H2O
Environments:
Plutonic igneous rocks
Volcanic igneous environments
Pegmatites
Carbonatites
Nepheline is a primary rock-forming
mineral that requires a
high soda and low silica environment, and it never occurs together with
quartz. It is the characteristic mineral of the alkaline rocks and it is the most
common of the feldspathoid minerals. It is associated with alkali
feldspars
in nepheline syenite and nepheline
gneiss, and with
plagioclase in alkaline
gabbro
(DHZ4 p258).
In alkaline rocks nepheline is associated with olivine,
augite, diopside and sodium-rich
pyroxenes and amphiboles, but not with
orthopyroxene or pigeonite
(DHZ4 p258).
In some calcium-rich basic rocks nepheline occurs with melilite,
monticellite and wollastonite
(DHZ4 p258).
In some potassium-rich hypabyssal rocks (intrusive igneous rocks that originate at medium to shallow depths within
the crust) and volcanic rocks, nepheline occurs with leucite
(DHZ 4 p258).
Nepheline may be found in
andesite,
basalt,
diorite,
gabbro,
mafic igneous rocks (characteristic),
syenite and
trachyte.
For pure nepheline, the low temperature phase is stable up to about 900oC, when it inverts to the high
temperature phase, which is stable up to 1254oC
(DHZ 4 p231).
At Pudding Island, near Portobello, Dunedin, New Zealand, aluminoceladonite
forms partial pseudomorphs after nepheline embedded in an altered
trachyte
matrix, most likely formed after emplacement of the host trachyte dykes
into the surrounding tuffaceous siltstone and
sandstone
(AJM 19.2.21-25).
The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in
marble. The development of veins of large crystals probably occurred as
a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during
this primary event, with additional species resulting from the
subsequent alteration of scapolite. There seems to be little, if any,
secondary, late-stage mineralisation present.
Nepheline occurs as transparent, pale grey massive material in the walls of the
scapolite veins behind the euhedral crystals. It is associated with pink
corundum
(R&M 96.6.550-552).
Alteration
Nepheline frequently alters to analcime,
cancrinite,
sodalite,
natrolite and thomsonite.
In the nepheline gneiss of southeastern Ontario, Canada,
nepheline is altered by low temperature hydrothermal
activity to natrolite and muscovite
(DHZ4 p252).
albite to nepheline and quartz
Na(AlSi3O8) ⇌ NaAlSiO4 + 2SiO2
(JVW p143)
jadeite to nepheline and albite
2NaAlSi2O6 ⇌ NaAlSiO4 + NaAlSi3O8
At 20 kbar pressure the equilibrium temperature is about 1,000oC (eclogite facies), with equilibrium to the right at
higher temperatures and to the left at lower temperatures
(Minsoc America special paper 2, 151-161 (1969))
nepheline and NaCl from the fluid to sodalite
6NaAlSiO4 + NaCl ⇌ 2Na4(Si3Al3)O12Cl
(MM 55.459-463)
At the Igaliko Complex, South Greenland, sodalite is formed by replacement of
nepheline, leading to a
volume change which in turn causes a network
of fractures. Deep blue fluorescent fluorite forms in these fractures, because
the reaction of nepheline changing to
sodalite reduces the salinity
of the fluid, hence reducing the solubility of fluorite, so it precipitates
(MM 55.380-459)
nepheline and H4SiO4 (silicic acid) to analcime and
H2O
NaAlSiO4 + H4SiO4 ⇌ Na(AlSi2O6).H2O
+ H2O
(http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.613.9474&rep=rep1&type=pdf)
nepheline and diopside to åkermanite,
forsterite and albite
3NaAlSiO4 + 8CaMgSi2O6 ⇌ 4Ca2MgSi2O7 +
2Mg2SiO4 + 3NaAlSi3O8
This reaction is in equilibrium at about 1180oC, with lower temperatures favouring the forward reaction
(DHZ4 p251)
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