CHABAZITE

Chabazite – (Ca,K2,Na2)2[Al2Si4O12]2.12H2O – is a tectosilicate from the Zeolite Group. Typically occurs as a secondary mineral in vesicles and fractures of volcanic rocks and is uncommon, it is one of the rarest zeolites. It has no economic importance.

“Chabazite” is actually not a mineral, but just a generic term used for members of the Chabazite Series: chabazite-Ca, chabazite-Na, chabazite-K, chabazite-Mg and chabazite-Mr. The most common is chabazite-Ca. It is not possible to distinguish the members from each other by optical means. Specimens that have not been analyzed in detail are simply called “chabazite”.

1. Characteristics

The informations below refer to chabazite-(Ca), which is the most common.

Crystal system: Triclinic pinacoidal. 

Color: Colorless, white, more rarely yellow, orange, pink or red.     

Habit: Pseudo-rhombohedrons, pseudo-hexagons, pseudo-cubes. Tabular, massive. See twinning! 

Cleavage: {10-11} distinct.       

Tenacity: Brittle.        

Twinning: Twins are widespread. They can be on [0001], of interpenetration, simple or repeated. There are also contact twins by [10-11]. Depending on the twins, the crystals look like rhombohedrons, they can simulate cubes. Complex twins produce pseudo-hexagonal crystals, a variety called “phacolite”. Another variety, the “haydenite”, has scalenohedral modifications and orange colors.       

Fracture:  Irregular.      

Mohs Hardness: 4 – 5

Parting: No.         

Streak: White.         

Lustre: Vitreous.          

Diaphaneity: Transparent.           

Density (g/cm³):  2.05 – 2.2

          

2. Geology and Deposits

Chabazite typically occurs in volcanic rocks such as basalts and andesites, filling cavities (vesicles) or fractures. It also occurs in cavities of granites, granodiorites and other plutonic rocks. It can occur in limestones and shales. It is associated with hydrothermal ore veins, where it crystallizes in cavities and fractures. Cryptocrystalline it forms in layered tuffs deposited in water bodies, from the alteration of volcanic ash.

A detailed account of the occurrence, forms and other details of chabazite can be found in the book “Zeolites of the World” by Rudy Tschernich, available for download on the internet. Detailed information on the various ways in which chabazite occurs can be found on the website of the Commission on Natural Zeolites: http://www.iza-online.org/

 

3. Mineral Associations

It occurs with the minerals that form the host rocks: olivine, nepheline, plagioclases, melilite, amphiboles, pyroxenes, magnetite, ilmenite, pyrite, native copper, axinite, clinochlore, epidote and tridymite.

Among the secondary minerals with which it occurs are calcite, prehnite, datolite, apophyllite, clay minerals (smectites), chlorite, chalcedony, macrocrystalline quartz and albite.

Finally, among these secondary minerals there are several other zeolites such as natrolite, mesolite, scholecite, stilbite, phillipsite, chabazite, heulandite and analcime.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.478 – 1.487    nβ: 1.480 – 1.490    nγ: 1.480 – 1.493

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.  

Relief: Low or moderate (literature diverges). 

Cleavage: {10-11} distinct, which may or may not be visible under the microscope, following the rhombohedron. 

Habits: Usually idiomorphic (euhedral), forming complex twins that generate pseudo-rhombohedral crystals (rhombohedrons like calcite) with angles of almost 90º between edges (they look like cubes!).

It can occur in tabular or in rounded and complex twinning shapes.

More rarely anhedral (xenomorphic), massive or in polygonal clusters.

It also forms crystals with complex twins that resemble hexagonal bipyramids (the phacolite variety), always with reentrant angles.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: 0.002 to 0.006: dark gray to light gray colors.

Generally, the grains are subdivided into subgrains with distinct extinction positions and straight boundaries between them.           

Extinction: Symmetrical to grain edges and cleavage. Subgrains may occur, in this case the extinction is by sectors. 

Elongation sign:  Does not apply.           

Twins: Common, interpenetrating, simple and repeated. Contact twins by {10-11}. Basal sections can show six zones with different optical orientations.         

Zoning: Does not have, but can be divided into subgrains.

CONVERGENT LIGHT

Character:  B(+/-) or U(+/-), usually B(+).         

2V angle: from 0 to 32º         

Alterations: No information available.

May be confused with: its pseudo-cubic habit facilitates its recognition in relation to long prismatic (scolecite), fibrous (mordenite) and cubic (analcime) zeolites.

Analcime is similar, but has rounded shapes (trapezohedrons!) and tends to be isotropic.

Cristobalite also contains subgrains, but is extremely rare in thin section.

Gmelinite and levyn are similar and care is needed in the differentiation.

Apophyllite, nosean and haüyne have, among other differences, different habits and higher reliefs.

In XPL, rhombohedral, pseudo-cubic, chabazite-Ca crystals between calcite crystals, in cavity wall lining (vesicle) in acidic volcanic rock (rhyolite). The grains are divided into sub-grains, in an irregular pattern that changes from grain to grain. In PPL, chabazite and calcite are colorless; the relief of chabazite is lower than that of calcite.
In XPL, rhombohedral, pseudo-cubic, chabazite-Ca crystals between calcite crystals, in cavity wall lining (vesicle) in acidic volcanic rock (rhyolite). The grains are divided into sub-grains, in an irregular pattern that changes from grain to grain. In PPL, chabazite and calcite are colorless; the relief of chabazite is lower than that of calcite.
Chabazite 01: In PPL, a pseudo-cubic chabazite crystal grown in a cavity of a volcanic rock. The angles between the edges are not of 90 degrees.
Chabazite 01: The same section as of the previous image, now in XPL and slighlty rotated counterclockwise. Chabazite has very low birefringence and interference colors by sectors, showing that it is a complex twin and not a single crystal.
In XPL, between calcite crystals (beige), aggregate of several chabazite-Ca grains, each grain with its own division into sub-grains.
Chabazite 02: In PPL, several rhombohedral (pseudocubic) crystals of chabazite-Ca crystallized on calcite, within a vesicle (cavity) of a basic volcanic rock (basalt).
Chabazite 02: The same section as of the previous image, now in XPL. The chabazite-Ca crystals are divided into sectors due to the fact that they are complex twins. The thin section is a little too thick, which in this case helps to visualize the Ca-chabazites.
In XPL, chabazite-Ca crystals (gray/black) between calcite crystals (beige), showing the different distributions of sub-grains that can occur in the chabazites-Ca. In PPL, both minerals are colorless.
In XPL, chabazite-Ca crystals (gray/black) between calcite crystals (beige), showing the different distributions of sub-grains that can occur in the chabazites-Ca. In PPL, both minerals are colorless.

5. Reflected Light Microscopy

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

Sample preparation: polishing chabazite is easy; it has the same degree of difficulty as the polishing of calcite, which can occur in association.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, approximately the same shade as quartz and feldspar. Lighter than the lightest pleochroism color of calcite.       

Pleochroism: No.      

Reflectivity: Low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections: Widespread, clear, colorless, milky.      

May be confused with: many other minerals, including many from the same paragenesis, especially other zeolites. It is not possible to identify chabazite by Reflected Light.       

Chabazite 01: In the center of the image, chabzite-Ca grain and, around it, calcite. In dark, a hole in the polished section; it is a cavity (vesicle) wall lining in acidic volcanic rock (rhyolite). Macroscopically, this chabzite-Ca is colorless to white. Calcite shows its pleochroism between lighter gray and darker gray, while chabazite is lighter than the lighter color of calcite. Calcite already shows some internal reflections, chabazite-Ca does not.
Chabazite 01: The same section as of the previous image, now in XPL. Both calcite and chabazite-Ca show colorless to off-white reflections. The identification of chabazite-Ca is not possible.
Chabazite 02: In PPL, chabazite and quartz lining a cavity in rhyolite, but here the chabazite macroscopically is orange. Idiomorphic quartz has worse polishing and higher relief than chabazite-Ca (which fills the interstices between quartz crystals).
Chabazite 02: The same section as of the previous image, now in XPL. Chabazite-Ca shows an uneven distribution of colors, with internal reflections in various shades of orange and red. The camera did not correctly capture this color, which is less red and more orange than in the image. Quartz with milky/clear reflections.