CANCRINITE

Cancrinite – (Na,Ca,K)6-8(AlSiO4)(CO3,SO4,Cl)1-2.n1-5H2O – is a tectosilicate of the Feldspathoid Group. It is a rare mineral, from alkaline igneous rocks such as syenites. It has no economic importance.

Cancrinite forms a solid solution with vishnevite, forming a series. May contain Fe, Ti, Mg, K, Cl and S. Yellow cancrinite may fluoresce in intense citric green.

1. Characteristics

Crystal system: Hexagonal piramidal.         

Color: Colorless, white, pink, yellow, blue, orange, red or gray-green.     

Habit: Usually as interstitial xenomorphic grains. Rare hexagonal prismatic crystals.       

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

Tenacity: Brittle.        

Twinning: Lamellar, rare.       

Fracture: Irregular.       

Mohs Hardness: 5 – 6

Parting: No.         

Streak: White.         

Lustre:  Vitreous, resinous, pearly.         

Diaphaneity: Transparent.           

Density (g/cm³): 2.42 – 2.51

 

2. Geology and Deposits

Cancrinite is a rare feldspathoid that occurs in alkaline igneous rocks, including pegmatites and nepheline syenites. It also occurs as an alteration product of nepheline. With high partial pressures of CO2 cancrinite can form pseudomorphs on nepheline in alkaline rocks such as nepheline-syenites.

In volcanic rocks, cancrinite can form idiomorphic crystals – hexagonal prisms or acicular crystals – in cavities.

 

3. Mineral Associations

Cancrinite occurs with feldspars (albite, sanidine), feldspathoids (nepheline), minerals from the Sodalite Group, clinopyroxenes (aegirine, aegirine-augite), garnets (andradite, melanite), annite, zircon, biotite, natrolite, hackmanite and calcite.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 1.507 – 1.528     nε: 1.495 – 1.503

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, never shows color much less pleochroism.          

Relief:  Very low, negative.          

Cleavage: [10-10] perfect, only visible in larger crystals.           

Habits: Typically granular, forming anhedral grains in the interstitial matrix of the rock. Also massive, columnar short to acicular. In veins tends to be fibrous. Rarely like hexagonal sections.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Birefringence from 0.002 to 0.025: colors up to early 2nd order, ranging from pale yellow, yellow and orange 1st order to medium 2nd order colors.          

Extinction:  Parallel to cleavage and crystal form. In anhedral and small grains (the most common form of occurrence), classification of extinction is difficult or impossible.          

Elongation sign: ES(-) by cleavage (diagnostic!).

Twins:  Rare lamellar, usually not visible under the microscope. 

Zoning: No.             

CONVERGENT LIGHT

Character: U(-), may be anomalous biaxial (see angle 2V at right). 

2V angle: May have a small anomalous 2V angle (only a few degrees). 

Alterations: Cancrinite alters to zeolites and calcite.          

May be confused with: Diagnostic are low birefringence, negative relief and paragenesis: a colorless mineral of very low relief, but with strong interference colors, up to late 1st order, orange to bluish-red, in syenitic rocks.

Nepheline has normally lower birefringence.

Vishnevite differs from cancrinite by still lower birefringence, it is almost isotropic.         

Cancrinite 01: In PPL, cancrinite as small, anhedral, interstitial crystals between large grains of potassium feldspar. Its relief is lower than that of feldspars.
Cancrinite 01: The same section as of the previous image, now in XPL. The strong interference colors of cancrinite, of the end of the first order, stand out against the gray of feldspars.
In XPL, feldspar (gray), sodalite (isotropic – black) and cancrinite with its granular habit and typical interference colors.
In XPL, among others, a hexagonal section of cancrinite.
Cancrinite 02: Diagonally in the image, large crystal of cancrinite. In PPL, the very low relief and regular cleavage parallel to the elongation can be observed.
Cancrinite 02: The same section as of the previous image, now in XPL. Cancrinite shows superior 1st order strong (yellow-orange-reddish) interference colors. The crystal is extinct in the vertical position (parallel extinction).
Cancrinite 03: In PPL, diagonally on the image, two large cancrinite crystals. In PPL, colorless and with low relief. With some care, it is possible to observe the cleavage parallel to the elongation.
Cancrinite 03: The same section as of the previous image, now in XPL. Cancrinite shows intense colors, predominating the 1st order orange. In the vertical position, the crystals are extinct (parallel extinction).

5. Reflected Light Microscopy

Reflected Light microscopy is evidently not the recommended analytical method for the identification of cancrinite. However, it is easy to recognize cancrinite in this technique. In addition, it is important to make a polished thin section or section for the identification of the opaque minerals that occur associated with cancrinite.

Sample preparation: Due to its most common habit, which is granular, the polishing of cancrinite is quite difficult and is of low quality, with many holes, which end up being diagnostic, because the minerals that occur associated with cancrinite exhibit a much better polish.

PLANE POLARIZED LIGHT – PPL

Reflection color:  Very light gray with a well-defined yellowish hue, when the macro color of cancrinite is yellow to orange. 

Pleochroism: No.      

Reflectivity: Low (<10%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections:  Generalized and intense in the color of the mineral in hand specimen. When the color of the cancrinite is yellow to orange, the reflections exhibit various shades of the two colors. Colorless cancrinite, which is rarer, will naturally exhibit milky reflections.     

May be confused with:  few other minerals, because among the common minerals, those that are transparent and yellow are not frequent. Looking at paragenesis and habit, the recognition of cancrinite on polished blocks is straightforward, simpler than using thin sections with Transmitted Light, as long as the cancrinite exhibits its typical yellow to orange color.

Cancrinite 01: In sodalite-syenite, cancrinite (in the center, very light gray, many black holes = bad polish), sodalite (top and bottom right, very dark gray) and calcite (right, medium gray, pleochroism).
Cancrinite 01: The same secton as of the previous image, now in XPL. Cancrinite exhibits very evident yellow to orange reflections. Sodalite is blue, calcite shows strong anisotropy in gray.
Cancrinite 02: In the center of the image, cancrinite with a yellowish gray color and many holes (black). Below, very dark gray sodalite. On top, light gray calcite with polishing pits.
Cancrinite 02: The same section as of the previous image, now in XPL. Cancrinite shows yellow internal reflections and typical granular habit. Sodalite with blue internal reflections and calcite with milky reflections and anisotropy.
In XPL, cancrinite (yellow internal reflections) in larger crystals tending to develop a prismatic habit, which is rarer.