CELADONITE

Celadonite is a phyllosilicate from the Mica Group. It is a very common clay mineral in volcanic rocks. It has no economic importance.

For decades it was believed that celadonite and glauconite were the same mineral. In addition, celadonite was often confused with malachite because the colors, macroscopically, are indeed quite similar. Celadonite can form crusts several centimeters thick, with mammillated surfaces, on the walls of rock vesicles, quite similar to malachite.

Celadonite is isostructural with Al-celadonite, Cr-celadonite, Fe-Al-celadonite, Fe-celadonite and glauconite. It is the Mg analog of Fe-celadonite and forms a series with muscovite. It is friable and unctuous, and may contain Mn, Ca and Na.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Blue-green, olive green, apple green.     

Habit: Very small micaceous lamellae, also in earthy aggregates.

Cleavage: {001} perfect.      

Tenacity: Fragile.        

Twinning: No.      

Fracture: No.       

Mohs Hardness: 2

Parting: No.         

Streak: No information available.         

Lustre: Resinous, greasy.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.95 – 3.05

         

2. Geology and Deposits

Celadonite occurs almost exclusively in cavities (vesicles) and fractures of basic and intermediate volcanic rocks, such as basalts, diabases, and andesites.

It replaces ferromagnesian minerals such as olivine and pyroxenes when these rocks alter, and can form complete pseudomorphs.

It can occur in metamorphic rocks of the zeolite facies.

 

3. Mineral Associations

It occurs with primary minerals of igneous, volcanic, or hypabyssal rocks, of intermediate to basic composition: plagioclase, clinopyroxenes (augite, pigeonite), olivine, magnetite, ilmenite, apatite, and others.

In the vesicles of rocks, it is associated with the secondary minerals that filled these vesicles. It frequently forms the first mineral layer next to the vesicle wall or is intergrown with the minerals that filled the vesicle, which include silica minerals (quartz, amethyst quartz, rose quartz, chalcedony, opal), calcite, fluor-apophyllite, zeolites (heulandite, laumontite, stilbite), prehnite, chlorite and clinoptilolite, clay minerals, and others.

It can also occur associated with hematite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.606 – 1.625    nβ: 1.630 – 1.662     nγ: 1.579 – 1.661

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Light green with defined pleochroism between yellow-green and blue-green, difficult to see because the crystals are usually very small.

Relief: Moderate.           

Cleavage: {001} perfect, characteristic of micas. Due to the submicroscopic size of the crystals, this cleavage is difficult or impossible to observe. 

Habits:  It always occurs in the form of aggregates formed by submicroscopic crystals with a micaceous habit. It can be earthy. It tends to form rounded aggregates.           

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: maximum birefringence of 0.027, corresponding to strong, intense colors of up to 2nd order. However, the strong color of the celadonite and the very small size of the crystals mask the observation of these colors, so that the color in XPL is almost the same, just a little more intense, than the color in PPL.         

Extinction: Oblique with an angle of 2-3º, it simulates to be paralell.           

Elongation sign: ES(+)            

Twins: No.         

Zoning: No.             

CONVERGENT LIGHT

Character: B(-) simulates being uniaxial, but the minute size of the crystals makes it impossible to obtain the interference figure.

2V angle: 5 – 8º         

Alterations: It easily alters to “limonite,” a generic term for iron hydroxides, often containing goethite. They are masses of yellow to orange color in PPL.          

May be confused with:  few other minerals.

Chlorite also exhibits green colors, but has a lower relief and shows anomalous interference colors in XPL, which celadonite does not.

Chalcedony has similar structures, but does not have a green color.

Green biotite develops large crystals, has other interference colors, and occurs in other parageneses.

Glauconite occurs in other parageneses.        

Celadonite 01: In PPL, in basaltic rock, a vesicle (former gas bubble) filled with secondary minerals, including quartz (colorless) and celadonite (green). The celadonite crystals, as always, are extremely small and their pleochroism is subtle.
Celadonite 01: The same section as of the previous image, now in XPL. Celadonite shows a stronger green color.
Celadonite 02: In PPL, in a basaltic rock, celadonite filling former gas bubbles (vesicles). Typical characteristics include strong colors, tiny flakes, and a slight color difference between PPL and XPL.
Celadonite 02: The same section as of the previous image, now in XPL. Green colors are very diagnostic and unique for celadonite in volcanic rocks.
In PPL, celadonite with zeolites filling a vesicle in basaltic rock. The spherical aggregates of the celadonite crystals is typical.
In XPL, volcanic rock (in black in the image) with a vesicle whose filling began with a layer of celadonite (green) and continued with zeolites (gray).
Celadonite 03: In PPL, chlorite and celadonite in a basaltic rock. The left half of the image is the rock (dark brown), crossed by a horizontal vein filled with chlorite (an aggregate of light green flakes). The right half of the image is part of a vesicle filled with celadonite (green) and zeolites (yellowish). The difference between the "green" of chlorite and the "green" of celadonite is clearly visible in PPL.
Celadonite 03: The same section as of the previous image, now in XPL. Chlorite exhibits anomalous gray interference colors, while celadonite shows brighter green colors and a yellowish tint.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying celadonite, but due to its very specific occurrence (basic and intermediate volcanic and hypabyssal rocks), recognizing celadonite is quite simple. It is important to prepare a polished slide or section to identify the opaque minerals associated with celadonite, which in volcanic rocks can be native copper and its alteration minerals, magnetite, ilmenite, pyrrhotite, hematite, goethite, chalcopyrite, and others.

Sample preparation: The extremely low hardness of celadonite and its typical habit of forming very small scales or lamellae make any type of polishing impossible. This forms a very irregular and rough surface that does not reflect light and, therefore, appears very dark in PPL. However, the typical colors of internal reflections in XPL practically eliminate this deficiency and allow celadonite to be recognized.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Because the surface of celadonite is not polished but rather worn, there is no light reflection, and the impression is of a dark, almost black color with a subtle greenish tint. 

Pleochroism: No.      

Reflectivity: No.        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: There is no anisotropy.        

Internal reflections: Widespread across various shades of green.

May be confused with: several other minerals.

Glauconite is very similar, but it only occurs in marine sedimentary rocks, where celadonite does not occur.

Secondary copper minerals of green colors, such as malachite, antlerite, atacamite, and others, which can also occur in volcanic rocks, may be very similar, but their polishing is much better. The use of other analytical techniques may be necessary to define the mineralogy.

Prehnite may be similar, but the green is lighter and the polishing is of much better quality.

This image was obtained with Oblique Light, illuminating the sample from above. It is possible to see how irregular the surface of celadonite (on the left) is in contrast to basaltic rock (on the right). It is not possible to polish celadonite.
Celadonite 01: In PPL, vesicle in basaltic rock filled with celadonite and other seconday minerals.
Celadonite 01: The same section as of the previous image, now in XPL. The purest celadonite is the green band immediately adjacent to the vesicle wall.
Celadonite 02: In PPL, basaltic rock with a martitized magnetite at left and celadonite infilling of a vesicle at right. The green colors and the spherical aggregates of celadonite are clearly visible.
Celadonite 02: The same section as of the previous image, now in XPL. Celadonite displays its green color and globular shape (spherical aggregates of micaceous crystals).
In XPL, in a vitreous volcanic rock of basaltic composition, a vesicle filled with very pure celadonite.