Nepheline

tectosilicate

albite

Formula: NaAlSiO4
Tectosilicate (framework silicate), feldspathoid
Nepheline forms partial solid solutions with both albite and anorthite.
Specific gravity: 2.6 to 2.7
Hardness: 5 - 6
Streak: White
Colour: Colourless, white, grey, yellow to brownish, reddish and greenish
Solubility: Readily soluble in hydrochloric acid
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).

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)

Common impurities: Mg,Ca,H2O

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