SODALITE

Sodalite – Na8Al6Si6O24Cl2 – is a feldspathoid, a tectosilicate of more restricted occurrence, occurring in rocks undersaturated in silica. When blue, it constitutes rocks of high commercial value, such as “Granito Azul Bahia” (which is not granite, of course).

It is classified in the Sodalite Group. May contain K, Ca, Fe, Mn, Br, S and H2O. There is a variety with Mo and another, synthetic. The twins form pseudohexagonal prisms. When broken, it can smell like a rotten egg due to the release of H2S.

The literature reports that it is fluorescent: under short UV waves it has vivid yellow, white and orange colors; under long UV waves it has yellowish-white, reddish-orange, blue, white and pink colors. However, this fluorescence only occurs in some cases, generally there is no fluorescence, not even in hand samples, much less in sodalites on thin sections.

1. Characteristics

Crystal system: Cubic hexaoctahedral.          

Color: Many possible colors: colorless, white, yellowish, greenish, various shades of blue, pink, violet, etc.

Habit: Usually massive, interstitial, granular. Rare crystals such as rhombododecahedrons.       

Cleavage: {110} poor.       

Tenacity: Brittle.        

Twinning: On {111} common.      

Fracture: Irregular, conchoidal.       

Mohs Hardness: 5.5 – 6

Parting: No.        

Streak: White.         

Lustre: Vitreous, resinous.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.31

          

2. Geology and Deposits

Sodalite occurs in undersaturated igneous rocks, both intrusive (nepheline-syenites, monzonites, monzodiorites, diorites, gabbros, anorthosites) and extrusive (phonolites, trachytes, porphyries, etc.). Also in cavities of ejected volcanic blocks.

In metamorphic rocks it can be found in limestones (marbles) that have undergone metasomatism.

 

3. Mineral Associations

Sodalite occur with other minerals typical of low silica paragenesis: nepheline, orthoclase, cancrinite, garnet (andradite), aegirine and pyrite.

In addition, with microcline, sanidine, albite, calcite, fluorite, ankerite, analcime, annite, phlogopite, baryte and ussingite.

Sodalite never occurs stably together with quartz!

 

4. Transmitted Light Microscopy

Refraction indices:  n: 1.483 – 1.487

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless to gray; does not have pleochroism.         

Relief: Low.           

Cleavage: {110} poor, normally not observable in thin sections. Alteration along the cleavage planes sometimes occur, in which case the cleavage becomes visible. Depending on the section, one or two cleavage directions are observed.           

Habits:  In intrusive rocks, sodalite occurs as interstitial grains, anhedral masses between larger grains.

In volcanic rocks, sodalite may show polygonal shapes with 4, 6 or 10 sides, may be distorted and/or have rounded corners.           

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Isotropic           

Extinction: Isotropic           

Elongation sign: Isotropic            

Twins:  Isotropic        

Zoning:  Isotropic            

CONVERGENT LIGHT

Character: Isotropic          

2V angle: Isotropic         

Alterations: the alteration coronas (altered margins) are quite characteristic.          

May be confused with: Sodalite Group minerals are difficult to distinguish under a microscope.

Haüyne and nosean do not occur in plutonic rocks such as sodalite, generally exhibit black edges and contain inclusions more often.

Leucite almost always features sector polysynthetic twins, which are more easily visible using the compensator.

Nepheline is U(-) and sodalite will never show interference figure.

Analcime is similar, but has a bad cubic cleavage, extinction by sectors (subgrains, use compensator to see!), can show twins and very often occurs as cavity filling.

Fluorite has much higher relief, may show cubic shapes, may exhibit cubic cleavage (which can be excellent) and/or irregular zonation (spots, stripes, etc.) of colors (violet color is very common).

Unaltered interstitial sodalite can be mistaken for a hole in the thin section! To distinguish sodalite from a hole in the thin section it is possible to:

          (1) observe the contacts along the perimeter between the dark area in CPL and neighboring grains. Intact contacts (well-defined boundaries of neighboring grains) indicate the presence of sodalite; broken contacts are indicative that this black portion is just a hole.

          (2) verify if there are no tenuous signs of cleavage in the black area in CPL. If any, it is indicative of the presence of sodalite with incipient alteration along the cleavage planes.

          (3) verify if there is no irregular network of small anisotropic grains in the dark area at NC. If any, these could be inclusions within the sodalite, which therefore indicate the presence of sodalite.

          (4) observe the slide with the diaphragm completely closed: normally the texture of the glass (= hole in the thin section) is different from the texture of sodalite.

          (5) It is also possible to turn off the microscope light and illuminate the slide obliquely from above with an intense LED light. You can usually detect what is a hole and what is sodalite.         

 

Sodalite 01: Syenite with sodalite in PPL. The green grains with the highest relief are aegirine. Feldspars, cancrinite and sodalite are colorless and of low-relief.
Sodalite 01: The same section as of the previous image, now in XPL. Sodalite is isotropic (black), feldspars are gray, and cancrinite has strong colors (orange, etc.).
In XPL, in a syenite, sodalite (black), cancrinite (granular, colored or white), feldspar (gray) and aegirine (green). Sodalite usually has no idiomorphic contours, but is interstitial.
In XPL, sodalite (black), feldspar (gray, yellowish), cancrinite (small, colored grains). The presence of small anisotropic grains in the middle of the black area indicates sodalite, they probably are inclusions in the sodalite
In XPL, in a syenite, sodalite (black), cancrinite (granular, colored or white), feldspar (gray) and aegirine (green). Sodalite usually has no idiomorphic contours, but is interstitial.
In XPL, it is possible to perceive in these (black) sodalites signs of cleavage (thin white diagonal lines) due to incipient alteration. Here is only one set of lines (diagonally, parallel to the neighboring aegirine (yellow-greenish) cleavage).
In XPL, sodalite (isotropic) with two well-marked cleavage directions. This cleavage is visible only in the altered portions of this sodalite grain.
In XPL, sodalite (black) and feldspar (grey). At the contact, alteration material is developed, which the literature calls “alteration corona”. These coronas are another indication of the presence of sodalite and help to distinguish sodalite from holes in the thin section.
In XPL, detail of the alteration material developed in sodalite (isotropic = black) at the contact with feldspar (gray).

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of sodalite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with sodalite, like pyrite, very typical of this paragenesis.

Sample preparation: sodalite is easy to prepare and acquires a very good polish. The accompanying minerals (feldspars, calcite, etc.) do not pose any problems either.

PLANE POLARIZED LIGHT – PPL

Reflection color: Very dark gray, much darker than common silicates (quartz, feldspars, etc.), it resembles the reflection color of fluorite.       

Pleochroism:  No.     

Reflectivity: Very low (4%?).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.       

Internal reflections: Generalized in the color of the mineral in hand specimen. Blue sodalite has blue reflections and is easy to recognize, but light colored sodalites have a dark gray reflection color as the most diagnostic feature.      

May be confused with: many other light colored transparent minerals if not blue. When blue, it resembles glaucophane and dumortierite, but the paragenesis is obviously different.       

Sodalite 01: In PPL, calcite (light gray, whitish) and sodalite (very dark gray).
Sodalite 01: The same section as in the previous image, now in XPL. Calcite has clear internal reflections and typical anisotropy and sodalite, in this case blue sodalite, has internal reflections in various shades of blue, from very light blue to deep blue, almost black.