NEPHELINE

Nepheline – Na3K(Al4Si4O16) – is a tectosilicate, an important feldspathoid, typical of silica-undersaturated rocks, both plutonic, hypabyssal and volcanic. It is also an industrial mineral with many different uses.   

Prismatic crystals of nepheline can reach 10 cm. Nepheline may contain Mg, Ca and H2O.   

There are two varieties. “Elaeolite” is a resinous gray, green or blue nepheline found in Norway. “Eleolite” is a massive, translucent nepheline with a resinous, greenish or reddish luster. A synthetic nepheline is germanate-nepheline, with Ge replacing Si.

1. Characteristics

Crystal system: Hexagonal piramidal. 

Color: White, gray, yellowish.

Habit: Short hexagonal prisms with 6 or 12 sides, granular, massive, compact.

Cleavage: {10-10} poor, {0001} poor.

Tenacity: Brittle.

Twinning: No.

Fracture: Sub-conchoidal.

Mohs Hardness: 5.5 – 6

Parting: No.

Streak: White.

Lustre: Vitreous, resinous.

Diaphaneity: Transparent.

Density (g/cm³): 2.55 – 2.66

 

2. Geology and Deposits

Nepheline is an important feldspathoid that occurs mainly in silica-subsaturated, alkali- and Na-rich rocks, in which it replaces the albite component. It is found in some metamorphic and metasomatic contact rocks, such as phenites.   

Nepheline occurs in aplitic granites and in a series of rocks with feldspathoids: syenites, monzonites, monzodiorites, diorites, dioritoids, monzogabbros, gabbros, gabbroic rocks, anorthosites (with sodalite), syenites, nephelinolites, clinopyroxenites, alkaline basalts, phonolites, tephrite phonolites , leucite-tephrites, basanitic nephelinites, leucitites, analcimites, nephelinites, nepheline phodrites, tephrite phodrites, porphyries with nepheline, rocks with melilite, lamprophyres and apatitolites.   

It also occurs in pegmatites and tuffs.

 

3. Mineral Associations

Nepheline occur with feldspathoids of the Sodalite Group (sodalite, leucite, nosean), alkali feldspars (microcline), micas (biotite, Ti-biotite, phlogopite), sodic plagioclase, olivine, amphiboles and sodic and calcosodic pyroxenes, cancrinite, melilite, fluorapatite, melanite garnet, clinohumite, magnetite, pyrrhotite and ilmenite.

Also with aegirine, eudyalite, schorlomite and zircon.

It almost never occurs associated with quartz. Some texts state that it never occurs with quartz.

 

4. Transmitted Light Microscopy

Refraction indices:  ne: 1.529 – 1.542       no: 1.534 – 1.547

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. In plutonic rocks it can be milky (“elaeolite”).

It may appear “foggy” at the edges of the crystals.

Relief: Low.

Cleavage: {10-10} and {0001} imperfect. They are not visible in thin section.

Habits: Usually in anhedral, interstitial masses. In volcanic rocks, it can form short hexagonal prisms, with square or hexagonal basal sections. It also occurs in equidimensional or columnar grains. May have inclusions and sub-grains.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence is very low, from 0.003 to 0.005, which results in 1st-order medium colors: various shades of gray to white, can be grayish-blue.

Extinction: Tends to be parallel.

Elongation sign: ES(-) in the longitudinal sections.

Twins: Rare, according to {10-10}, {33-65} and {11-22}, but generally not present, which is a very diagnostic feature in relation to other feldspars (plagioclase, anorthoclase, orthoclase, sanidine) and feldspathoids (leucite).

Zoning: Frequently zoned, mainly in volcanic rocks.

CONVERGENT LIGHT

Character: U(-). Isogyres may be slightly separated in some samples; isogyres can be very wide or diffuse. Samples with Ca are U(+).

2V angle: When nepheline is anomalous B(-), it has an angle of 0 – 6º.

Alterations: Nepheline is commonly altered.

Nepheline can alter to clay minerals, analcime, sodalite, calcite and cancrinite.

It also alters to fibrous zeolites (natrolite) and sericite, even forming pseudomorphs of sericite on nepheline.

It can undergo albitization, with the development of areas composed of albite (plagioclase) in the nepheline crystals; these areas have the typical polysynthetic twins of the plagioclases.

May be confused with: several other minerals. Diagnostic for nepheline is the absence of twins, its uniaxial character and the fact that it is almost always cloudy to a greater or lesser degree due to the alteration. When it occurs as anhedral, interstitial material, it is difficult to identify, but the negative uniaxial (or anomalous biaxial!) interference figure is diagnostic.

Sanidine in volcanic rocks can be very difficult or even impossible to distinguish from nepheline, if sanidine displays a very low 2V angle, near to zero, and do not shows the typical Carlsbad twin.

Natrolite occur in the same paragenesis and also displays square or rectangular basal sections.

Quartz is U(+), orthoclase and other feldspars are biaxial.

Apatite and beryl are also U(-), but have much higher reliefs.

Melilite has higher relief and anomalous (bluish) interference colors.

Scapolite has higher birefringence.

Nepheline in volcanic rocks 01: Longitudinal section of nepheline in PPL. Colorless, low relief and without cleavage.
Nepheline in volcanic rocks 01: The same section as in the previous image, now in XPL. Nepheline shows interference colors between gray and white and must give an uniaxial negative interference figure.
Nepheline in volcanic rocks 02: In PPL, a basal hexagonal section of nepheline.
Nepheline in volcanic rocks 02: The same section as in the previous image, now in XPL. Basal sections are isotropic. and it is possible to obtain a good interference figure U(-). Around it, square basal sections of smaller nephelines of the matrix.
Nepheline in volcanic rocks 03: In PPL, square basal section of nepheline, with many other sections of the same type, but much smaller, in the rock matrix.
Nepheline in volcnic rocks 03: the same section as in the previous image, now in XPL. A clear parallelism of these square sections is observed in the thin section.
Nepheline in XPL with incipient albitization. Albite occupies square to rectangular areas with polysynthetic twins and progressively occupies the nepheline crystal.
Nepheline phenocryst in volcanic rock in XPL. Nepheline is altered to albite (albitization), which occupies square to rectangular areas with polysynthetic twins in a zone parallel to the grain margins. In addition, nepheline is changed to natrolite, which are the very fine, almost needle-like crystals that are concentrated in the center of the nepheline crystals.
Nepheline in nephelinite in XPL. The crystals are anhedral and very small: the image was taken with the medium magnification objective (10x) and zoom.
Basal section of nepheline in PPL tending to hexagonal with 6 sides. In XPL presents very dark gray colors, as it is almost in the isotropy section.
Nepheline in non-volcanic rocks 01: In nepheline syenite, on the diagonal of the images, large crystal of nepheline.
Nepheline in non-volcanic rocks 01: The same section as in the previous image, now in XPL. Nepheline shows a greyish-blue interference color, parallel extinction and a well-defined U(-) interference figure.
Nepheline in non-volcanic rocks 02: In eudialyte-syenite, on the diagonal of the images, longitudinal section of a long prismatic crystal of nepheline, partially altered. In PPL the form and the partial alteration are quite diagnostic.
Nepheline in non-volcanic rocks 02: The same section as in the previous image, now in XPL. Nepheline shows whitish colors, parallel extinction and a clear U(-) interference figure.
Nepheline in non-volcanic rocks 03: In eudialyte-syenite, hexagonal basal section of nepheline, greatly altered, in PPL. There are other nepheline grains in the image, in longitudinal sections, showing the same type of alteration.
Nepheline in non-volcanic rocks 03: The same section as of the previous image, now in XPL. The basal section is very dark (isotropy section!), but it gives a very good quality U(-) figure. On the top right, eudialyte with deep gray and blue interference colors.
Nepheline in non-volcanic rocks 04: In alkaline clinopyroxenite (jacupiranguito), in the center of the images, anhedral nepheline. Around it, olivine (colorless, high relief, on top), clinohumite (yellow, next to olivine), phlogopite (orange, cleavage) and opaques (magnetite and pyrrhotite). Calcite also occurs.
Nepheline in non-volcanic rocks 04: The same section as of the previous image, now in XPL. Nepheline is gray and it is necessary to check its interference figure to identify it.
In the same alkaline clinopyroxenite (jacupiranguite) of the previous images, in XPL, idiomorphic sections of nepheline that tend to present 6 sides.
In the same alkaline clinopyroxenite (jacupiranguite) of the previous images, in XPL, idiomorphic sections of nepheline that tend to present 6 sides. The black rhomb is a magnetite octahedron in longitudinal section.
Nepheline in non-volcanic rocks 05: Ijolite in PPL, with nepheline and augite. Nepheline sometimes is very cloudy, sometimes shows no alteration. The augite in PPL has a deep green color, but only a weak pleochroism.
Nepheline in non-volcanic rocks 05: The same section as of the previous images, now in XPL. Nephelines are not idiomorphic and its necessary to confirm them with the interference figure.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of nepheline. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with nepheline, like magnetite, pyrrhotite and ilmenite.

Sample preparation: Nepheline acquires a good polish without any difficulty, like feldspar and quartz, showing the same reflection color.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, like quartz and feldspars.

Pleochroism: No.

Reflectivity: Low (<<10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not observed.

Internal reflections: Generalized in light colors, they can be colorless, milky to white, vary with the grain thickness at the point considered.

May be confused with: many other light colored transparent minerals. It is not possible to identify nepheline under Reflected Light, unless the paragenesis is known and the crystals present show defined forms (idiomorphic).

Nepheline 01: Nepheline (dark gray), aegirine (light gray), and holes (black) in syenite. Nepheline has no diagnostic features.
Nepheline 01: The same section as in the previous image, now in XPL. Nepheline has clear to white reflections. Aegirine with very dark internal reflections. It is only possible to confirm nepheline under Reflected Light if it presents its typical forms.
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