NOSEAN

Nosean – Na8(Al6Si6O24)SO4.H2O – is a tectosilicate, a feldspathoid from the Sodalite Group. It is a characteristic mineral of alkaline volcanic rocks and has no economic importance other than a mineral for collection. Powdered alkaline rocks, on the other hand, are used in the glass and ceramics industry as fluxes.

Macroscopically, nosean can be confused with pale-colored hauynite; if it presents blue colors it is easily confused with lazurite.

1. Characteristics

Crystal system: Cubic hexaoctahedral.

Color: Colorless, white, grey, brown-gray, blue to black.

Habit: Dodecahedral crystals, may occur granular or massive.

Cleavage: {110} poor.

Tenacity: Brittle.

Twinning: On {111}.

Fracture: Irregular.

Mohs Hardness: 5.5

Parting: No.

Streak: White.

Lustre: Vitreous.

Diaphaneity: Transparent.

Density (g/cm³): 2.3 – 2.4

 

2. Geology and Deposits

Nosean is restricted to alkaline volcanic rocks, mainly phonolites, leucitites and nephelinites. The Sodalite Group occurs in rocks such as phonolites, alkali-basalts, trachytes and feldspathoid basalts.

 

3. Mineral Associations

Nosean occur associated with potassic feldspars (sanidine), other feldspathoids (nepheline, leucite, sodalite), micas (biotite, phlogopite), clinopyroxenes (augite, aegirine-augite), zeolites (natrolite), sulfides (pyrite), oxides (magnetite, ilmenite), garnet, zircon, titanite and calcite.              

More rarely with schäferite, liebenbergite, colusite and minerals from the Whitlockite Group.

 

4. Transmitted Light Microscopy

Refraction indices:  nε:  1,488    no: 1,495

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Usually colorless to gray. It may be pale blue, but it is not pleochroic.

Relief: Very low (negative).

Cleavage: {110} poor, usually not visible under a microscope.

Habits: Dodecahedral, elongated pseudohexagonal prisms (sections with 6 sides), anhedral grains, clouded by inclusions.

Edges may be black. There may be inclusions in concentric zones.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Isotropic, but can simulate weak birefringence if it contains inclusions.

Extinction: Isotropic.

Elongation sign: Isotropic.

Twins: Simple twins.

Zoning: Frequently zoned.

CONVERGENT LIGHT

Character: Isotropic.

2V angle: Isotropic.

Alterations: alters to fibrous zeolites, usually natrolite, which develop and propagate from fractures. It also alters, by weathering, to clay minerals, calcite and cancrinite.

May be confused with: the Sodalite Group also includes sodalite and haüyne. It may be impossible to differentiate members of the Sodalite Group solely by their optical characteristics.

Sodalite is normally colorless, does not have abundant inclusions, and is the only one that occurs in plutonic rocks.

Haüyne is normally blue (pale or strong) on a thin section and usually contains dark inclusions that may be concentrated in sectors, planes, cores or edges.

Fluorite has excellent cleavage, much higher relief and occurs in other paragenesis.

 

Nosean 01: In PPL. pseudohexagonal nosean with its low relief, hexagonal shape or polyhedral outlines, without cleavage and black edges. Around it, smaller nosean crystals and colorless leucite crystals.
Nosean 01: the same section as in the previous image, but now in XPL. Nosean is isotropic. Leucites are very dark, but not isotropic; they show twins.
Nosean 02: Very typical for nosean (and for instance, also for haüyne!) are such partially resorbed phenocrysts with these dark rims (due to exsolved Fe-sulfides and Fe-oxydes), rounded contours and embayed edges. The rectangular patterns inside the crystals the literature explains as alignments of submicroscopic exsolved Fe-sulfides and oxydes. The rock is a sanidinite-nepheline.
Nosean 02: in PPL, another view from the same thin section as of the previous image. The colorless crystals is nepheline.
Most often, the exsolved submicroscopic Fe-sulfides and Fe-oxydes follow concentric zonal patterns that produce dark rims. However, clear rims as in this image (in PPL) also are possible. In XPL, the dark portions of the crystals and the clear rims both are isotropic, revealing that it is a single crystal.
Another nosean crystal with clear rims in PPL. In XPL, the entire crystal is black.
In addition to hexagonal and irregular shapes, nosean may show other shapes, suggesting the occurrence of combined cubic forms. PPL.
In addition to hexagonal and irregular shapes, nosean may show other shapes, suggesting the occurrence of combined cubic forms. PPL.
Nosean 03: In a phonolite, pseudohexagonal nosean crystal with many inclusions, but with clean rims. PPL.
Nosean 03: the same crystal as of the previous image, now in XPL. Leucites show their typical irregular twinning.
In PPL, nosean crystal with alteration developing along fracture planes. The difference in relief between the nosean remains and the alteration minerals is clearly observed.
In PPL, leucite grain in phonolite with idiomorphic colorless inclusions of nosean or haüyne. The predominant form of these inclusions, with a few microns in diameter, is the rhombododecahedron of the cubic system. Often, only a hexagonal outline is observed or the appearance suggests the occurrence of combined cubic forms.

5. Reflected Light Microscopy

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

Sample preparation: the polishing of nosean is simple, it follows the polishing of other feldspathoids and the associated feldspars.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, darker then the color of feldspars.

Pleochroism: No.

Reflectivity: Low (<<10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Isotropic, completely black.

Internal reflections: Nosean becomes completely black in XPL.

May be confused with: few other minerals. The behavior under Reflected Light, considering the paragenesis, is so unique that the mineral is easy to recognize.

General Characteristics: 

Grain shape tends to polygonal; hexagons are possible.

Polishing scratches do not occur.

Polishing pits do not occur.

Inclusions are common; they can be opaque or transparent.

Zonation can be observed without difficulties.

Nosean 01: In PPL, with good polish, nosean grains in a phonolite. The color of the nosean is sligthly darker than the color of feldspars and feldspathoids.
Nosean 01: the same crystals as of the previous image, now in XPL. Nosean is isotropic.
In XPL, in phonolite, leucite crystals (above, rounded, colorless and clear reflections) and a nosean crystal (black) with very evident zonation: a clear rim and a dark core, an image similar to that in Transmitted Light.
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